Incompatibility, Stamen Movement and Pollen Economy in a Heterostyled Tropical Forest Tree, Cratoxylum formosum (Guttiferae)

Incompatibility, Stamen Movement and Pollen Economy in a Heterostyled Tropical ForestTree, Cratoxylum formosum (Guttiferae)Author(s): D. LewisSource: Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 214, No.1195 (Jan. 22, 1982), pp. 273-283Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/35529 .

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Proc. R. Soc. Lond. B 214, 273-283 (1982) Printed in Great Britain

Incompatibility, stamen movement and pollen economy in a heterostyled tropical forest tree,

Cratoxylum formosum (Guttiferae)

BY D. LEWIS, F.R.S. Department of Plant Biology and Microbiology, Queen Mary College,

University of London, Mile End Road, London El 4NS, U.K.

(Received 30 July 1981)

[Plate 1]

Cratoxylum formosum shows all the classical features of a distylic species. The two types are: long-styled plants with short stamens and small pollen grains and short-styled plants with long stamens and large pollen grains. Compatible pollinations are only between the two types; incompatible pollen tubes are inhibited in the style. A significant morphological feature distinguishes Cratoxylum from distylic plants in other families. Instead of having a small number of anthers making well separated narrow discs in the two types, Cratoxylum has many anthers (144) and they are arranged on staminal bundles that produce long cylinders of anthers that partially occupy similar height zones in the two types of flower.

A novel method of separation of the two height zones is achieved by the bending of the stamens of the long-styled type when the flower opens, which converts the cylinder to a narrow disc of anthers at the same height as the 'short' stigma. The bending coincides with anther dehiscence and is slightly later than the first daily insect visitation. The anthers return to the upright position later in the day, when the pollination is complete.

There was a 20-fold difference between the amounts of pollen deposited on the two types of stigmas. The 'long' stigmas received 1200 pollen grains per flower, in the ratio of 46 'long' to 54 'short', which is close to the ratio of two types of pollen produced in the population. This random deposition of pollen on 'long' stigmas is, however, more than adequate for the 36 seeds produced per flower. 'Short' stigmas received only 64 pollen grains per flower, in the ratio of 90 'long' to 10 'short', and several flowers had below the critical level of 36 compatible pollen grains for full seed production. Pollen loads of the pollinating bee, Apis javana, consisted of 'long' and 'short' pollen on the thorax in the ratio found on the 'long' stigma, and on the head of the bee in a ratio close to the 9: 1 found on the 'short' stigma. The corbicular loads reflected accurately the pollen of the tree in which the bee was caught.

For Cratoxylum the accurate positioning of the anthers of the long-styled plant in relation to the visiting bees head was an important evolutionary step in the effective pollination of the short-styled form, which, at least in this species, is one critical and highly selected feature of the system.

[ 273 l



274 D. Lewis

1. INTRODUCTION

Heteromorphic self incompatibility in flowering plants is accompanied by differen- tially positioned sex organs. In the distylic system high stigmas (Y) with low anthers (J) are found in the long-styled form and low stigmas with high anthers in the short-styled form. The self incompatibility, which is controlled by two alleles, is separate from the morphology and is caused by the inhibition of pollen germination or pollen-tube penetration of the style. The biological significance of the self incompatibility is obvious; it is to regulate inbreeding caused by self fertilization. The biological significance of the morphological differences also appeared to be obvious as pointed out by Charles Darwin (I877): 'There can hardly be a doubt that the relative positions of these organs is an adaptation for the safe transportal by insects of the pollen from one form to the other'. With a shift of interest from the morphological to the self incompatibility aspect the relative positions of stigmas and anthers have been considered to be of secondary importance and there has even been doubt of the adaptive significance proposed by Darwin.

The efficiency of distyly in pollen transport has recently been tested in 13 distylic species, summarized by Ganders (I 979). The results in several of the species show certain consistencies and support the adaptive hypothesis but only by a marginal improvement above random transport of the compatