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5 DDIISSCCUUSSSSIIOONN

During the past few decades, the study of pollen morphology has

assumed great significance in the realm of morphological and comparative

botany due to the realization that the pollen grains reflect in them, to a fair

degree, the facts and facets of the biology of the mother plant they belong

to. The structural uniqueness of the pollen wall, the exine has no parallel in

any other morphosystem comprising the plant body. The developmental

morphology itself is unique as the sporopollenin material constituting the

exine has its origin in the gametophytic pollen protoplasm initially, and in

the sporophytic tapetal tissue in succession. The high degree of strength

and tenacity of the exine which is acknowledged as one of the most

resistant material in the organic world is necessitated by the fact that the

function of the exine is the protection of the most vital life material, the

male gametophyte, a critical partner in the biology of reproduction.

The advancements in microscopy, and particularly electron

microscopy, have its significance in our understanding of the structure and

sculpturing of the pollen wall, which has led to the effective use of new

morphological parameters to taxonomic and phylogenetic purposes. The

propounding of the triphyletic theory of angiosperms as the basis of the

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morphological evolution of apertural characters in pollen (Nair, 1979) is an

outstanding example of the effective application of pollen morphology in

plant phylogeny as well as in straightening the ambiguities and problems of

taxonomy. In making morphological analysis of exine characters relevant for

applying them in plant taxonomy and evolution, it is held that the characters

relating to the aperture are primary, exine ornamentation secondary and the

others tertiary in their degree of importance. Principles have been laid and

methods evolved for the evolutionary interpretation of each morphological

category of exine characters (Nair, 1970b) which have facilitated the

increasing use of pollen morphology in comparative botany. The present

study is expected to provide adequate pollen exine information for use in the

taxonomy and evolution of the myrtalean families.

Based on the palynological information gathered in a sizeable number

of taxa of the six families of the Myrtales from the South Indian region, a

brief discussion on the pollen exine morphosystem of these families is made.

In addition aspects such as pollen morphological evolution in the group, role

of palynology in the systematics of the order and their interrelationships,

evolution and affinities are also considered.

5.1 Pollen Morphological analysis

Pollen grains, the male gametophytes of flowering plants have two

main concentric layers in the wall, the inner intine and the outer exine. The

107

morphological characteristics of the pollen grains are manifested in the

outer layer, the exine. A brief discussion of the different pollen

morphological features of the order Myrtales based on the data obtained

from the present investigation is attempted (Table 1). Morphological

analysis of pollen is made in relation to the degree of importance of the

various characters in plant taxonomy, phylogeny and evolution; the

germinal aperture being of primary importance, exine ornamentation

secondary, and the other characters such as exine strata, size and shape

tertiary (Tewari and Nair, 1979).

5.1.1 Pollen aperture

Apertures are specially delimited, generally thin-walled areas in the

outer pollen wall or exine, through which the pollen tube usually emerges

during germination. Although most apertures seem to represent thinner

areas in the exine or aperture membranes, in some pollen grains they are as

thick or thicker than the exine on nonapertural areas on the same grain

(Walker and Doyle, 1975). Apertures serve a second major function in that

they allow for volume-change accommodations (harmomegathy) in pollen

grains subjected to changes in humidity (Wodehouse, 1935; Payne, 1972).

Some apertures may serve both for pollen tube exitus and harmomegathic

changes, while some pollen grains have “pseudo-apertures” which appear to

function solely in a harmomegathic capacity. Harmomegathic mechanisms

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involve the reaction of the complete pollen wall to the turgor pressure of

the cytoplasm (Blackmore and Barnes, 1986). During harmomegathic

movements, different stresses are exerted on the pollen grain wall

depending on whether cytoplasmic volume changes are accomodated

mostly by apertural or mesocolpial areas of the exine (Crane, 1986). This

phenomenon could contribute to the diversity of exine substructures as has

been observed in Eperua. In contrast to the pollen of most other legume

taxa, where volume changes are accommodated by apertural exine, in

Eperua the mesocolpial exine tends to curve inwards when dehydrated.

This may be correlated with the relatively small aperture size (Banks and

Rico, 1999). As discussed by Blackmore and Barnes (1986), it is important

to consider the effects of harmomegathy on the wall structure in the

context of an overview of all functions, phylogenetic history and

ontogenetic constraints and further study including experimental

observations on living grains are required for a better understanding of

this.

Almost all palynological discussions of plant relationships and

phylogeny are based on the aperture form, their number and distribution,

and position (designated as the NPC; N=Number; P=position;

C=character). Erdtman (1969) proposed the NPC-system for the

classification of pollen grains based on the number, position and character

of the aperture.

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With respect to the number of apertures, pollen may be classified as

inaperturate (without aperture), mono-aperturate (with one aperture), di-

aperturate (with two apertures), tri-aperturate (with three apertures), or

poly-aperturate (with more than three apertures). Number of apertures and

their distribution in the pollen grain forms one among many trends in the

evolution of pollen grain morphology (Walker and Doyle, 1975; Van

Campo, 1976; Chanda et al., 1979). An increase in the number of apertures

from the first angiosperm pollen grains to more recent ones appear to be a

trend (Van Campo, 1976; Chanda et al., 1979). The more apertures a

pollen grain has, the more quickly it loses its ability to germinate, and

shorter its life expectancy (Dajos et al., 1992). The more apertures a grain

has, the more is it susceptible to external stresses. Consequently many

apertured pollen grains probably dry up much more quickly than few-

apertured ones, since the pollen wall is very much reduced in the aperture

region (Dajos et al., 1992). In a number of taxa it has been observed that

there is a correlation between the ploidy level and the size of the aperture.

As the ploidy increases the aperture size also increases correspondingly

(Nair and Ravikumar, 1984).

The external characters of pollen, such as size, number of pores and

sculpturing of the exine, are widely used for identification purposes, and

the number of pores on periporate pollen has been used as a diagnostic

character for taxonomic purposes in several families such as the

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Caryophyllaceae, Plantaginaceae and Chenopodiaceae (McAndrews and

Swanson, 1967). Davarynejad et al. (1995), in their study, used pollen

morphology as an indicator for the identification of male pistachio trees

for Pistacio vera varieties. However, it is recognized that these characters

are more or less variable, particularly as a result of the effects of climate

on pollen-forming plant (Kurtz and Liverman, 1958), and several studies

have attempted to demonstrate the relationship between pollen

characteristics and various climatic factors. Species with many pores tend

to have larger pollen grains than species with few pores. This increase in

size may be caused by an increased adaptation to different systems of

animal pollination as well as the level of polyploidy that may be

manifested by larger grains as the ploidy level increases. Mineral nutrition

as well as the genetic factors may also cause variation in pollen size. In

addition, a correlation with climatic factors has been postulated for the

genus Ilex, in which the pollen size increases both with latitude and

altitude (Lobreau-Callen, 1975). There is a trend towards a larger size of

pollen grains from the semi-arid sites to the more arid sites, and which is

also negatively correlated to the site altitude (Belhadj et al., 2007). This is

not in accordance with Kurtz and Liverman (1958) who reported the

occurrence of smaller pollen grains caused by aridity. Another perspective

is given by Do¨mnez and Pinar (2001), who reported a trend towards

greater pollen size from the less to the more specialized taxa in Iris

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species, supporting the general idea that pollen size increases with

evolution. The pollen of angiosperms for instance, has evolved towards an

increasing number of apertures that offers a potential selective advantage,

because it increases the likelihood of contact between the germination site

and the stigmatic surface (Dajoz et al., 1991). Pollen fertility, as well, may

provide additional information of taxonomic importance when correlated

with morphological traits and may help to determine whether a plant

species is successfully adapted to certain ecological conditions (Qureshi et

al., 2002). Crane et al. (1974) reported that pollen fertility could vary

between species, types and years. This variation may be due to different

ecological conditions between the different sampling regions as has been

suggested for Pistacio species (Ozeker et al., 2006).

Beaulieu et al. (2008) found a strong positive relationship between

genome size and cell size, suggesting that genome size may partly

determine, or be correlated with pollen size. The correlation between the

pollen size and genome size was studied in 464 species by (Knight et al.,

2010), but found only a weak relationship between the two in their

analysis.

The members of Myrtales generally are triaperturate. The aperture

number does not show any variation in Rhizophoraceae. The number of

apertures is three, except in Calycopteris floribunda (Combretaceae), and

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Clidemia hirta and Dissotis rotundifolia (Melastomaceae) where

occasional four-apertured grains occur. Myrtaceae showed variation in

aperture number in Psidium 3, 4 and 5 apertured; 3 and 4 apertured grains

in Pimenta, Melaleuca, species of Eugenia and Syzygium. In Lythraceae 3

and 4 apertured grains are met with in Lagerstroemia indica, Cuphea

ignea and Rotala malampuzhensis. In Rotala densiflora the pollen grains

are 4-zonoaperturate which agrees with the previous report by (Graham et

al., 1990). In Onagraceae 3 and 4 apertured pollen grains are present in

Ludwigia suffruticosa; in Fuchsia arborescens the pollen grains are

2-zonoaperturate and in F.fulgens 3, 4 and 5 apertured grains occur.

Apertures may be elongated (furrow-like) or circular (pore-like). The

elongated apertures are termed as colpi, and the circular ones as pori.

Pollen grains with colpus are colpate and with porus, the porate type.

Colpate pollen is essentially restricted to dicotyledonous angiosperms. The

apertures are situated either in the outer layer of the exine (sexine) or in the

inner layer (nexine) or both. An aperture which is a feature of the sexine is

termed as the ectoaperture (ectocolpus, ectoporus), and the one which is a

feature of the nexine is termed as the endoaperture (endocolpus,

endoporus). The outer and the inner faces of the apertures in the colpate

and porate grains may be congruent or incongruent. When the outer and

inner faces are incongruent the apertures are termed colporate and

pororate. The frequency of the three major apertural types (colpate,

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colporate, porate) observed in the present group show that colporate one is

the predominant type (101 species, 36 genera, 6 families) followed by

porate (4 species, 2 genera, 2 families) and the least being colpate type (3

species, 2genera, 2 families) as shown in Table 2 and Fig. 2.

Table 2. Distribution of aperture forms in South Indian Myrtales.

Colpate Colporate Porate

3-co

lpat

e

3-sy

n co

lpat

e

3,4-

Col

pora

te

3,4-

Syn

colp

orat

e

3,4-

hete

ro

colp

orat

e

2-Po

rate

3-Po

rate

3,4,

5-po

rate

Family

G S G S G S G S G S G S G S G S Rhizophoraceae - - - - 5 7 - - - - - - - - - -

Combretaceae - - - - - - - - 6 12 - - - - - -

Myrtaceae - - 1 2 4 5 8 27 - - - - - - - -

Melastomaceae - - - - - - - - 8 26 - - - - - -

Lythraceae - - - - 5 8 1 3 4 6 - - 1 2 - -

Onagraceae 1 1 - - 1 7 - - - - 1 1 - - 1 1

Although the prominent aperture type is colporate in Myrtales,

variations occur in aperture structure. Myrtaceae have syncolpate apertures

in Barringtonia racemosa and B.acutangula but colporate in others. A few

Species of Eugenia and Syzygium, Melaleuca leucodendron, Pimenta

pubescens, Callistemon citrinus, Eucalyptus, Careya arborea and

Barringtonia asiatica (Myrtaceae) and Cuphea species (Lythraceae) have

syncolporate apertures while, Eugenia uniflora, Syzygium jambos,

S.hemisphericum, S.zeylanicum, S.occidentalis, and S.aromaticum have

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parasyncolporate apertures. Exclusively porate apertures have been met

with, in Sonneratia (Lythraceae) and, species of Fuchsia (Onagraceae),

while species of Ludwigia have brevicolpate apertures and in all the

species of Onagraceae studied they are protruding or aspidote type. In

Trapa (Onagraceae) the aperture is of the colpate type.

Fig. 2. Distribution of aperture type in South Indian Myrtales

The apertures usually have aperture membranes. The membranes are

psilate (smooth), granulate (provided with granules), crustate (thickly beset

with coarse granules), etc. In certain apertures the membranes are

sometimes conspicuously thickened so as to assume almost the same

appearance as the interapertural parts of the pollen grains. The

subcategories of the basic apertural morphoform is formed on the basis of

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the nature and form of endoapertures (endocolpium, endoporium), the

apertural membranes, operculum etc. in addition to the number, position

and distribution of the apertures. In the colporate grain the endocolpium

(ora) is usually circular or it may be extended or elongated transversely

(lalongate) or elongated longitudinally (lolongate). The distribution of the

endoaperture type in the taxa of the order Myrtales studied presently is

shown in (Table 3 and Fig.3). Lalongate endoaperture had the highest

frequency with (35 species, 14 genera, 3 families); the endoaperture being

rectangular-lolongate in all the 7 species of Rhizophoraceae presently

studied, the endoapertures being fused in species of Rhizophora and

Bruguiera, a feature not met with in any other myrtalean families and faint

type was exhibited by (25 species, 13 genera, 4 families). Circular

endoaperture was observed in (26 species, 12 genera and 5 families) and

lolongate in (8 species, 5 genera and 2 families) with the lowest frequency.

Table 3. Distribution of endoaperture types in South Indian Myrtales

Faint Circular Lalongate Lolongate Rectangular lolongate Family

G S G S G S G S G S Rhizophoraceae - - - - - - - - 5 7 Combretaceae 2 2 1 2 5 8 - - - -

Myrtaceae 7 17 2 2 3 7 4 6 - -

Melastomaceae 3 5 1 1 6 20 - - - -

Lythraceae 1 1 7 14 - - 1 2 - -

Onagraceae - - 1 7 - - - - - -

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Fig. 3. Distribution of endoaperture type in South Indian Myrtales

In some taxa, the aperture is covered by a cap-like sporopollenin

termed as the operculum. Operculate grains have been observed in

Myrtaceae (Rhodomyrtus) Melastomaceae (Memecylon, Melastoma) and

Lythraceae (Rotala, Nesaea) in the present study. Operculate grains are

characteristic of Gramineae and supposed to be highly evolved.

According to Clarke (1977) the term heterocolpate covers pollen

grains with ectocolpi bearing endoapertures alternating with ectocolpi

which do not. “Subsidiary colpi or the pseudocolpi differ from a normal

furrow in that it is not an exit for the pollen tube” (Faegri and Iversen,

1964) while, according to Erdtman (1971) it is the “Colpoid streaks not

functioning as apertures”. Muller (1981) considers the term to be a

misnomer because pseudocolpi function in volume changes of the pollen

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grain during expansion and contraction in response to moisture content

(harmomegathy of Wodehouse, 1935) therefore he recommends the term

“subsidiary colpi”. Subsidiary colpi or pseudocolpi, generally have a

thinner ektexine than the surrounding mesocolpial areas but in contrast to

the colpi all exine layers are usually represented. The thinning of the ektexine

is gradual, and thus the subsidiary colpi often are not as clearly delimited as

the colpi, although the endexine is increased in thickness just as in the colpi.

The surface sculpture in the pseudocolpi is often different from that of the

colpi. They have been observed in Lythraceae, Acanthaceae, Boraginaceae,

Hydrophyllaceae, Leguminosae and Verbinaceae (Erdtman, 1971; Nowicke

and Skvarla, 1974; Faegri and Iversen, 1975; Ferguson and Skvarla, 1981,

1983; Raj, 1983), Crypteroniaceae (Muller, 1975) and Melastomaceae

(Vasanthy, 1976). Subsidiary colpi have also been observed in Lythraceae,

Combretaceae, Melastomaceae (Patel et al., 1984).

In the present investigation pseudocolpi have been observed in

families such as, Combretaceae (6 genera and 12 species),

Melastomaceae (8 genera and 26 species), and Lythraceae (3 genera and

4 species). Subsidiary colpi or pseudocolpi are either equal to the number

of colpi (isomerous), alternating with them as in Combretaceae and

Melastomaceae or there can be additional subsidiary colpi, as observed in

Rotala, Ammania and Nesaea (Lythraceae), where they are twice the

number of aperture. Distributon of pseudocolpi in the myrtalean families

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is given in Table 4 and Fig. 4. Subsidiary colpi have been reported in

members of Lythraceae by Erdtman (1952), Campos (1964), Graham

(1977), Patel et al., (1984) and Graham et al. (1990). They are reported

to be located on just one polar face of the grain in the Oliniaceae (Patel et

al., 1984), and hence considered as half subsidiary colpi, while in some

members of Alzateaceae, they are reported to be rudimentary or

incipient.

Table 4. Distribution of Pseudocolpi in Myrtalean Families of South India

Number of Pseudocolpi

Three Six

Family

G S G S

Rhizophoraceae - - - -

Combretaceae 6 12 - -

Myrtaceae - - - -

Melastomaceae 8 26 - -

Lythraceae - - 3 4

Onagraceae - - - -

Intercolpar concavities were originally reported by Wodehouse

(1928) in the tribe Mutisieae of Compositae. These are structurally and

functionally similar to subsidiary colpi, distinguished only in that they are

markedly larger. Intercolpar concavities have also been described in

Calyceraceae (Skvarla et al., 1977) and Hoplestigmataceae, Verbenaceae

and Olacaceae by Erdtman (1971) and Raj (1983). In the Myrtales

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intercolpar concavities have been observed in Myrtaceae (2 genera, 2

species) and in Melastomaceae (1 genus, 1 species).

Fig. 4. Distribution of pseudocolpi in South Indian myrtalean familien

Position of the apertures is determined quite early and may be

correlated with the position of the meiotic spindle poles (Blackmore and

Barnes, 1990). With respect to the position of the aperture, pollen grains

are described as polar when the apertures are located on the poles. Polar

apertures may be either proximal or distal. The proximal apertures are met

with in the pteridophytes. Some primitive angiosperms also have proximal

apertures (Nair, 1968). Zonal or equatorial apertures are located on the

equator and global apertures scattered over the surface of the pollen grain.

The zonal and global positions of the apertures are restricted to the

angiosperms alone. The position of the aperture in the order of

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evolutionary progress is proximal, distal, zonal and global. All the taxa

presently studied have zonal apertures.

Viscin threads are sporopollenin containing threads and form most

remarkable structural features of pollen grains in some angiosperms.

Viscin threads are themselves nonsticky, thin, long, flexible, nonelastic

fibers and basically located on the surface of the pollen tetrads or single

grains. One end is free and the other end is attached to the exine of the

pollen. Viscin threads occur in some unrelated families viz., Onagraceae,

Ericaceae, and Caesalpiniaceae (Skvarla et al., 1978; Graham et al., 1980;

Waha, 1984; Hesse, 1984; Takahashi and Skvarla, 1990). Morphological

and chemical variations of viscin threads are notable, smooth pattern

(Rhododendron) and beaded pattern (Oenothera) (Yamada, 1988). The

threads in Onagraceae, Ericaceae and Caesalpiniaceae chemically contain

sporopollenin (Skvarla et al, 1978; Graham et al., 1980; Waha, 1984;

Hesse, 1984), elastic fibers devoid of sporopollenin occur in some

Orchidaceae (Vijayaraghavan and Shukla, 1980) and probably different in

some Caesalpiniaceae (e.g. Delonix: (Cruden and Jensen, 1979; Hesse,

1984). Since the viscin threads hold large number of pollen grains together

from dehisced anther, it is presumed that these threads play an important

role in pollination. Viscin threads may tie the pollen grains like ropes to

insect hairs and bristles.

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Viscin threads have been observed in Onagraceae in the present

study in 2 genera and 4 species. The structure of these threads appear to be

different in these genera, being smooth and tufted (L.adscendens and

L.peruviana), long and twisted (L.suffruticosa) and long and mycelia-like

(F.fulgens).

Permanent tetrad formation represents an advanced character over

solitary grains. In some instances however monads may have evolved from

tetrads (Walker, 1971), and in such cases solitary grains represent an

advanced rather than a primitive character state. Compound pollen grains

in the form of dyad, triad, tetrad, polyad or pollinium occur in more than

56 families of angiosperms (Knox and McConchie, 1986). Among

dicotyledons pollen tetrads are found both, in ancient families possessing a

number of primitive features such as Winteraceae, Monimiaceae,

Annonaceae, and in advanced families like Onagraceae, Ericaceae,

Proseraceae and Mimosaceae. Recently they have also been reported in

Amaryllidaceae (Meerow et al., 1986) and Empetraceae (Kim et al.,

1988). Two common binding systems in the permanent tetrads are

recognized in about 20 families that have been examined (Knox and

McConchie, 1986): the cross wall cohesion mechanism where exine

bridges occur between the grains as in Acacia (Kenrick and Knox, 1979;

Knox and McConchie, 1986), Calluma (Dahl and Rowley, 1991), Drosera

(Takahashi and Sohma, 1982), Hedycarya (Sampson, 1977), Onagraceae

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(Skvarla et al., 1975) and Winteraceae (Sampson, 1981) etc. The simple

cohesion mechanism, refers to the binding of adjacent microspores simply

at the tectal/ bacular level, without the presence of the exine bridges, as in

Asimina (Waha, 1987; Gabarayeva, 1992, 1993), Elaeocharis (Dunbar,

1973), Leshenaulti (Knox and Frederich, 1974), Pyrola (Takahashi and

Sohma, 1980) and Typha (Skvarla and Larson, 1963; Takahashi and

Sohma, 1984). The mechanism of tetrad pollen formation in two species

of Annona by the rotation of microspores has been observed by Tsou

and Fu (2002).

Microspores are unique entities providing important morphological

criteria for taxonomic considerations of plants. Palynologically the various

taxa are either stenopalynous or eurypalynous. In the stenopalynous taxa,

the different sporomorphs are of the same basic type, while in the

eurypalynous ones, they are of more than one type.

Distribution of stenopalynous and eurypalynous conditions in the

myrtalean families presently studied is as follows:-

Eurypalynous families:

COMBRETACEAE 3-zonocolporate

4-zonocolporate

MYRTACEAE 3-zonosyncolpate

3-zonocolporate

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3,4-zonocolporate

3,4,5-zonocolporate

3-zonosyncolporate

3,4-zonosyncolporate

3-zonoparasyncolporate

3,4-zonoparasyncolporate

MELASTOMACEAE 3-zoncolporate

3,4-zonocolporate

LYTHRACEAE 3-zonocolporate

3,4-zonocolporate

4-zonocolporate

3-zonosyncolporate

3,4-zonosyncolporate

3-zonoporate

ONAGRACEAE 3-zonocolpate

3-zonocolporate

3,4-zonocolporate

2-zonoporate

3,4,5-zonoporate

In the eurypalynous families, further groupings at generic level may be

possible based on aperture character. The proposed groupings are as below.

124

COMBRETACEAE

Grains 3-zonoheterocolporate Terminalia

Anogeissus

Lumnitzera

Combretum

Quisqualis

Grains 3,4-zonohetercolporate Calycopteris

MYRTACEAE

Grains 3-zonosyncolpate Barringtonia

Grains 3-zonocolporate Syzygium

Grains 3-zonocolporate (longicolpate) Rhodomyrtus

Grains 3-zonocolporate (brevicolpate) Eugenia

Grains 3,4-zonocolporate Syzygium

Grains 3,4,5-zonocolporate Psidium

Grains 3-zonosyncolporate Eucalyptus

Melaleuca

Syzygium

Careya

Barringtonia

Grains 3,4-zonosyncolporate Callistemon

Psidium

Syzygium

Pimenta

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Grains 3-zonoparasyncolporate Syzygium

Grains 3,4-zonoparasyncolporate Syzygium

MELASTOMACEAE

Grains 3-zonoheterocolporate Osbeckia

Melastoma

Sonerila

Medinilla

Memecylon

Tibouchina

Grains 3,4-zonoheterocolporate Clidemia

Dissotis

LYTHRACEAE

Grains 3-zonocolporate Rotala

Woodfordia

Lawsonia

Lagerstroemia

Punica

Grains 3-zonoheterocolporate Rotala

Ammania

Nesaea

Grains 4-zonocolporate Rotala

Grains 3,4-zonocolporate Rotala

Lagerstroemia

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Grains 3-zonosyncolporate Cuphea

Grains 3,4-zonosyncolporate Cuphea

Grains 3-zonoporate Sonneratia

ONAGRACEAE

Grains 3-colpate Trapa

Grains 3-zonocolporate Ludwigia

Grains 3,4-zonocolporate Ludwigia

Grains 2-zonoporate Fuchsia

Grains 3,4,5-zonoporate Fuchsia

Stenopalynous Family:

RHIZOPHORACEAE

Grains 3-zonocolporate Rhizophora

Kandelia

Bruguiera

Carallia

Blepharistemma

5.1.2 Exine sculpturing

Pollen wall is made up of two layers, the inner intine which surrounds

the cytoplasm and the outer exine. The exine is exceedingly hard and

resistant. Chemically seen the exine is composed of sporopollenin, a

127

substance resembling lignin. It shows morphological features that are of

high diagnostic value which is beautifully revealed by means of scanning

electron microscopy (Nilsson, 1992). The exine generally consists of two

layers, inner homogeneous, nonsculptured (nexine) and an outer variously

sculptured (sexine) layer. The exine is highly resistant and durable. The

exine is also known to be associated with mode of pollination (Knox,

1984).

The pattern of pollen wall sculpturing is species specific and in

majority of cases is determined by the sporophyte (Quiros, 1975).

Blueprint for different sculpturing patterns is laid down at the time of

meiosis, while the tetrads are still encased in the callose wall. Pollen wall

morphology has often been shown to contain taxonomically interesting

information (e.g. Walker and Skvarla 1975, Skvarla et al.,1977).

The surface ornamentation of the exine is a significant morphological

character helping a great deal in the categorization of the various genera and

species within the family and genera (Nair and Sharma, 1965). The

morphoform categorization based on exine ornamentation is particularly

useful in the grains of the unipalynous taxa. The exine surface often presents

various types of ornamentation forms. The two broad categories of

ornamentation patterns are – the depression type and the excrescence

(projection) type. The basic forms are the spinate (spinulate), baculate,

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clavate, gemmate, verrucate and granulate among the excrescence types, the

reticulate (retipilate, foveolate, fossulate, scrobiculate), lophate, striate and

the rugulate among the depression types.

The excrescence type and depression type represent two parallel lines

of evolution with their possible genesis in morphoforms without any

ornamentation. The excrescence form and depression form of ornamentations

are the end products of the same developmental phenomenon and the result of

a secondary activity in exine organization. The primitive angiosperms have

pollen with more or less psilate pattern. Regarding the phylogeny of the

ornamentation pattern it has been argued that a character which would

serve to increase the protection of the protoplast of the spore or pollen

should be considered more primitive. The phylogenetic trend may be from

the psilate to the reticulate to the spiny to the echinate.

The taxa belonging to the order Myrtales presently investigated

possess various types of exine sculpturing. Although the taxa have both

depression type as well as excrescence type of exine ornamentation, the

depression type predominates in the majority of the taxa. The psilate or

smooth exine occurs in (7 species, 5 genera, 4 families), while the different

excrescence types, such as spinulate have been observed in one taxa only,

Myriophyllum tuberculatum, (Haloragaceae, which has been included in

Myrtales by Takhtajan, 1980); granulate in (27 species, 11 genera and 5

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families), while tuberculate in only one species of Myrtaceae. The taxa

belonging to the order exhibited a higher frequency of the depression type

sculpturing such as reticulate (5 species, 3 genera and 2 families); the

reticulate pattern is met with in only one member of Combretaceae

studied, (Lumnitzera racemosa) and four taxa of Myrtaceae (Barringtonia

species and Careya arborea, which were later on shifted to a separate

family Lecythidaceae); scrobiculate in one species of Rhizophoraceae;

punctate in 6 species and 4 genera of Rhizophoraceae; foveolate (9

species, 5 genera and 3 families); striate (14 species, 4 genera and 2

families); rugulate (33 species, 12 genera and 5 families); areolate (4

species and 3 genera and 2 families), the highest frequency of exine

sculpturing being the rugulate (depression type) followed by granulate

(excrescence type). The distribution of exine sculpturing patterns in the

South Indian taxa of Myrtales presently studied has been given in Table 5,

Fig. 5.

130

131

132

5.1.3 Exine stratification

In certain mosses and ferns the sporoderm consists of perine, exine

and intine. The exine comprises an inner, usually homogeneous layer

(nexine), an outer layer (sexine) composed of more or less radial processes

and one or several layers (tegillum), lying more or less parallel to the

general surface of the nexine. The processes may be classified according to

their shape (verrucae, gemmae, baculae, pila, etc.). In certain cases

(bacula) they may have a root in the nexine, protrude as infrategillar

elements; penetrate (or in certain cases by amalgamation) form a tegillum,

continue (if there are two tegilla) as infrategillar elements penetrate the

upper tegillum, and come to an end with a distal suprategillar part. Certain

processes (spinae, spinulae) are generally borne on tegilla only. A tegillum

may be defined as a layer or (layers) formed whenever two or more

processes are united by the deposition of material upon and/or between

their distal parts. According to the areal extension of the tegilla (tegillum),

the sexine is generally striate, reticulate or tectate (areolate). When a

tegillum is present, the tegillum and everything connected with its outer

surface may be referred to as ectosexine, whereas the supporting bacula or

the layer that may be found in their place may be referred to as

endosexine. The nexine often seems to consist of an outer thicker and less

refracting part (ectonexine) and an inner, thinner, probably less resistant

more refracting part (endonexine). In some plants, e.g. Echinops, the outer

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layer (referred to as foot layer by Faegri) of the ectonexine differs from the

inner layer particularly with regard to the staining properties. The

endonexine can be distinctly seen as an individual stratum e.g. Epilobium

species.

Pollen wall enclosing the protoplasm of the grains consists of two

fundamentally different layers, the inner intine and the outer exine. The

intine is hyaline and completely covers the pollen protoplasm. The exine

covers the pollen surface excepting the germinal apertures where it is

absent or greatly reduced. The exine of pollen grains can be divided into

an inner homogeneous layer, endine (syn. nexine, Erdtman, 1952) and an

outer heterogeneous layer, the ectine (syn. sexine, Erdtman, 1952). The

ectine is composed of radial rods, the columellae, which are either free at

their tips or are united to form a layer called tegillum. It appears that the

lumina and spaces between the columellae provide storage space for exine-

bound substances, such as lipids and recognition proteins (Heslop-

Harrison, 1976), as in many pollen grains. Evolution of similar traits in

different lineages can be considered as evidence for convergent adaptive

change (Brooks and McLennan, 1991).

The diversity of pollen surface patterns has been interpreted as a

result of an adaptation for rapid germination, protection from desiccation

and harmomegathic mechanisms within the closely related taxa (Nowicke

  134

and Skvarla, 1979). In order to see if a correlation exists between pollen

sculpturing patterns and habitat within sect. Tithymalopsis, the tectal

variation and habitats of the species were mapped on the phylogenetic

tree proposed by the morphological data (Park, unpubl. data). The

resulting tree shows that the reticulate pollen grains have arisen twice

through independent evolution within section Tithymalopsis. In addition,

reticulate or microreticulate pollen grains are commonly associated with

dry, sandy habitats, because the foveolate patterns only appear in mesic

habitats.

The subject of exine stratification and its nomenclature has long been a

moot point in palynology. But, it has been very aptly dealt with by Faegri and

Iversen (1975), who distinguished between the massive and lamellate

endexine on the one hand, and the ectexine on the other. Ectexine most

often comprised a foot-layer and a tectum separated by columellae. As is

now well-known, it may also exhibit a granular structure below a more or

less obvious tectum. Endexine and ectexine, in Faegri's opinion, differed in

their ontogeny. In both gymno- and angiosperms the endexine is indeed

deposited on periclinal membranes that are apparently produced by the

plasmalemma, and share in tripartite structure (Rowley and Dunbar 1967,

Rowley and Southworth 1967, Dickinson and Heslop-Harrison 1968,

Willemse and Keznickova 1980). A review of most of the work on

endexine ontogeny has been presented by Nabli (1979) who himself

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studied it in several Labiatae. In angiosperms the membranes are short,

and in most cases sporopollenin accretion is such that they are no longer

apparent in the mature grain.

5.1.4 Pollen size and shape

Pollen and spore shapes and size do not apparently possess much

phylogenetic significance. Walker (1976); Le Thomas (1980, 1981), and

Walker and Walker (1984) argued that large boat-shaped, granular and

monosulcate pollen grains are the primitive type in angiosperms. Variation

in pollen size and shape are of less diagnostic value. However, this has

been shown to be of value in certaian instances, especially as an index of

aneuploidy (Nissen, 1950), and for correlating with chromosome number.

The value of pollen size in taxonomy has been emphasized in the

angiosperm family Rubiaceae (Mathew and Philip, 1983). The size of

pollen grains may be affected by the method of preparation and hence can

be rather an unstable character. Acetolysis is known to swell grains to

varying extents depending on the duration of the treatment (Reitsma,

1969). The angiosperm pollen exhibit a tremendous size range, from about

2 to 5µm in Myosotis (Boraginaceae) to over 300µm in Cymbopetalum

(Annonaceae). The pollen size classes are determined based on length of

the longest axis and are minute when less than 10µm, small (10-24µm),

(25-49µm) medium, (50-99µm) large, (100-199µ) very large and greater

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than 200 gigantic ones. Approximately a dozen angiosperm families have

species with pollen grains as large as or larger than 200µm.

Data of pollen size in the myrtalean families studied have shown wide

variation. In the order Myrtales pollen grains of the taxa of the constituent

families are generally medium sized with the exception of a few taxa with

large grains such as Lythraceae (Sonneratia, Punica, Lagerstroemia),

Myrtaceae (Barringtonia, Careya) and Onagraceae (Ludwigia, Fuchsia,

Trapa), while those in the herbaceous members of Lythraceae and members

of Myrtaceae are generally of smaller size. In some cases significant size

variation of pollen grain has been noticed in the same taxon. Pollen size

variation is exhibited by Lagerstroemia indica, L.speciosa, Rotala species

(Lythraceae), Osbeckia species (Melastomaceae) etc.

The shape of the pollen grain is unfixed and hence this character is

not generally considered as a reliable parameter in morphological

analysis of pollen (Nair, 1970). Pollen grain shape is correlated largely

to aperture type which in turn is correlated to polarity and symmetry.

The shape of the pollen grains varies from species to species.

Angiosperm pollen may be either nonfixiform (without definite shape)

or fixiform (with definite shape). Nonfixiform shape is rare in

angiosperms and occur in marine angiosperms such as Zostera, where

the pollen is long (2000µ) and thread-like. Normal fixiform angiosperm

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pollen is divided into two basic shape classes, boat-shaped and globose.

The boat-shaped pollen may be further divided into a number of

subtypes depending on the ratio of the longer equatorial axis to the

shorter.

The outline in polar view or amb, is circular, triangular, or in other

geometrical shapes while in the equatorial view the ratio between the

polar and the equatorial diameters multiplied by 100 gives an indication

of the shape. Based on the shape angiosperm pollen can be grouped into

nine classes such as peroblate grains, oblate, suboblate, oblate-

speroidal, spheroidal, prolate-spheroidal, subprolate, prolate and

perprolate grains (Walker and Doyle, 1975). Most of the different shape

categories were noticed among the myrtalean families except

perprolate. Generally the pollen is oblate or peroblate in Myrtaceae and

Cuphea species, while Careya, Barringtonia and Couroupita have

spheroidal to prolate shape; least frequency is observed in prolate and

spheroidal shape classes.

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Table 6. Distribution of pollen shapes in South Indian Myrtales

Pero

blat

e

Obl

ate

Subo

blat

e

Obl

ate-

sphe

roid

al

Sphe

roid

al

Prol

ate-

sp

hero

idal

Subp

rola

te

Prol

ate

Family

G S G S G S G S G S G S G S G S

Rhizophoraceae - - - - - - 1 1 - - 3 4 2 2 - -

Combretaceae - - - - - - 5 5 - - 3 6 1 1 - -

Myrtaceae 3 7 7 22 - - - - 1 1 2 2 1 1 1 1

Melastomaceae - - - - 1 4 2 3 1 1 6 10 3 8 - -

Lythraceae 1 2 1 1 - - - - - - 2 2 6 11 3 3

Onagraceae - - 2 3 1 2 1 1 1 2 1 1 - - - -

Data of pollen size and shape are made use of in constructing pollen

keys for taxa at various levels. The primitive shape of angiosperm pollen is

clearly boat-shaped, with globose pollen representing an advanced

character-state. The presently studied taxa of Myrtales exhibited much

variation with regard to pollen shape. All the six families of the order have

globose pollen grains with a range of shapes. With regard to the shape of

the pollen grains, Clarke (1980) has reported that in related species of

dicotyledons oblate grains are more advanced than the prolate ones. The

pollen possess peroblate, oblate (Myrtaceae and genus Cuphea of

Lythraceae), oblate-speroidal, prolate-spheroidal, spheroidal, subprolate

and prolate shapes in different families.The distribution of different shapes

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of pollen in the South Indian Myrtales presently studied are given in Table

6 and Fig. 6.

Fig. 6. Distribution of pollen shapes in South Indian Myrtales

5.2 Pollen features in myrtalean families

A general scenario of the pollen morphology of the Myrtales has

been dealt in detail by Patel et al. (1984). The pollen grains in the core

families of the order seem to be basically 3-zonocolporate, and less

frequently two to more than four apertures have been reported. Further the

pollen grains are basically tectate and frequently characterised by pseudocolpi

(intercolpate furrows or “rugae” Erdtman, 1952). The pseudocolpi are not

actual apertures, but conspicuous colpus-like thin parts of exine. In their

early ontogeny, they differ from true apertures by not being subtended by a

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thick layer of intine. Pollen tetrads occur in some groups, especially in

Onagraceae, where another peculiarity is the occurrence of viscin threads.

Pollen data so far known in the group show that pseudocolpi are

present in families like Lythraceae, Oliniaceae, Combretaceae and

Melastomaceae, but are absent or very indistinct in many Lythraceae, (incl.

Punicoideae, Sonneratioideae and Duabungoideae), Trapaceae, Alzateaceae,

Psiloxylaceae, Heteropyxidaceae, Myrtaceae and Onagraceae. More or less

distinct intercolpate depressions in the pollen grains connect the distinctly

heterocolpate pollen grains with other types and make the feature

somewhat vaguely defined (Dahlgren and Thorne, 1984). However,

presence of pseudocolpi is so significant in the Myrtales, and so rare

outside the order, that it may be possible to attach great phylogenetic

significance to its distribution.

Distribution of pseudocolpi in the families of Myrtales presently

studied (Table 4 and Fig.4) show that the occurrence is most common in

Combretaceae followed by Melastomaceae and Lythraceae, while in a few

(Rhizophoraceae, Myrtaceae and Onagraceae) they are nonexistent.

The family Lythraceae shows great variation in the occurrence of

pseudocolpi (Patel et al., 1984). Lythraceae generally posses 3-colporate

pollen grains, several genera with pseudocolpi (as in Ammania, Rotala and

Nesaea in the present study). In Lythraceae, there are six pseudocolpi, two

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alternating with each aperture, as in Ammania, Rotala and Nesaea. A few

other genera lack pseudocolpi. Lawsonia (present study; Campos, 1964)

has incipient or rudimentary pseudocolpi. Those and the pollen grains of

Lafoensia, where the pseudocolpi are very faint, could be interpreted as

incipient or reduced. A study of the Lythraceae pollen morphology may

indicate that distinct or faint pseudocolpi seem to occur in derived genera,

as contented by Dahlgren and Thorne (1984). As far as variational

occurrence of pseudocolpi is concerned, Lythraceae are outstanding in the

order, as it may be argued whether absence of pseudocolpi represents an

original or advanced state.

Sonneratia and Duabunga have anguloaperturate pollen grains with

short colpi (Muller, 1969, 1978). In Sonneratia intercolpate depressions

resembling pseudocolpi occur. The pollen grains of Punica resemble those

of Lythraceae. They are 3 or rarely 4-colporate and lack pseudocolpi.

Whether the characteristic pollen grains of Trapa with their

meridional crests of folded exinous material meeting at the poles represent

a heterocolpate type or rather a type with pronounced intercolpate

depressions need to be ascertained. The heterocolpte pollen grain type

represents a distinct category in the order. They include very peculiar

shapes such as that in Oliniaceae where the pseudocolpi are restricted to

one hemisphere (Patel et al., 1984). It appears evident that the families or

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subfamilies with clear heterocolpate pollen grains form a coherent group

along with some others where pseudocolpi are absent or very indistinct

(Lythraceae, subfamily, Punicoideae, Duabungoideae and Sonneratioideae,

Combretaceae, subfamily Strephonematoideae and Alzateaceae) or where

pseudocolpi are “missing or doubled” in number (Lythraceae, subfamily

Lythroideae). None of the families outside Myrtales showing considerable

similarities to them possess pseudocopi.

A critical overview of the aperture scenario of the various myrtalean

families is highlighted as under:

Rhizophoraceae

The pollen morphological studies on the Rhizophoraceae, as a part of

floristic or general morphologic surveys has been done by (Erdtman, 1952;

Kubitzky, 1965; Huang, 1968; Bonnefille, 1971; Guers, 1974; Geh and Keng,

1974; Sowunmi,1973; Straka and Friedrich,1984; Thanikaimoni,1986, 1987).

Comparative palynology in Rhizophoraceae has focused on the mangrove

genus Rhizophora, primarily in connection with the recognition and study

of paleo-shorelines (Kuprianova, 1959; Langenheim et al., 1967;

Assemien, 1969; Rakosi, 1978; Sowunmi, 1981). Light Microscopic,

Scanning Electron Microscopic and Transmission Electron Microscopic

study on the pollen morphology of 39 species belonging to Anisophylleaceae

and Rhizophoraceae has been carried out by Vezey et al., (1988). SEM

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studies have been centred on the tribe Rhizophoreae (Tissot, 1979; Bertrand,

1983; Ludlow-Wiechers and Alvarado, 1983). Blepharistemma membranifolia

was examined by Murthy (1992); and the present study showed similar

results to that of the previous observations.

The Rhizophoraceae have tricolporate, radially symmetrical and

isopolar pollen in all the species presently studied. The shape of pollen

grains differ from subprolate (2 species), to prolate-spheroidal (4 species)

and oblate spheroidal (1 species). The colpi are long with smooth

membrane in (Rhizophora mucronata, Kandelia candel, Blepharistemma

serratum) and granular membrane in (R.apiculata, Bruguiera gymnorhiza,

B.cylindrica, Carallia bracheata). Endoaperture in the family has been

rectangular-lolongate in all the species studied, a feature not met with in

members of any other myrtalean families. Vezey et al., (1988) observed

fusion of the endoapertures in varying degrees which could be observed in

the species of Bruguiera and Rhizophora in the present investigation. The

exine sculpturing in the majority of the species has been punctate

(R.mucronata, Kandelia, Bruguiera, Blepharistemma), scrobiculate in

R.apiculata and faintly granular in Carallia. Thus the family appears to be

more or less homogeneous palynologically.

Light and ultrastructural data on Myrtales pollen (Patel et al., 1985)

are generally comparable to the data of Vezey et al., (1988). The majority

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of pollen morphological characteristics in Anisophylleaceae and

Rhizophoraceae occur in a broad range of families throughout the

angiosperms. It seems significant that none of these families could be

connected to Myrtales on palynological grounds.The colporoidate or fused

endoapertures possessed by Rhizophoraceae or Anisophylleaceae have no

counterpart in Myrtales. However, the pseudocolpi of Myrtales pollen do

not correspond to features of either Anisophylleaceae or Rhizophoraceae

pollen (Vezey et al., 1988).

Combretaceae

There are no modern detailed studies on the pollen morphology of

Combretaceae (Thanikaimoni, 1984). Earlier palynological work in the

family is scanty except that of Erdtman (1971) on Quisqualis indica as

having colpi alternating with “pseudocolpoid thin walled areas” and made

comparisons with, Cacoucia, Combretum and Terminalia. He also studied

Laguncularia racemosa, but did not mention about the presence or

absence of subsidiary colpi. Q.latialata was described as having three

pseudocolpi by Lobreau et al. (1969). Sowunmi (1973) described pollen of

Combretum glutinosum and four species of Terminalia in general as

resembling each other and possessing a characteristic shape, aperture

system and exine. All grains were noted to have “colpoid streaks” or

subsidiary colpi alternating with colpi. Guers et al. (1971) and Guers

(1974) also showed a fundamental similarity in the pollen of Combretum

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(6 species), Conocarpus erectus, Pteleopsis diptera and Terminalia (3

species). They reported colpi alternating with subsidiary colpi in all of

them, and the SEM of Combretum aculeatum was similar to that of

C.cacoucia.

From a comparative point of view, Erdtman (1971) held that pollen

grains more or less similar to Combretaceae occur in Melastomaceae, (also

Lythraceae and Penaeaceae). The grains in Haloragaceae, Hernandiaceae,

Myrtaceae, Punicaceae, Sonneratiaceae etc. are different. The SEM

observations of Patel et al. (1984) indicate several rather distinctive groups

possessing subsidiary colpi, or not, and other exine ornamentation

features. Their first group comprises taxa having colpi alternating with

subsidiary colpi possessing various surface patterns. The second group is

characterized by an echinate surface and subsidiary colpi. A third group

consists of a single genus with echinate surface, but lacks subsidiary colpi.

The fourth group also consisits of a single genus and have punctate surface

and lacks subsidiary colpi. The fifth group represented by a single genus

lacks subsidiary colpi and is different from the rest of the Combretaceae in

having a reticulate surface which has not been observed in any other

member of Myrtales core families (Patel et al., 1984). But such a

distinction could not be made out in the present study of 12 taxa. The

details of the pollen characters in the taxa studied here by and large

correspond to the first group of Patel et al. (1984).

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In the Combretaceae presently studied, pollen is tricolporate (rarely

tetracolporate as in Calycopteris floribunda), heterocolpate, radially

symmetrical and isopolar. The shape of the grains is prolate-spheroidal in

majority of the species (Terminalia catappa, T.chebula, T.elliptica,

T.paniculata, C.floribunda and Quiqualis indica) subprolate in (T.cuneata

and T.bellirica) and in a few species oblate-spheroidal (Anogeissus latifolia,

Lumnitzera racemosa, Combretum latifolium and Q.malabarica). The colpi

are long with acute ends and granular surface (T.catappa, Calycopteris) and

smooth surface (T.bellirica, Combretum and Quisqualis). Endoapertures are

lalongate in most of the species (10 species) and circular in two species

(T.catappa, T.elliptica). Mesocolpial extensions over the endoaperture have

been observed in T.catappa, T.elliptica and Calycopteris. The predominant

exine surface ornamentation met with in Combretaceae has been the

rugulate type (all species of Terminalia and Anogeissus), rugulate-

verrucate in Calycopteris, granulate in Combretum, reticulate in

Lumnitzera (a condition not met with in any other core Myrtales) and

psilate in Quisqualis.

Generalized comparison with the pollen of the Myrtales core families

agree with Erdtman’s (1971) suggestions. The pollen of the first group of

Patel et al. (1984) comprising six genera having colpi alternating with

subsidiary colpi showed resemblances with that of Penaeaceae and

Melastomaceae and some Crypteroniaceae, and the rest of the groups

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showed resemblance to other Myrtales (Patel et al., 1984). The heterocolpate

pollen of Combretaceae show broad similarities to the heterocolpate members

of Melastomaceae observed in the present investigation of Myrtales such as

(Memecylon, Osbeckia, Tibouchina and Dissotis) which is in consensus

with the reports of Erdtman (1971).

Myrtaceae

In the extensive bibliography of myrtalean palynology, Thanikaimoni

(1984) lists only three references that allude to electron microscopy of

Myrtaceae pollen, making it obvious that SEM-TEM investigations are

greatly needed in the family. The most comprehensive taxonomic light

microscopic study of Myrtaceae pollen was that of Pike (1956) from the

Southwest Pacific area including 71 genera and 300 species, and McIntyre

(1963) examined pollen of 18 New Zealand taxa and found that most of

them could be recognized upto genera. Electron microscopic studies were

limited to that of Gadek and Martin (1981), who examined the pollen of 28

species in seven genera of subtribe Metrosiderinae with Light Microscope

and SEM. They found a greater range of pollen morphology within the

family than was heretofore recognized, and in some instances pollen could

be identified to the generic and specific levels. Transmission Electron

Microscopic studies were carried out by Lugarden and van Campo (1978)

and Gadek and Martin (1982). Some studies on pollen morphology and

wall organization have been done in a few species of Eucalyptus (Gadek

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and Martin, 1982; Patel et al., 1984; Heslop Harison and Heslop Harrison,

1985; Zhou and Heusser, 1996; Pickett and Newsome, 1997). The

ultrastructure and cytochemstry of the pollen of E.globulus has been

studied in detail (Eliseu and Dinis, 2008) and the pollen is 3-

parasyncolporate with rugulate mesocolpia, and psilate apocolpium and

margo. In E.globulus (Eliseu and Dinis, 2008) and in E.rhodantha (Heslop

Harrison and Heslop-Harrison, 1985) the intine is three-layered under the

apertures forming a complex oncus. The outer layer is pectic and compact,

and according to Heslop-Harison and Heslop-Harrison (1985, 1991) is

bordered by a cap or operculum that represents an extension of the foot

layer of the ektexine. The same was reported by Gadek and Martin (1982)

who considered the remnant of the foot layer to form a colpus membrane.

In general, Myrtaceae pollen is tricolporate (triporate in Tristania

nereifolia), radially symmetrical and isopolar or heteropolar, and in lateral view

the pollen is oblate-peroblate, elliptic with obtuse or truncate sides (Reitsma,

1970). In polar view it is triangular, goniotreme with straight or curved sides

and with acute or obtuse corners. Colpi vary in length. In the taxa studied

presently, the number of apertures varied from three to four as in Psidium,

Melaleuca, Callistemon, Pimenta, Rhodomyrtus, S.calophyllifolium,

S.salicifolium, S.cumini, S.samarangense, S.hemisphericum, S.malaccensis,

S.aromaticum and S.jambos and three, four or five in P.cattleianum. In polar

view it is triangular with straight or curved sides and with acute or obtuse

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corners. Colpi vary in length, and are either syncolpate or parasyncolpate,

and generally have smooth surface. Endoapertures are lalongate.

Intercolpar concavities are present in species of Callistemon, Eucalyptus,

Syzygium jambos and S.occidentalis. Heteropolar grains are due either to

the nature of the colpi (e.g., long colpi on one pole and syncolpate on the

other pole) of the two polar faces as observed in Psidium guajava or to the

different shapes (concave, convex, straight) of the two polar faces. In

Eugenia capuli and in some grains of Luma chequen and Ugni molinae,

one polar face is concave (or straight, in Chamaelaucium) and the other

face convex. The pollen is free except in Myrtus communis and Psidium

littorale where tetrahedral tetrads are present along with monads (Patel et

al., 1984). Operculate grains have been observed in Rhodomyrtus (present

study).

Based on the nature of colpi, Pike (1956) recognized three pollen

types in the Myrtaceae: (1) longicolpate grains; (2) syn- or parasyncolpate

grains, and (3) brevi- or brevissimicolpate grains. All these types are

present in the taxa examined in the present study.

1) Longicolpate type: here the colpi are long; “colporate grains are

longicolpate, when the colpi are longer than the distance between

their apices and the poles” (Pike, 1956). Hypocalymna angustifolium,

Myrceugenella apiculata and Ugni molinae belong to this group. The

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pollen shape is also similar in Myrceugenella and Ugni: triangular

grains with convex side and slightly protruding apertures in polar

view and an elliptic shape with acute ends in lateral view. The

surface in Myrceugenella is verrucate-granular with some larger

verrucate elements at the poles. Ugni shows unique surface pattern

with multiangular units that have acute corners. These units are large

and scattered at the poles, but become smaller and more tightly

packed toward the equator. The surface appeared granular around the

endoapertures (Patel et al., 1984). In the present study longicolpate

grains have been observed in Psidium, Rhodomyrtus and Pimenta.

The exine surface was rugulate in Psidium; granulate in Rhodomyrtus

and areolate in Pimenta. The shape of the grains was triangular with

acute corners; the colpus narrow with a smooth surface.

2) Syncolpate and parasyncolpate type: In syncolpate grains, the colpi

are either straight, meeting at the poles without becoming wider, or

curved where they are wider at the poles and form a triangular area.

In parasyncolpate grains, the colpi bifurcate at the poles and their

branches meet and outline a triangular apocolpium. Some grains are

syncolpate on one pole and parasyncolpate on the other pole while

others also combine with longicolpate grains and result in long/syn/

and long/para forms. These combinations were reported in Myrtus

communis, Psidium littorale, Eucalyptus ficifolia, E.robusta,

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Heteropyxis natalensis, Temu divaricatum and Eugenia elliptifolia

(Patel et al., 1984). In the present investigation, pollen was

longicolpate on one pole and syncolpate on the other pole has been

observed in Psidium guajava. Syncolpate grains have been observed

in Callistemon, Eucalyptus, Melaleuca, Eugenia uniflora, Syzygium

benthamianum, S.rubicundum, S.gardneri, S.calophyllifolium,

S.travancoricum, S.salicifolium, S.cumini, S.samarangense,

S.aqueum, S.malaccensis, S.laetum, S.macrosepala and S.occidentalis

and parasyncolpate in Pimenta, S.zeylanicum, S.hemisphericum,

S.jambos, and S.aromaticum.

The taxa included in the syncolpate and parasyncolpate type are

further grouped according to the presence or absence of intercolpar

concavities. Pollen with clearly defined intercolpar concavities have

been observed in Acmena smithii, Callistemon citrinus,

C.teretifolius, Calothamnus validus, Eucalyptus ficifolia, E.robusta,

Heteropyxis natalensis, Melaleuca hypericifolia, M.raphiophylla,

Tristania conferta and T.lactiflua while the “depressions” are not as

markedly depressed as in the above mentioned taxa in Eugenia

elliptifolia, Metrosideros nervulosa and M.polymorpha. Melaleuca

hypericifolia has a verrucate-granular surface in the intercolpar

concavities, and the surrounding areas are scabrate. In Callistemon

teretifolius, C.citrinus and Heteropyxis the surface of the mesocolpia

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is rugulate, and it is rugulate-verrucate in the intercolpar concavities

(Patel et al., 1984). Syncolpate pollen with intercolpar concavities

has been observed in the present study in Callistemon, E.tereticornis,

S.jambos and S.occidentalis; exine surface ornamentation was

granulate; and the exine surface ornamentation being different in the

intercolpar concavities and in the rest of the mesocolpium.

In pollen without intercolpar concavities the surface is more or less

uniform over the entire surface of the grain. The surface in Myrtus

communis, Psidium littorale, Eremaea pauciflora, Melaleuca

preissiana and M.decussata the surface is granular-verrucate-

rugulate. There are fine, branched, irregular channels separating the

surface elements (Patel et al., 1984). Pollen without intercolpar

concavities occur in the majority of the taxa presently studied. The

surface in Luma chequen, Pilidiostigma glabrum and Rhodamnia

argentia is verrucate-granular. In Luma, the colpi are curved and

form a large thin walled triangular area at the poles that has

irregularly scattered verrucae and granules on it. In Pilidiostigma, at

the parasyncolpate pole, scattered granules form a triangular

apocolpium. The surface is smooth near the endoapertures in both the

taxa. In Rhodamnia larger verrucate elements occur along the

margins of the mesocolpia, even near the endoapertures (Patel et al.,

1984). The exine surface in the taxa studied is variable; it is psilate

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(Psidium cattleianum, Syzygium rubicundum, S.gardneri,

S.caryophyllatum), granular (Callistemon, Eucalyptus, Rhodomyrtus,

Syzygium zeylanicum, S.salicifoliu, S.hemisphericum, S.jambos,

S.macrosepala and Couroupita) areolate (Pimenta, S.salicifolium,

S.aqueum), foveolate (Melaleuca), macroreticulate (Barringtonia,

Careya), rugulate-verrucate in Eugenia uniflora or rugulate (Psidium

guajava, Eugenia mooniana, Syzygium benthamianum,

S.calophyllifolium, S.samarangense, S.malaccensis, S.aromaticum,

S.mundagam).

3) Brevicolpate type: In the brevicolpate pollen, the length of the colpi

is equal to or less than the distance between their ends and the poles.

In the brevissimicolpate grains, the colpus length is less than that of

underlying endoaperture (Erdtman, 1971). Chamaelaucium

uncinatum has brevissimicolpate grains (Patel et al., 1984).The

brevicolpate grains occur in Eugenia mooniana (present study) and

the surface sculpturing is rugulate-verrucate.

In the present study, endoapertures are faint in most of the species

(17), lalongate in nine species and lolongate in four species. Exine

surface ornamentation in the Myrtaceae studied have been different;

psilate pattern has been observed in four species, granular in 12

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species, foveolate in one, rugulate in eight species, areolate in three

species, macroreticulate in four species and rugulate-verrucate in two.

In Barringtonia, Careya and Couroupita of Myrtaceae in the present

study, the pollen is 3-zonoaperturate, and the pollen shape varies from

prolate, prolate-spheroidal to spheroidal. The pollen grains of

Barringtonia, B.acutangula and B.racemosa are 3-zonosyncolpate and

large in size; but B.asiatica and Careya has 3-zonosyncolporate grains

with lolongate endoaperture. In Couroupita guainensis 3-zonocolporate

grains have been observed which have comparatively smaller size. In

Careya the aperture membrane bears two rows of gemmate-verrucose

sculptural elements flanking the underlying endoaperture (apertural

verrucae). The apertural verrucae in B.asiatica are less prominent. In

B.asiatica and Careya the endoapertures are bordered by marginal ridges

which in turn are having marginal grooves to the mesocolpial side. In the

polar areas the exine is thickened forming the polar cushions. The exine

surface ornamentation in Barringtonia and Careya has been

macroreticulate, while in Couroupita it is minutely granulate.

The three genera (Barringtonia, Careya and Couroupita) have pollen

quite different from the rest of the Myrtaceae as well as other core

Myrtales. These three genera itself may be categorized into two groups

based on pollen morphology: the first group including Barringtonia and

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Careya with comparatively larger pollen, macroreticulate exine sculpturing

and syncolpate or syncolporatae pollen; the second group in the present

study includes only Couroupita with smaller grains, 3-zonocolporate

aperture and granular exine sculpturing. The first group including the

species of Barringtonia and Careya and the second group (Couroupita

only in the present study) is comparable to the Planconia type and

Lecythis type proposed by Erdtman (1952).

Although the pollen of Myrtaceae is essentially uniform, in some

taxa minor differences make it possible to recognize particular genera or

species. Within the family there is no particular feature that separates

pollen of the Myrtoideae from that of Leptospermoideae, but the pollen of

Chamaelaucieae differs markedly from that of all other tribes in the family

(Pike, 1956). Within a tribe the taxa are usually similar, but it is possible

for closely related genera to have quite distinct pollen. On the other hand

pollen of widely separated genera may show some similarities. Grains of

the different species of the same genera are usually indistinguishable or

they may be similar in general features but show a comparatively large

range in size.

The Myrtaceae pollen does not appear to have any close similarities

to taxa from the other core families of the Myrtales. Some superficial

similarity exists with Onagraceae pollen (through the shape of the grains

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as well as the short colpi in some members of the two families) but the two

are clearly recognizable. Some similarity has been observed between the

pollen of Myrtaceae and Lythraceae as suggested by Erdtman (1971)

through Cuphea, but more studies are required to establish the

relationship. Pollen of Lecythidaceae and Sapindaceae has been examined

using SEM (Muller, 1972, 1973; Muller and Leenhouts, 1976; Mori et al.,

1980) and are not similar to Myrtaceae pollen. However, in Sapindaceae

there are several taxa with triangular, parasyncolpate grains (Muller and

Leenhouts, 1976).

Melastomaceae

The pollen morphological studies in the Melastomaceae have not

gone beyond the light microscopic level before Guers (1974) who

examined Osbeckia decandra, Dicellandra barteri and Calvoa orientalis

electron microscopically, which was part of a light microscope study of

eight genera and twenty species from tropical Africa. Of these Dicellandra

and Calvoa compare favourably with the second group of pollen in

possessing intercolpar concavities and O.decandra with the first pollen

group having heterocolpate grains with subsidiary colpi of Patel et al.,

(1984). Earlier palynological studies of Melastomataceae have been

undertaken by Erdtman (1952), Nair (1965b), Huang (1972) and Patel et

al. (1984). Detailed pollen morphological investigations in relation to

taxonomy were initiated by Erdtman (1952). He stated that Melastomataceae

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pollen grains are prolate-spheroidal, have three ori (orial) colpi and three

pseudocolpi, with the sexine as thick as the nexine or slightly thinner and an

obscure pattern. He also suggested that the pollen morphology did not

support the opinion that Memecylon and related genera should be referred

to a separate family and that the pollen grains were similar to the pollen

grains of Combretaceae and Lythraceae, but not to Myrtaceae.

A more detailed paper on this family was produced by Patel et al.

(1984) in which the pollen morphology of 19 genera was studied. The

pollen grains are generally monads although tetrads and polyads occur in

Miconia melanotricha and Tococa spadiciflora. The grains are

tricolporate, radially symmetrical, isopolar and spheroidal to subprolate in

equatorial view with a circular, hexagonal or triangular shape in polar

view. The exine sculpturing in the mesocolpia is variable, striate and

rugulate. They also suggested that the Melastomataceae pollen can be

divided into three pollen groups; heterocolpate with subsidiary colpi,

heterocolpate with intercolpar concavities and tricolporate.

In the heterocolpate group with subsidiary colpi, the three colpi (four

in Votomita) alternate with three (four in Votomita) elongate, narrow

subsidiary colpi. This type of pollen is present in (Trembleya flogiformis,

Tibouchina urvilleana, Tristemma littorale, Dissotis brazzae, Marumia

nervosa, Dissochaeta celebica, Osbeckia polycephala, Acanthella sprucie,

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Memecylon normandii, Mouriri glazioviana, Votomita monadelpha,

Miconia hondurensis, M.alypifolia, M.caesia, Comolia stenodon and

Tococa broadwayi). The surface sculpture is variable; smooth, punctate,

rugulate-punctate, verrucate-rugulate, or striate (Patel et al., 1984).

Colpi are long and some grains are syncolpate in Dissotis, Acanthella

and Dissochaeta with acute ends and a smooth surface or the surface is

granular. Endoapertures are lalongate and elliptic. Extensions of the

mesocolpia over the endoapertures are present. A horizontal bar is often

persistent over the open endoapertures (Patel et al., 1984).

In the second group “heterocolpate” with intercolpar concavities,

three intercolpar concavities - large elliptic, thin-walled, depressed areas

are present on the mesocolpia. The remaining thick-walled portion of the

mesocolpia forms a more or less narrow band around the intercolpar

concavities (Adelobotrys tessmannii, Allomorphia caudata, Bredia hirsuta,

Astronia cumingiana, Oxyspora paniculata and Miconia melanotricha).

The grains are spheroidal in lateral view and circular to triangular or

hexagonal in polar view. Colpi are long and narrow with acute ends and a

more or less smooth surface. Extensions of the mesocolpia are present over

the lalongate-elliptic endoapertures. The surface of the meso and

endocolpia is variable; it is psilate, striate-rugulate, fine rugulate-verrucate,

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smooth-punctate, coarse with many channels and pits, verrucate or

granular-verrucate (Patel et al., 1984).

The third group, tricolporate is represented by Tococa stephanotricha

and T.formicaria, which have a rugulate surface and very short colpi.

Another species of Tococa, T.spadiciflora could not be assigned to any of

the three groups described above. It consists of polyads composed of basic

units of tetrahedral tetrads. Internal bridges on the proximal faces of

individual pollen grains maintain tetrad unity, while the external bridges

on the distal faces of the tetrads, frequently along the aperture margins,

maintain polyad unity (Patel et al., 1984).

Pollen grains of Melastomaceae in the present investigation are

tricolporate in all the taxa studied except in Clidemia and Dissotis where it

is tri and/or tetracolporate, radially symmetrical and isopolar. Grains are

prolate-spheroidal (10 species), subprolate (9 species), oblate-spheroidal (3

species), suboblate (3) and spheroidal (1 species). All species of

Melastomaceae studied are heterocolpate with subsidiary colpi except one

(Medinilla) in which intercolpar concavities occur. On the basis of pollen

morphology the grains of the family could be classified into two groups

such as, heterocolpate with subsidiary colpi which comprise all the taxa

studied except Medinilla and heterocolpate with intercolpar concavities

consisting of Medinilla in the presently studied taxa.

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In the heterocolpate group with subsidiary colpi, three colpi alternate

with three elongate, narrow subsidiary colpi. Surface of the colpus is

granular in Sonerila species while it is smooth in Osbeckia, Dissotis,

Melastoma, Memecylon and Tibouchina. Pollen grains seem to be

operculate in M.malabathricum and Memecylon. The colpi are very long

and almost meet at the poles in Osbeckia and Melastoma. The endoaperture

is lalongate in all the species studied except Memecylon flavescens where it

is circular, and faint in M.grande. Mesocolpial extensions over the

endoapertures were observed; and often it forms a horizontal bar over the

endoaperture in (Osbeckia, Sonerila and Tibouchina).

The surface of the meso- and apocolpia is variable; psilate in the

mesocolpium and granulate in the intercolpar concavities in Medinilla;

foveolate in (O.aspera var.aspera, O.courtallensis, O.octandra,

O.leshenaultiana, O.cupularis, M.flavescens), rugulate-fossulate (O.aspera

var.travancorica, O.reticulata), rugulate in (O.wynadensis, Dissotis),

granulate-rugulate (O.parviflora), matted in Melastoma, striate in all

species of Sonerila studied and granulate in all species of Memecylon

studied (except M.flavescens), Clidemia and Tibouchina.

The ornamentation can be used for the identification of pollen grains

at the type or subtype levels. At the LM level the pollen morphology of

Melastomaceae is in agreement with the observation of Patel et al. (1984)

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which indicates that pollen grains with subsidiary colpi or with intercolpar

concavities are not important for group identification. The range of

variation in Melastomataceae pollen is pronounced. It is, therefore, not

possible to separate the genera (of Thai Melastomataceae) using pollen

morphology except for one genus, Pternandra. However, all grains studied

can be divided into three groups by LM and five types using both LM and

SEM (Chantaranothai, 1997).

Generalized comparisons with pollen from the core families of the

Myrtales show that pollen of the Melastomaceae, Combretaceae and

Lythraceae have a number of features in common and indicate that these

three families are closely related. However, Melastomaceae pollen is more

like that of Combretaceae than of Lythraceae. On the basis of pollen

morphology there is no evident difference between the subfamily

Memecyloideae and the others and so this study follows Erdtman (1952) in

keeping this subfamily in the Melastomaceae. Palynological evidence does

not confirm the tribal and subfamily classification except in the case of the

genus Pternandra in the tribe Kibessieae and subfamily Astroniodeae,

according to Clarke (1879), Krasser (1893) and Hutchinson (1969).

Among the genera examined by Patel et al. (1984) Mouriri, Votomita

and Memecylon have been segregated with several other genera from

Melastomaceae sensu stricto as Memecylaceae or subfamily Memecyloideae

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(eg.Dahlgren and Thorne, 1984; Johnson and Briggs, 1984). All three of

these genera belong to the heterocolpate group, but are not delimited as a

group in anyway fom the other heterocolpate genera of Melastomaceae,

although Votomita is unusual in having four colpi and subsidiary colpi, and

a bead-like colpus surface. Further studies on this group are essential to a

proper understanding of the relationships between Melastomaceae sensu

strico and Memecyloideae. Memecylon belong to the group heterocolpate

with three subsidiary colpi and does not show enough pollen morphological

evidence in favour of its segregation from Melastomaceae sensu stricto.

Within the limited context of this study the heterocolpate pollen with

subsidiary colpi show some similarities to other core families. At the SEM

level some resemblance is evident with Combretum, Bucida, Conocarpus,

Pteleopsis, Terminalia, Ramatuella, Guiera, Poivera and Lumnitzera (Patel

et al., 1984). In the present investigation also resemblance is observed in the

pollen grains of Melastomaceae and Combretaceae in the grains possessing

heterocolpate pollen with subsidiary colpi. Also Calycopteris floribunda

(Combretaceae) and Clidemia hirta and Dissotis rotundifolia

(Melastomaceae) have 3, 4-zono heterocolpate pollen grains.

Lythraceae

The available pollen morphological data including the present

findings indicate that Lyhraceae have the most diverse pollen morphology

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in the Myrtales. Much of the diversity concern the aperture system for e.g.

tricolporate grains is the dominant condition exhibited by species

belonging to several genera such as Physocalymma, (Cos Campos, 1964),

Rotala (Guers, 1970), Heimia (Graham, 1977), Adenaria, Pleurophora,

Galpinia (Erdtman, 1971) and Diplusodon (Muller, 1981) and also in

members of several genera presently studied such as Rotala, Lawsonia,

Lagerstroemia, Woodfordia and Punica, heterocolpate grains with

isomerous subsidiary colpi are reported in Lythrum (Cos Campos, 1964;

Guers, 1970; Huesser, 1971) and also heterocolpate with additional

subsidiary colpi in three genera (Rotala, Ammania, Nesaea). The same

condition was reported in a few genera by Erdtman (1971) and Muller

(1981). Grains with three meridional ridges that alternate with apertural

fields are reported in Lafoensia, Crenea and Lagerstroemia (Muller, 1981)

and also in Lagerstroemia indica and L.speciosa presently observed.

The family is a palynologically interesting group in the order

Myrtales as evidenced from the present study as well as earlier reports on

the pollen morphology of Lyhraceae by Cos Campos (1964), Graham et

al. (1968), Guers (1970), Erdtman (1971), Graham and Graham (1971),

Graham (1977), Muller (1981) and Graham et al., (1990). Much diversity is

exhibited in the aperture structure of the members. Most of the genera have

tricolporate grains (Rotala, Woodfordia, Lawsonia, Lagerstroemia

microcarpa and Punica), heterocolporate grains with six subsidiary colpi

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have been observed in (Rotala, Ammania and Nesaea) while grains with three

meridional ridges alternating with apertural fields (Lawsonia, L.speciosa,

L.indica), syncolporate grains (Cuphea) and triporate grains in Sonneratia.

4-apertured grains have been noticed along with 3-apertured ones in L.indica,

C.ignea, R.malampuzhensis while R.densiflora have exclusively 4-apertured

grains, and this is in consensus with the report by (Graham, 1990). The

Lythraceous pollen morphology may indicate that pseudocolpi seem to occur

in derived genera. The family is outstanding in the order, and it may be

argued whether absence of pseudocolpi represents an original or, by

secondary loss an advanced state (Dahlgren and Thorne, 1984).

Pollen morphological diversity is also evident in the shape of the

pollen grains. Majority of the grains have prolate and subprolate shape

while in Cuphea the shape is oblate to peroblate. Size of the grains also

show variation as large grains occur in (Sonneratia, Lagerstroemia and

Punica) in contrast to the small to very small grains in Ammania, Rotala,

Nesaea etc. Exine ornamentation also showed much variation from psilate

(Cuphea), pebblate (Woodfordia), rugulate (Lagerstroemia, Lawsonia),

rugulate-verrucate (Sonneratia), striate (Cuphea, Nesaea, Rotala) and

granulate (Punica).

Pollen grains of Sonneratia are triporate with meridional ridges

alternating with apertural fields and polar caps as has been observed by

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Muller (1969, 1978). The three well-developed meridional ridges alternate

with three apertural fields. The apertural field has a protruding pore in the

centre. The surface is verrucate in the apertural fields, verrucate-rugulate

on the ridges and the polar caps are psilate. In addition to the three large

ridges, six smaller ridges are present, two in each apertural field. The

meridional ridges and smaller ridges are prominent in S.alba and not

observed in S.caseolaris. In the present study the pollen of S.alba observed

resembles S.caseolaris of Patel et al. (1984) (i.e. S.alba of Muller, 1978)

in contrast to their reports that their “SEM results show that at least

superficially, S.caseolaris appears more similar to Muller’s SEM of S.alba

than to his SEM of S.caseolaris. However, it should be noted that the

similarity is observed in the well-developed meridional ridges and

apertural fields in S.alba of the present study and Muller’s S.alba”.

Pollen diversity at the infrageneric level has been documented in the

genus Cuphea in which wide variation has been reported with pollen

ranging from basic tricolporate-spheroidal to tricolporate-syncolporate-

oblate, triangular (Graham et al.1968; Guers, 1970). The species of the

genus presently studied exhibited tricolporate-syncolpate-oblate, triangular

grains. Starting with a basic oblate, oblate-tricolporate, striate, tectate grain

the genus Cuphea was shown to be remarkably eurypalynous with great

variation at sectional, subsectional, specific and varietal level. Moreover

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Cos Campos (1964) have described multiple pollen types produced by

individual plants or anthers in the genus.

A comprehensive study by Lee (1979) including 26 genera of the

family has reported subsidiary colpi and three major pollen groups in the

family – (1) 3-pseudocolpate – Lythrum, Pemphis, Peplis, Rhynchocalyx;

(2) 6-pseudocolpate – Ammania, Capuronia, Crenea, Lagerstroemia,

Lawsonia and Rotala; of these, the same condition was noticed in

Ammania and Rotala; and 3) nonpseudocolpate in Adenaria, Alzatea,

Cuphea, Decodon, Didyplis, and Heimia.

In his study of the Lythraceae, Muller (1981) examined pollen grains

of Crenea, Diplusodon, Lafoensia and Lagerstroemia as well as

Sonneratia and compared the results on a harmomegathic functional

standpoint, and several structural pollen types were established, starting

with a tricolporate longiax prototype. He has postulated that the relation

between pollen form and function was indicative of adaptive radiation in a

few directions such as (1) there is a trend towards increasing the number of

colpi (i.e. subsidiary colpi), (2) harmomegathic functions are transferred

from individual colpi to flexible apertural fields alternating with

meridional ridges, (3) harmomegathic function is transferred to prominent

poles and (4) harmomegathic function is lost in the ecto- and endo-apertures.

In his concept of usefulness of pollen morphology, Muller (1981) has

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emphasized the need for ecologic interpretations of function and for detailed

ultrastructural studies to uncover those characters which reflect ancient

phylogenetic links. He has held that this can best be illustrated by considering

the possible affinities between the genera Lafoensia, Lagerstroemia, and

Sonneratia assuming that the latter may be placed in the Lythraceae. If,

convergences in the recent pollen morphology is stressed Lafoensia will be

considered closely related but, the differences in ultrastructure and

heterocolpate nature in some pollen types of this genus would argue against

affinity with Sonneratia. He has further pointed out that if fossil evidence is

taken into consideration, the genus Lagerstroemia appears a much stronger

candidate, although its present-day type show less similarity with a living

Sonneratia type. In a more general sense, Lafoensia is considered to be

similar to Sonneratia of subfamily Sonneratioideae (Muller, 1981);

Diplusodon similar to Duabunga of subfamily Duabungoideae; while

Lagerstroemia bears resemblance to Punica of subfamily Punicoideae. These

similarities would strongly support the view of Dahlgren and Thorne (1984)

that Sonneratia, Duabunga and Punica should be regarded as separate

subfamilies independently related to Lythraceae.

Onagraceae

Onagraceae pollen has been the object of studies using light

(Mitroiu, 1961-1962; Ting, 1966; Brown, 1967) and electron microscopy

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(Skvarla et al., 1975, 1976, 1978). These studies have shown that the

pollen of the 17 genera and approximately 650 species possess distinctive

features such as viscin threads, a spongy paracrystalline ektexine, apertural

protrusions and a large central body; tetrads and /or polyads in certain

groups; and an exine surface composed of circular, globular, and elongate

or rod-like elements.

The pollen of Onagraceae studied here have 3-zonoaperturate

condition as the predominant feature with variations in aperture number

such as 2 (Fuchsia arborescens), 3, 4 and 5-aperturate (F.fulgens). The

grains are 3-colpate in Trapa, brevicolpate in Ludwigia and porate in

Fuchsia. In all the taxa of Onagraceae presently studied the pollen have

certain distinctive features such as a large central body and apertural

protrusions; viscin threads have been observed in L.adscendens,

L.suffruticosa, L.peruviana and F.fulgens; Viscin threads have different

surface patterns: smooth in L.peruviana, L.adscendens, F.fulgens twisted

in L.suffruticosa; pollen tetrads occur in L.perennis. L.prostrata and

L.suffruticosa, and polyads in L.peruviana.The exine surface has rugulate

ornamentation in all the species of Ludwigia studied except L.adscendens

where it is areolate, and granulate in Fuchsia and Trapa.

Trapa is distinguished by the possession of 3-zonocolpate grains with

three prominent meridional ridges on the grain and apertures that are

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protruding and swollen as elongated domes. The lens-shaped colpus is

covered by the meridional ridge which is formed by the folding of

ektexine, and they are united at the poles to form a greatly enlarged

triangular base. The ridges enclose a cavity also. The extremely thick

endexine, surface sculpturing and protruding apertures of Ludwigia pollen

resemble Trapa in a remote sense although there are no difficulties in

recognizing both. Trapa thus bears unique features that are quite different

from the rest of the Onagraceae presently studied, and its separation from

Onagraceae into a distinct family could be justified on palynological

grounds.

The Onagraceae pollen is distinctive in the order by considering the

extremely thick endexine, surface sculpturing, and the protruding apertures

and the pollen of Ludwigia resemble that of Trapa in a remote sense. The

short colpi of Hauya, Gongylocarpus and Boisduvalia (Patel et al., 1984) and

Lopezia (Skvarla et al., 1976) and the brevicolpate condition in Ludwigia in

the present study bears a distant similarity with that of Myrtaceae. Meridional

ridges observed in Ludwigia pollen, L.alternifolia, (Patel et al., 1984) and

L.adscendens and L.peruviana of the present study are unique and should not

be confused with other taxa in the Myrtales possessing meridional ridges.

Most genera of Onagraceae shed their pollen exclusively as monads

but some shed their pollen as tetrads or polyads. Pollen tetrads are

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characteristic of Boisduvallia, most species of Epilobium and some species

of Camissonia and Ludwigia. Presence of viscin threads, as noticed in

Ludwigia, L.adscendens, L.suffruticosa and L.peruviana and Fuchsia

fulgens in the present study has been reported earlier in several taxa of the

family by Patel et al. (1984). These are extensions of the exine surface

which has different surface patterns like smooth or segmented, tightly

compound-twisted and incised-compound (present study and Patel et al.,

1984). According to Skvarla et al. (1978) some of the patterns are difficult

to categorise and may represent intermediate or transitional forms. For

example the obliquely inclined-segmented-beeded threads as observed in

Fuchsia by Patel et al. (1984) or as mycelial threads (present observation).

It is considered that the viscin threads are an integral part of the pollen

grain wall, namely the ektexine as documented by Skvarla et al. (1978)

and Nowicke et al. (1979). There are different interpretations regarding its

relationship with the exine such as “an attachment point” indicating a

specific area on the exine surface where the threads emerge, or it is not an

inherent part of the exine, but as a later addition. For avoiding these

confusions, a different term in the place of “attachment point” is used

namely, emergence area (Patel et al., 1984). Structures similar to viscin

threads in Onagraceae are known in other families like Ericaceae (Skvarla et

al., 1978), Leguminosae (Graham et al., 1980). Later study of Patel et al.

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(1985) interpreted these threads ends attached to different pollen grains

indicating that they are exinal connections rather than viscin threads.

Sculpturing of exine surface is variable in the family. Taxa studied

here had rugulate, heteromorphic-rugulate, areolate and granulate exine

ornamentation types.

Other surface features of significance in the family are long colpi and

prominent ridges in Ludwigia as reported by Patel et al. (1984) as against

short colpi in the present study of the genus. Prominent meridional ridges

are of two types such as those occurring on the polar faces and extend to

the equator in the area midway between the apertural protrusions, and

lateral ridges which occur between the apertural protrusions which are

joined with the meridional ridge. Those noticed in the present study is of

the former type.

Trapa is a small genus with three species accommodated in the

Onagraceae of sensu Bentham and Hooker (1865) of which one species,

T.natans has been studied here. It is distinguished by the possession of 3-

zonocolpate grains with three prominent meridional ridges on the grain

and apertures, that are protruding and swollen as elongated domes. The

other species of the genus, namely T.japonica, was studied by Patel et al.

(1984); almost similar pollen morphological features were observed except

the better developed domes.

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Structurally the meridional ridge of Trapa is different from that

found elsewhere in the order. In contrast, the ridge formed in Lythraceae

(Lagerstroemia and Lawsonia) and Onagraceae (Ludwigia) are formed by

the increase in thickness of the exine. Moreover, the ridge passes over the

colpi in Trapa, whereas in others it alternates with the colpi. The

distinctiveness of Trapa pollen was recognised by Erdtman (1971) who

examined the species and felt that the genus merited family status. Trapa

pollen shows a distant resemblance to Onagraceae in surface sculpturing

and in the nature of protruding apertures. Similarities to Onagraceae are

further evident in the very thick endexine and indistinguishable or at least

very thin foot layer.

Haloragaceae

The pollen of Myriophyllum tuberculatum of the Haloragaceae has

been studied as this family has been included in the Myrtales order by

different classificatory treatments such as (Emberger, 1960; Melchior,

1964; Soo, 1967; Takhtajan, 1980) etc. The pollen grains are isopolar,

radially symmetrical, porate, aspidote and crassimarginate, and the exine

sculpturing spinulate. These features seem to be deviating from that of the

core Myrtales especially the spinulate ornamentation. The porate, aspidote

aperture, though present in Onagraceae, the Onagraceae pollen do have a

number of distinctive features such as thick endexine, large central body,

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viscin threads etc. quite different from the Myriophyllum (Haloragaceae)

pollen and stands out from the other Myrtales.

5.3 Morphological evolution of pollen grains

A perusal of the literature of the pollen morphology of angiosperms

shows that schemes of evolution of palynological characters have varied

with individual workers. Of the different morphological characters of the

exine, the germinal apertures have been the focus of attention of

palynologists. Wodehouse (1935) has viewed phylogenetic changes in the

germinal aperture in relation to the function. Apart from the monocolpate

form, the trilete and 3-zonocolpate forms have been held by him to be of

basic significance in aperture evolution. Accordingly the major

evolutionary changes in aperture have been considered to have started

from the one-furrowed forms of primitive gymnosperms by the process of

“modification, protection or elimination of the wide open furrow”. For

evaluating the significance of pollen morphology in the monocotyledons,

Kuprianova (1948) considered the aperture form as a major feature.

According to Erdtman and Vishnu-Mittre (1957), pollen grains having

pore-like or illdefined proximal apertures have given rise to those with

zonal, distal or global apertures. Vishnu Mittre (1964) has viewed the

trilete form as the ancestral type from which inaperturate type followed by

aperturate ones evolved. Walker (1974) has contended the inaperturate

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pollen grain as primitive form from which evolved the aperturate ones, of

which the tricolpate type is the progenitor of all the apertural forms. Nair

(1970c) has proposed that the evolution of the dicotyledons has taken

place along two lines: one along the line of the Magnoliaceae and the other

along the Ranunculaceae, while the Monocots with the predominantly

monocolpate form evolved along an independent line. According to him

“the Magno-Ranalian complex have originated from the trilete or

trichotomocolpate form of progenitor characteristic of the preangiosperms

by which the trilete aperture is considered to be the most primitive type

and the other forms and the inaperturates advanced types in the scale of

morphological evolution of pollen. Forms such as colporate, porate,

pororate and spiraperturate are considered to have been evolved from the

colpate type by reduction and specialization from the aperturates. The

change in form has also been accompanied by increase in the number of

apertures. According to Nair (1985) the other palynological features such

as exine ornamentation, exine strata and pollen size also constitute various

structural entities of pollen.

  Evolution of similar traits in different lineages can be considered as

evidence for convergent adaptive change (Brooks and McLennan, 1991).

The diversity of pollen surface patterns has been interpreted as a result of

an adaptation for rapid germination, protection from desiccation and

harmomegathic mechanisms within the closely related taxa (Nowicke and

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Skvarla, 1979). In addition to the germinal aperture which is of primary

importance in the morphology of pollen, there are also other

morphological characters in which evolutionary phenomina are expressed.

Wodehouse (1935) has observed gradual reduction in the excrescences in

the pollen grains of Asteraceae. In the case of saccate pollen forms those

with two or more sacci are considered to have originated from forms with

a single saccus. The pollen types with thick and heavily ornamented exine

are considered to be primitive, while those with thin and unornamented

exine advanced.

From the palynological data of myrtalean families in the South

Indian region the distribution of various aperture forms in the member

families of the order sensu Bentham and Hooker (1865) presented, it is

found that the order is fairly eurypalynous with different aperture forms

occurring. The different forms come under three major categories such as

colpate, colporate and porate, the distribution of which is represented

below.

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3-colpate Myrtaceae

Onagraceae

Colpate 3-syncolpate Myrtaceae

3-colporate Rhizophoraceae

Combretaceae

Myrtacaeae

Melastomaceae

Lythraceae

Onagraceae

Colporate 4-colporate Combretaceae

Myrtaceae

Melastomaceae

Lythraceae

Onagraceae

3-syncolporate Myrtaceae

Lythraceae

4-syncolporate Myrtaceae

Lythraceae

Para-syncolporate Myrtaceae

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2-porate Onagraceae

3-porate Lythraceae

Porate Onagraceae

4-porate Onagraceae

5-porate Onagraceae

It may be noted that the most primitive 3-colpate condition occurs

exclusively in the members of the Myrtaceae and one member of

Onagraceae, while the 3-colporate in all the six families and together with

4-colporate forms in most of them. The most advanced porate condition (2

to 5-porate) in members of the Onagraceae and in one genus of Lythraceae

(Sonneratia). It appears that aperture evolution has most operated in the

Lythraceae followed by Onagraceae. It is also interesting to note that

syncolpate, syncolporate and parasyncolporate conditions occur frequently

in the Myrtaceae and sporadically in the Lythraceae.

A tentative scheme of evolution of apertures in the myrtalean complex

is presented in the chart.

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Fig. 7. Tentative scheme of evolution of apertures in the myrtalean complex

  179

From the chart it may be seen that the most primitive 3-colpate

condition occurs only in the Myrtaceae and Onagraceae. Still more

advanced 3-colporate apertures in all the six families, Rhizophoraceae,

Combretaceae, Myrtaceae, Melastomaceae, Lythraceae and Onagraceae.

Relatively more advanced syncolpate form occurs in two species of

Myrtaceae; syncolporate in Myrtaceae and Lythraceae and parasyncolporate

in Myrtaceae alone. Still more advanced porate condition occurs in two

families (3-porate) in Lythraceae and (2 to 5-porate) in Onagraceae. Some

degree of heterogeneity is noticed in three families; in the Myrtaceae

possessing six different aperture forms (3-syncolpate, 3-colporate,

4-colporate, 3-syncolporate, 4-syncolporate, 3-parasyncolporate); Lythraceae

(3-colporate, 4-colporate, 3-syncolporate, 4-syncolporate, 3-porate) and in

Onagraceae (3-colporate, 4-colporate and 2 to 5-porate) conditions.

5.4 Systematic considerations

The myrtalean group of families is characterized primarily by two

distinctive wood anatomical features that are not commonly found together

in any other angiosperm groups (Conti et al, 1996). All plant families that

have been regarded as indubitable members of the Myrtales are

characterized by the combination of these anatomical features which is

uncommon in any other dicot families outside the Myrtales. They are

bicollateral vascular bundles in the primary stem and vestures in the

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bordered pits of the secondary xylem. The combined occurrence of these

two anatomical characters is very rare outside the Myrtales. This is

considered to strengthen the value of these characters for the delimitation

of the Myrtales (perceived from the anatomical point of view). The

families like Rhizophoraceae, Lecythidaceae etc. which are included by

some taxonomists in the Myrtales fail to gain anatomical approval (Vliet

and Baas, 1984). The members of the order consist of woody and

herbaceous plants characterized by production of tannins (Cronquist, 1981).

Leaves are typically opposite, simple and with entire margin. The flowers

are bisexual, mostly 4 or 5-merous, actinomorphic or weakly zygomorphic,

often distinguished by the presence of numerous stamens. The 2 to 5 carpels

are fused to form a syncarpous pistil and ovary with several locules; the

position of the ovary varies from perigynous to epigynous, often with a

short to long hypanthium. Pollen grains are predominantly tricolporate often

with pseudocolpi; pollination usually mediated by insects or birds. Other

notable features almost typical of the order include unilacunar nodes, starch-

accumulated plastids in the sieve tube elements, and chromosome numbers

in multiples of 11 or 12 (Raven, 1975).

This is one of the intensely investigated orders from various biological

angles, and despite this a number of systematic issues remain unsolved

concerning the circumscription, composition and interrelationships, both

intra- and interordinal. A great deal of controversy exists between and

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among the various systematic treatments both classical and modern.

Bentham and Hooker’s (1865) treatment of the Myrtales is known to be

the most comprehensive among the classical, and according to this the

order comprises six families such as the Rhizophoraceae, Combretaceae,

Myrtaceae, Melastomaceae, Lythraceae and Onagraceae. This composition

has been subjected to significant degree of shuffling and reorganization in

subsequent taxonomic treatments based on evidence from various

disciplines like morphology, embryology, anatomy, palynology,

phytochemistry and molecular biology (reviewed in Dahlgren and Thorne,

1984). Notwithstanding the wealth of taxonomic studies the order is

subjected with, considerable degree of uncertainty still exists. The

distribution of myrtalean families according to Bentham and Hooker and

other major treatments is furnished in Table - 7 below.

 182

 183

  184

From the distribution of families in the order as projected in the

various treatments, it may be noted that the families of the Myrtales sensu

Bentham and Hooker that are most tampered with are Rhizophoraceae,

Myrtaceae and the Lythraceae, and the others are marginally shuffled. A

modest attempt is made here to view the merits and feasibility of the

existing major classificatory treatments of the order in the light of

evidence from available palynological information including the ones

gathered presently from a good sample of taxa of the order distributed in

the South Indian region which covers 108 species in 42 genera

representing the six families of Bentham and Hooker.

Some general conclusions can be drawn by comparing the

distribution of families in the various treatments. Almost all modern

authors agree in including families such as Myrtaceae, Heteropyxidaceae,

Psiloxylaceae, Lythraceae, Punicaceae, Sonneratiaceae, Combretaceae,

Trapaceae, Crypteroniaceae, Alzateaceae, Melastomaceae, Memecylaceae,

Onagraceae and Rhynchocalycaceae in the Myrtales. Most of them agree

in including Oliniaceae and Penaeaceae in the Myrtales with some

exceptions. Emberger (1960) excluded these two families from his

Myrtales, and has placed both in the order Thymelaeales with

Geissolomataceae, Thymelaeaceae and Elaeagnaceae; Melchior (1964)

and Soo (1975) have by and large followed Emberger’s treatment, but placed

Oliniaceae in the Myrtales. Takhtajan’s treatment (1980) conformed to those

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of Emberger (1960) and Melchior (1964) and Soo (1975) in keeping

Rhizophoraceae, Lecythidaceae and Haloragaceae in Myrtales. However, a

few of the still recent treatments as that of Cronquist (1981), Dahlgren et al

(1981) and Thorne (1983) have excluded these families from their Myrtales.

Thymelaeaceae has been retained in the order by Cronquist (1981).

Of the various modern treatments, the most comprehensive work

relying on evidences from morphological and chemical characters of

Myrtales and associated families is that of Dahlgren and Thorne (1984).

They did not, however, furnish an explicit phylogenetic analysis of

relationships in the group. The more recent work of Conti et al. (1996) has

attempted a cladistic analysis of circumscription of relationships of

Myrtales based on molecular data. They have prescribed a phylogenetic

analysis of molecular sequence data from the chloroplast gene encoding

rbcL of Myrtales and putatively related families. The main objective of

their work is (1) to establish whether the Mytales sensu Dahlgren and

Thorne (1984) constitute a monophyletic order (2) to determine whether

some controversial families (Thymelaeaceae, Rhizophoraceae,

Lecythidaceae, Haloragaceae) should be included or excluded from the

Myrtales and (3) to identify the sister group of the Mytales. Conclusion

from the phylogenetic analysis of rbcL sequence data is that no single

cluster of morphological or chemical characters can be used to

unequivocally determine the circumscription of Myrtales, based on

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morphological and anatomical characters, notably the combined

occurrence of bicollateral vascular bundles and vestured pits in the vessel

elements. Their findings based on the parsimony analysis of DNA sequence

data from the chloroplast gene has supported the monophyly of a myrtalean

clade consisting of 13 families: Alzateaceae, Rhynchocalycaceae,

Penaeaceae, Oliniaceae, Memecylaceae, Melastomaceae, Heteropyxidaceae,

Myrtaceae, Vochysiaceae, Onagraceae, Lythraceae, Trapaceae and

Combretaceae (Conti et al., 1996). Moreover the circumscription of

Myrtales defined by the rbcL tree largely correspond to that proposed by

Dahlgren and Thorne (1984) based on morphological and anatomical

characters, notably the combined occurrence of bicollateral vascular

bundles and vestured pits in the vessel elements. The finding based on the

rbcL data is shown to be strongly supportive of the exclusion of the

Thymelaeaceae and Lecythidaceae from the myrtalean complex.

Rhizophoraceae

This family is treated with a premier position in the distribution of

families of Bentham and Hooker’s Myrtales. The relationship of this

tropical family which includes four mangrove genera has been a matter of

great controversy. The questionable monophyly of this family, with the

debatable placement of Anisophyllea seems to further complicate the issue

in establishing its relationship to other families. Some of the modern

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treatments given in Table - 7, such as Emberger (1960), Melchior (1964),

Soo (1975) and Takhtajan (1980) have retained this family in their

Myrtales, while in the relatively more recent treatments (Cronquist, 1981;

Dahlgren and Thorne, 1984; Briggs and Johnson, 1979), this family is

excluded from the Myrtales. Characters suggesting inclusion of

Rhizophoraceae in the Myrtales include the well-developed hypanthium in

the genera with epigynous flowers, tricolporate pollen grains (a condition

predominant in the core families of the Myrtales) and abundance of tannin

(Dahlgren and Thorne, 1984). However, the lack of vestured pits and

internal phloem argues against a close affinity of this family with the

Myrtales. The unique characteristics of Rhizophoraceae seem to isolate

them from any other rosid group despite the palynological attribute of

tricolporate pollen grain consistently noted in all taxa of the family

presently studied. Cronquist (1981) has assigned the family to a separate

ordinal rank of its own, the Rhizophorales. Thorne placed Rhizophoraceae

in the Cornales with Haloragaceae, while Dahlgren argued that the lack of

iridoid compounds and abundance of tannins do not support the placement

(Dahlgren and Thorne, 1984). In a later revised classification, Thorne

(1992), as also Cronquist (1981) accommodated the family in the

Rhizophorales that was included in the super order Geranianae together

with Linales, Geraniales and Malpighiales.

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In the present study of the Rhizophoraceae (five genera and seven

species) of which five species are with mangrove habit and others

terrestrial. Palynologically all the taxa studied here are consistently

tricolporate, radially symmetrical and isopolar. However, as regards the

secondary character most of them possess punctate ornamentation which is

seldom known in any of the myrtalean taxa. Another palynological feature

almost consistently noticed here is the fused endoaperture which is

rectangular-lolongate in shape, a condition so far unknown in any of the other

families of Myrtales. Moreover, absence of pseudocolpi in all members of the

family studied here is a condition known to predominate in other families of

Myrtales and appears to be a factor against retention of Rhizophoraceae in the

Myrtales. In this connection the findings of Vezey et al. (1988) who studied

39 species of the family seems to be against the retention of Rhizophoraceae

in the myrtalean complex. Their main findings on palynological features are:

(1) fused endoapertures and (2) lack of pseudocolpi. Based on this and their

molecular study, they divided the Rhizophoraceae into two families such as

Rhizophoraceae and Anisophylleaceae, both being given a position outside

the Myrtales. The main objection raised by most of the modern taxonomic

treatments of the Myrtales for inclusion of Rhizophoraceae in the Myrtales is

the absence of the vestured pits, internal phloem and bicollateral vascular

bundles. This together with the aforesaid palynological features present in the

taxa of the Rhizophoraceae swings more against its retention in the myrtalean

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complex, despite the solitary and seemingly favourable attribute of

tricolporate pollen grain which is not a single palynological character that can

be taken into consideration.

Combretaceae

This is one of the large families of the order Myrtales, almost

uniformly treated as a core member of the Myrtales in all taxonomic

treatments, both classical and modern. The character attributes studied

from various biological systems including pollen morphology show a high

degree of resemblance with other sister families of the order. In the present

study of 12 species representing six genera, the pollen is characterized by

tricolporate, heterocolpate, radially symmetrical and isopolar grains with

shape being prolate-spheroidal in majority of species. The heterocolpate

pollen grains of Combretaceae show broad similarities to the heterocolpate

pollen of Melastomaceae (Erdtman, 1971) and also to Lythraceae. One of

the genera namely, Strephonema is shown to be so distinct from all the

members of Combretaceae, and based on this a separate subfamilial status

(Strephonematoideae) has been suggested by Dahlgren and Thorne (1984)

and others. The genus differs from other members of Combretaceae due to

the absence of fibrous exotegmen in the seeds. Apart from this minor

difference, all the known treatments agree in considering Combretaceae as

a coherent family in the myrtalean group.

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As regards pollen morphology as well the genus Strephonema stands

out in having pollen which lacks pseudocolpi and having reticulate exine

surface in contrast to the condition in the rest of the members of the

family. However, a considerable number of features are common to the

subfamilies of Combretoideae and Strephonematoideae which include the

hair type, racemose inflorescence, obdiplostemony and unilocular ovary.

But Strephonema differs from other Combretaceae by having

hemiepigynous flowers and several wood anatomical features. These

together with palynological distinction may be argued in support of the

subfamilial status to Strephonema, a view highly favoured by Dahlgren

and Thorne (1984).

Myrtaceae

The family is a fairly distinct one, characterized in particular by

having gland-dotted leaves, stems and floral parts, inflorescence basically

paniculate, the flowers always epigynous, bisexual and actinimorphic, 4 or

5-merous, the androecium is usually polystemonous, the pollen grains

generally triangular, often syncolpate and lack pseudocolpi. Except for

Psiloxylaceae and Heteropyxidaceae, which Schmid (1980) included in

Myrtaceae the family shows no connections with other families of the

order Myrtales. A phylogenetic link with Lecythidaceae has been proposed,

but not supported (Dahlgren and Thorne, 1984). Bentham and Hooker’s

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Myrtaceae has been drastically reshuffled in most of the modern treatments.

In the largely accepted treatments, the Myrtaceae has been subdivided into a

number of families, most of them splinter ones and they are (in addition to

Myrtaceae) Lecythidaceae, Rhynchocalycaceae, Penaeaceae, Oliniaceae,

Crypteroniaceae, Psiloxylaceae and Heteropyxidaceae.

The Myrtaceae lineage as defined in the parsimony consensus rbcL

tree comprises a Heteropyxis/Psiloxylon subclade sister to the subclade in

which the inclusion of Vochysiaceae is only weakly supported, but would

imply a paraphyletic Myrtaceae sensu stricto. The taxonomic rank of

Psiloxylon and Heteropyxis has long been debated. Essentially the

discussion has focused whether these two genera should be treated as part

of Myrtaceae or as separate families. Melchior, 1964; Soo, 1975;

Cronquist, 1981; Schmid, 1980 and Dahlgren, 1981 (in Dahlgren and

Thorne, 1984) regards both Heteropyxis and Psiloxylon as segregate

monophyletic families. Psiloxylon differs from other parasyncolpate taxa

of Myrtaceae by having unusually large apocolpia and larger rugulate

elements in the equator; but otherwise similar to the rest of the families.

Heteropyxis is not easily distinguished from other genera of the syncolpate

and parasyncolpate group with intercolpar concavities. Myrtaceous pollen

as documemted by SEM (Patel et al, 1984) does not appear to have any

close similarity to taxa from other core Myrtales except for some

superficial similarities existing between Onagraceae pollen. (Erdtman,

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1971) has suggested similarity to Lythraceae through Cuphea. Both

Heteropyxis and Psiloxylon are narrow endemics, with the former

restricted to South- West Africa and the latter confined to the Mascarhenas

Island off the coast of Madagascar (these two genera are not represented in

the present study). The two genera differ in ovary, stigma and fruit

morphology (Schmid, 1980), and based on the phenotypic data and

embryological characters, the sister group relationship between

Heteropyxis and Psiloxylon is supported by other exomorphic features

(Conti et al., 1997).

Preliminary analysis of ndhF sequence across the order (Systma et

al., 1996) supports a second scenario, i.e. Heteropyxis and Psiloxylon

forms a sister clade to Myrtaceae sensu stricto where the Vochysiaceae is

sister to the larger clade. In view of the general exomorphic, embryologic

and molecular distinction combined with pollen morphological distinction,

separation of Psiloxylon and Heteropyxis from the Myrtaceae seems

relevant assigning both of them separate family rank.

The genus Barringtonia, Careya and Couroupita have been

associated with Myrtales sensu Bentham and Hooker. However, several

differences such as a number of embryological features, (summarised by

Tobe and Raven, 1983), presence of alternate leaves, bitegmic tenuinucellar

ovules, absence of internal phloem and vestured pits (Cronquist, 1981;

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Dahlgren and Thorne, 1984; Briggs and Johnson, 1979) strongly argue

against a myrtalean affinity to them. On these morphological grounds,

Cronquist (1981) has assigned a family status to these genera

(Lecythidaceae). Another alternative was to include the family in the order

Theales on the basis of numerous stamens and staminodia that are

symmetrically or asymmetrically disposed and developing in centrifugal

sequence as in most Theales. The tenuinucellate ovules with thicker

integument and more rapidly destroyed nucellus during its development

and absence of a parietal cell are attributes of Lecythidaceae which are

Thealean. The fruits are of various types and often very large, (Prance and

Mori, 1978) and the seeds are sometimes conspicuously large. The

endosperm is almost completely absorbed during seed development and

the embryo is large and rich in fat, as in myrtalean families.

The chemical contents of members of Lecythidaceae largely agree

with those of myrtalean families, but some agree with those in Theales

also. Lack of internal phloem, relatively more primitive vessels without

vestured pitting, presence of wedge-shaped phloem rays, alternate leaf

arrangement, polymerous, centrifugally developing androecium and

tenuinucellate ovules, all indicate Thealean affinity. A funicular aril, as found

in some Lecythidaceae has its counterpart in the Thealean Clusiaceae. The

pollen grains in certain genera are trinucleate when dispersed and the

tapetum is amoeboid which strongly deviate from the myrtalean pattern.

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The pollen grains are tricolpate, often syncolpate (Erdtman, 1952) or

tricolporoidate, whereas the simply colpate condition is not known in the

Myrtales. Based on this in addition to the characters mentioned by

(Dahlgren, 1980; Thorne, 1981; Conti et al., 1996) have suggested the

inclusion of Lecythidaceae in Theales or more appropriately in its own

suborder. Analysis of rbcL sequence data suggests that Lecythidaceae are

best placed at the base of the Asteridae near Sapotaceae and Ebenaceae,

among others (Morton et al., 1995).

The present study of the genera and species that are held as the

present day Lecythidaceae revealed syncolpate, syncolporate and colporate

pollen and macroreticulate exine sculpturing and apertural verrucae in

some species of one genus thus appear to support the inclusion of these

genera into a separate family, Lecythidaceae as earlier suggested by

several taxonomists.

Melastomaceae

This family has been treated as a major core family of the Myrtales in

all the taxonomic treatments of the order both classical and modern. The

family is considered to possess stable character states with respect to various

biosystems which include exomorphology, anatomy, embryology, palynology

and molecular biology. Based on molecular study, Conti et al. (1997) have

projected a melastomaceous lineage consisting of a subclade formed of

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Oliniaceae, Peneaeaceae, Rhynchocalycaceae and Alzateaceae sister to a

subclade formed by Memecylaceae and Melastomaceae sensu stricto. Based

on phenotypic data, Johnson and Briggs (1984) held that the ancestor of

melastomaceous lineage is characterized by fixation of opposite phyllotaxy, a

character paralleled by the Lythraceae lineage. Support of individual

clades within the Melastomaceae lineage has implications in the

taxonomic ranking in the family. Earlier debates on the classification of

Melastomaceae has focused on whether Memecylaceae should be

recognized as a separate family as suggested by Johnson and Briggs

(1984), Renner (1993), Takhtajan (1997) or treated as a subfamily of

Melastomaceae as suggested by Emberger (1964) Soo (1975), Cronquist

(1981) and Dahlgren and Thorne (1984). The distinctiveness of

Melastomaceae sensu stricto is supported by molecular data and is in

strong support for the clade comprising the three African families

(Oliniaceae, Rhynchocalycaceae and Penaeaceae) plus Central/South

American. This is suggestive of a Gondwanian origin and vicariant

evolution of these families. Morphology of the clade is corroborated by

three phenotypic synapomorphies (Johnson and Briggs, 1984). However,

uniquely derived character states for the African clade and the low

measuring of branch support suggest that the relationship between the

three African families and Alzatea remain uncertain (Conti et al, 1997). It

should be noted, as phenotypic characters suggest, that a close relationship

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of Crypteronia with the Melastomaceae lineage exists, but no clear

affinities within it evident (Dahlgren and Thorne, 1984).

Several taxa of the family have been studied here representing 8

genera. In all of them the pollen is tricolporate, heterocolpate, isopolar and

radiosymmetric. Palynological evidence is very much in favour of

assigning Melastomaceae and Memecylaceae in a comfortably single

family rank as against their suggested segregation on other grounds

(Dahlgren and Thorne, 1984).

Lythraceae

The Lythraceae sensu Bentham and Hooker has been subjected to a

great deal of taxonomic restructuring based on evidences from various

disciplines including palynology and molecular biology. During the present

study, 19 species representing 9 genera have been examined, and the taxa

showed greatest amount of variation in the order with diversity evident at all

levels, including shape of pollen, aperture system and exine sculpturing.

Infrageneric variation in palynological characters was remarkable as in

Cuphea. Major deviation of taxonomic treatment of both Bentham and

Hooker (1865) from the modern ones is the recognition of familial and/or

subfamilial rank given to genera such as Alzatea, Sonneratia, Duabunga and

Punica. In his taxonomic treatment, Dahlgren (1981) assigned familial rank

to Punica and Sonneratia. Similar family status for these genera was

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proposed by several modern treatments (Emberger, 1960; Melchior, 1964;

Soo, 1975; Hutchinson, 1973; Stebbins, 1974; Briggs and Johnson, 1979;

Cronquist, 1981; Takhtajan, 1981). However, in a later treatment Dahlgren

(1984) opted to assign these, only subfamilial status. From the results of Patel

et al. (1984)’s study it is pointed out that Lafoensia is similar to Sonneratia

(Sonneratiaceae), Diplusodon similar to Duabunga (Duabungaceae) and

Lagerstroemia bears resemblance to Punica (Punicaceae). These similarities

strongly support the view of Dahlgren and Thorne (1984) that Sonneratia,

Punica and Duabunga should be regarded as separate subfamilies

independently related to Lythraceae.

From the present examination of palynological features of Punica and

Sonneratia it may be noted that in Punica the pollen grains are tricolporate

without pseudicolpi, and are comparable to the pollen of Lagerstroemia

species. While, in Sonneratia the pollen is anguloaperturate, triporate with very

prominent psilate polar cushions and rugulate-verrucate mesocolpia, prominent

meridional ridges and apertural fields. Palynological attributes of Sonneratia

and Punica stand out from the rest of the genera of the Lythraceae, and more so

with Sonneratia, and hence on palynological grounds, familial or subfamilial

rank assigned to them appears very much justifiable. It may be noted that

palynological distinction in Sonneratia appears to be strong enough to

even this being given a separate family rank rather than subfamily.

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Onagraceae

This is a very distinctive family, and different from other Myrtales on

several features. Bentham and Hooker (1865) and most of the modern

treatments have included Onagraceae as a constituent family of the

myrtalean complex.The close relation with Lythraceae suggested has been

in view of the teeth structure and marginal ciliation of leaves (Hickey,

1981). Fibrous exotegmen of seeds and similar petal venation are some

other conspicuous attributes which may indicate close connection between

the two families (Dahlgren and Thorne, 1984). The relationship of

Onagraceae has been widely debated, the most parsimonious phenotypic

trees (Johnson and Briggs, 1984) have consistently supported a clade between

Onagraceae and Trapaceae, but no characters on the common branch were

largely homoplastic. The phenotypic analysis further showed Lythraceae

(including Sonneratiaceae, Punicaceae and Duabungaceae) are the sister clade

to Trapaceae + Onagraceae. Detailed morphological study of Lythraceae by

Graham et al (1993) favours the sister group relation between the Onagraceae

and Lythraceae. The interrelationships of Onagraceae were discussed by

Conti et al. (1993), and they stressed monophyly of the family. The

distinctiveness of Onagraceae is further supported by several morphological

synapomorphies pertaining especially to pollen morphology (pollen grains

with viscin threads and unique exine sculpturing and embryology, Oenothera

type embryo sac) Johnson and Briggs (1984).

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The genus Trapa formerly included in the Onagraceae by Bentham

and Hooker (1865) has temperate to tropical distribution. Out of the

existing three species of the genus, one species has been presently studied.

The plants are aquatic floating herbs. The pollen grains triangular and have

three meridional ridges. They can be interpreted as possessing intercolpate

depressions. It lacks viscin threads on the pollen grain; flowers are

epigynous; 4-nucleate Oenothera type embryo sac. In view of the

distinction it has with members of Onagraceae, especially absence of

viscin threads on the pollen grains which is a characteristic feature of

Onagraceae, the possession of meridional ridges formed by the exine

folding, their position over the colpus in contrast to the meridional ridges

formed by mere exine thickenings that alternate with colpus in

Onagraceae, a separate family rank given to Trapa (Trapaceae) can be

confidently vindicated.

Haloragaceae

This family has often been placed in Myrtales due to the possession

of opposite leaves with minute stipules, 4-merous, diplostemonous flowers

with four carpels, similar embryology and endospermous seeds (Orchard,

1975). Hickey and Wolf (1975) placed it in Hippuridales due to the

occurrence of Rosoid type leaf teeth. But these are met within some

Onagraceae and Lythraceae too (Hickey, 1981). Vestigial kind of stipules

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as in Myrtales are present in the Haloragaceae. The floral anatomy of the

family (Orchard, 1975) shows similarity to that in Cornales and Araliales

rather than myrtalean families. The pollen grains are shed in the tricellular

stage as in Araliales and unlike Myrtales. The endosperm formation in

Haloragis and in some species of Myriophyllum is of the cellular type that

is never found in Myrtales. In Polygonum type of embryo sac development

and the Caryophyllad type embryogeny, the family differs from all

Myrtales (Kapil, 1962; Kapil and Bala-Bava, 1968). The endosperm-rich

seeds of Haloragaceae are not common in Myrtales but, the seeds in

Araliales, though rich in endosperm the embryo is proportionately smaller

than in the former one.

Haloragaceae seems to be a fairly isolated family, and though it

possesses a number of myrtalean attributes, the dissimilarities count more.

The view of Orchard (1975) that Cornaceae is the closest family of

Haloragaceae is not supported by Dahlgren as the typical Cornales characters

such as the unitegmic, tenuicucellate ovules, lack of tannins, presence of

iridoids etc. are not met within the latter. Haloragaceae may better be placed

in an order separate from, but near Myrtales. Its position may be closer to

Araliales than to Cornales as in Engler and Prantle’s treatment. Thorne agrees

with Orchard in placing the family in Cornales and considers Gunneraceae

and Hippuridaceae as related families in the Haloragineae. In the present

study of a single taxon (Myriophyllum tuberculatum) of Haloragaceae the

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pollen grains are triporate, aspidote and with spinulate exine sculpturing, a

condition not observed in any of the myrtalean families included in this

study. So the separation of the family from Myrtales and its inclusion in a

separate order seems feasible.

5.5 Interrelationships, Evolution and Affinities of the Myrtales

The Myrtales are one of the few larger orders that have a rather

uncontroversial circumscription as regards the “nucleus” or “core

families”. The widely accepted such core families of the order are

Onagraceae, Trapaceae, Lythraceae (incl. Punicaceae and Sonneratiaceae),

Oliniaceae, Combretaceae, Alzateaceae, Penaeaceae, Rhynchocalycaceae,

Crypteroniaceae, Memecylaceae, Melastomaceae, Psiloxylaceae,

Heteropyxidaceae and Myrtaceae. All of these may not necessarily be

entitled familial status (Dahlgren and Thorne, 1984). In an attempt to

approach the interrelationships between families of the order, and its

evolution and affinities it may be profitable to deduce a probable original

state for their common ancestor. This has been done by comparing the

character states of myrtalean families inter se, and by examining those of

probably related order, Rosales being taken as the out group. Dahlgren and

Thorne (1984), in their attempt to trace the myrtalean ancestry have listed

a constellation of character attributes, mostly morphological and

anatomical. They have pointed out that myrtalean ancestors were probably

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woody plants with alternate or opposite leaves with teethy margins. Stems

had evolved bicollateral vascular strands, and the vessel elements had

alternate vestured pits. Moreover, they consider that the immediate

ancestral forms were tannin plants with a flavanoid spectrum. Though

none of the extant families exhibit this combination of character attributes,

they hold that the ancestral forms of Myrtales would have been fairly

similar to certain extant Lythraceae, and that the position of Lythraceae in

the order is central, and they show close relationship with several of the

families, including Penaeaceae, Rhynchocalycaceae and Onagraceae. A

number of primitive states are concentrated, to a higher degree than in

Lythraceae in Psiloxylaceae, Heteropyxidaceae, Myrtaceae and

Strephonematoideae, although each of these is specialized in various

respects. It may be noted that varied rates of evolutionary specializations

seen throughout in Myrtales occur in Psiloxylon, which has retained the

presumably ancestral condition (Dahlgren and Thorne, 1984).

Johnson and Briggs (1984) have given a cladistic presentation of the

probable evolution of Myrtales. Supporting their contention, Dahlgren and

Thorne (1984) viewed that an evolutionary line that probably diverged

very early from the myrtalean ancestors is represented in Psiloxylaceae,

Heteropyxidaceae and Myrtaceae. These share some conspicuous features

such as the characteristic shape and aperture condition (syncolpate of the

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pollen grains, (Patel et al., 1984) and the presence of schizogenous cavities

with essential oils visible as pellucid dots on the green parts.

The great concordance in pollen morphology among Psiloxylaceae,

Heteropyxidaceae and many Myrtaceae suggests that this rather peculiar

type evolved from protomyrtaceous ancestors, and later in many

Myrtaceae gave rise to superficially simpler kinds (Dahlgren and Thorne,

1984). The fact that this kind of pollen is known already in the Cretaceous,

before any other / certain Myrtales, also indicate that this group of families

may have differentiated from the myrtalean ancestors very early.

Onagraceae seem to deviate rather strongly from other Myrtales, and

probably may have evolved as a lateral evolutionary line at an early stage.

The evidence is somewhat contrary to in this respect (Dahlgren and

Thorne, 1984). The family is an unusually distinct one having the

combination of epigynous flowers, pollen with viscin threads, Oenothera

type embryo sac formation, and tissues with Calcium oxalate raphides. The

latter three characters are absent in nearly all other Myrtales, and it is

likely that all these character states are derived ones. Thus it is likely that

other families such as the Trapaceae could have evolved from an

Onagarceous evolutionary line after these attributes had been acquired. In

pollen grain shape and pollen wall structure Onagraceae shows some

general similarity (present observation) to Myrtaceae, Heteropyxidaceae

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and Psiloxylaceae; but more conspicuous are a number of characteristics

shown by Onagraceae and Lythraceae. Wood, with libriform and septate

fibres, leaves with lateral teeth, petals with similar pinnate venation etc.

(Corner, 1976). With the exception of the leaf teeth, these attributes seem

to represent a derived state. Convergent evolution of some of the derived

states is also unlikely; and more likely is the alternative that Onagraceae

diverged from proto-lythraceae ancestors after the wood and the seed coat

structure had already evolved.

The Onagraceae are palynologically very distinct in the order. The

pollen are special by virtue of the often triangular shape with three or more

protruding “papillose” apertures, the mechanism of tetrad cohesion,

(Skvarla et al., 1975), and the fine structure of the exine, in particular the

ektexine, which is granular, “beaded” delicately branched (Skvarla et al.,

1976), and especially by their constant presence of viscin threads (present

study, Skvarla et al., 1978). The relative number per pollen grain and the

surface structure of these threads are variable in Onagraceae which may be

considered to be interesting for the division of the family. It is possible

that the characters, to be associated with the pollen grains of the family,

along with many other distinctive features is suggestive that the family

was derived early from ancestral Myrtales (as suggested elsewhere). On

the whole the exine structure as well as pollen shape and aperture of

Onagraceae can suggest its closest resemblance in Myrtaceae as well as

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Psiloxylaceae and Heteropyxidaceae, a view upheld by Dahlgren and

Thorne (1984).

The remaining families of the Myrtales, (sensu Dahlgren and Thorne,

1984) form somewhat a coherent group, most families being characterized

by so-called “heterocolpate” pollen grains in which pseudocolpi are

present between the true apertures. These features are extremely rare in

angiosperms outside the Myrtales, and it would be highly likely that

pseudocolpi evolved independently in several phyletic lines within the

order. Hence, as Dahlgren and Thorne (1984) have contented, it can be

presumed that the genetic/gametic constitution for pseudocolpi (whether

expressed or not) may have evolved once in the common ancestor of these

families, but may have become lost subsequently that is, this attribute has

not come to expression in some lines. Thus, as in Lythraceae, pseudocolpi

occur in some, but not in all genera (Patel et al., 1984; present

observation). In some genera they are indistinct or inconsistently present.

In Lythraceae the genus Lythrum has pseudocolpi of the same number as

the true aperture, while in most herbaceous genera with pseudocolpi, the

number is twice as many as the number of the apertures (present study).

Pseudocolpi are absent or very indistinct in the pollen of Alzateaceae

and Lythraceae subfamily Punicoideae, Sonneratioideae and

Duabungoideae, in the Combretaceae subfamily, Strephonematoideae, and

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in Trapaceae (Patel et al., 1984), where the intercolpate depressions

dubiously correspond to pseudocolpi. The first five families which are all

few in species content show strong affinity (in various respects) to families

or subfamilies where, the pollen grains possess pseudocolpi, and hence it

is likely that their common ancestors had heterocolpate pollen.

The Trapacae are often associated with Lythracae by Miki (1959)

who derived Trapa from Lythrum through Hemitrapa. The family is

unique in the order in many respects. However, a specialized floating

aquatic, this deviates from the other Myrtales in many exomorphological

and embryological features. It is possible that Trapa may have diverged

strongly from other Myrtales and also from any plausible ancestral types.

The pollen morphology of Trapa may indicate affinity with the

heterocolpate condition if the intercolpate areas between the meridional

crests are considered homologous to intercolpate depressions, and hence to

pseudocolpi. But this is a disputed possibility.

Within the subfamily Lythroideae, the polymerous large-flowered

genera such as Lafoensia and Lagerstroemia have numerous stamens and

unspecialized pollen grains lacking pseudocolpi. These features are

generally considered to be primitive, a view challenged by Dahlgren and

Thorne (1984), who consider that increase in floral size have favoured an

increase in stamen number. Lack of pseudocolpi is found in a number of

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lythraceous genera, and does not seem to characterize natural group of

genera in subfamily Lythroideae. Dahlgren and Thorne (1984) have

considered reasons for the absence of pseudocolpi, and maintained this

feature to be a derived condition in the family. They contented that in

contrast to all other myrtalean families, the pseudocolpi, when present,

tend to be double the aperture number, a condition which should be

considered another derived character state.

Oliniaceae agree with Combretaceae in several features e.g. epigyny

and the frequent occurrence of small petal scales and common basic

chromosome number (x = 12). A number of families mostly with

heterocolpate pollen grains have seeds without a fibrous exotegmen.

Although Oliniaceae are not known in this respect, they probably belong

to the group sharing a number of features with Penaeaceae and

Rhynchocalycaceae and these families are Melastomaceae, Memecylaceae,

Crypteroniaceae, Rhynchocalycaceae, Penaeaceae and Alzateaceae.

Alzateaceae probably belongs to this group although their pollen grains

lack pseudocolpi.

The two unigeneric families, Alzateaceae and Rhynchocalycaceae,

share a number of wood anatomical characters (Vleit and Baaas, 1984)

which make them closely allied, and in which they differ from, especially

Penaeaceae, which have also retained a primitive wood anatomy, also

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closely connected with Oliniaceae, Rhynchocalycaceae and Alzateaceae in

various floral features. Many taxonomic experts, especially Dahlgren and

Thorne (1984) consider it difficult to speculate about the interrelationships

and evolutionary sequences for the Melastomaceae - Memecylaceae –

Crypteroniaceae – Penaeaceae – Oliniaceae - Rhynchocalycaceae –

Alzateaceae group, and refer to alternative interpretation prescribed by

others like Johnson and Briggs (1984). They are of the view that small

deviations in interpretation and small deviations from the most

parsimonious evolutionary courses may strongly change their evolutionary

model.

Families that are allegedly related to or in various respects

conspicuously similar to Myrtales are Thymelaeaceae, Haloragaceae,

Rhizophoraceae, Lecythidaceae, Elatinaceae etc. The Haloragaceae,

however has often been placed in Myrtales by virtue of its opposite leaves,

4-merous, basically diplostemonous flowers, similar embryological

features etc. Many features are in accordance with those of Myrtales

barring internal phloem and vestured pitting. The consensus in the circle of

the renowned taxonomists is that Haloragaceae seem to comprise a fairly

isolated family, although they possess a number of myrtalean attributes,

which however are counter balanced by several dissimilarities. Porate

pollen grains and the spinulate exine sculpturing in them differ which may

be suggestive that the family is not strongly allied to Myrtales. Thus it may

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be proper that Haloragaceae be better treated in an order separate from, but

near Myrtales.

There are a few families which are sometimes associated with, but

apparently distantly related to Myrtales. They are Rhamnaceae,

Marcgraviaceae, Theaceae, Clusiaceae, Myrsinaceae Malpighiaceae,

Columelliaceae, Loganiaceae etc. But there is no unanimity regarding this

among leading taxonomists.

Concerning the relationship and affinity of the Myrtales with other

orders there is no agreement. The position of the Myrtales has varied in

different classifications, and is still a matter of doubtful opinion. Cronquist

(1981) and Takhtajan (1980) are somewhat constrained by their division

into the subclass Dilleniidae and Rosidae of the majority of orders of

dicots. Myrtales in both classifications are placed in the Rosidae, where

they form the main order. It is generally agreed that the order is more or

less related to Rosiflorae including Rosales and Saxifragales. Other orders

sometimes recognized and associated with Myrtales are Elaeagnales and

Thymelaeales. Two attributes to which most leading modern taxonomists

attach great importance are occurrence of internal phloem and the presence

of bicollateral vascular bundles and vestured pitting in the vessel elements.

When the Myrtales are strictly circumscribed, these characteristics become

critical.

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Another order where opposite leaves are combined with internal

phloem and vestured pits is Gentianales, within which especially

Loganiaceae show some resemblance to Myrtales. But, because floral

morphology, embryology and chemistry are vastly different, Dahlgren

(Dahlgren and Thorne, 1984) does not consider any relationship between

Loganiaceae and Myrtales. The major orders with which affinities for

Myrtales have been contemplated by various taxonomic experts are

Rosales, Cunoniales, Saxifragales, Rutiflorales, Theales and Cornales.

However, arguments for and against these affinities also exist among them.

Numerous studies (morphological, anatomical, embryological, cytological,

palynological, phytochemical and molecular) have been conducted by a

host of workers on the Myrtales, and yet a convincing consensus

concerning inter relationships among the families of the order and its

phylogeny and evolution and affinities still remain elusive.

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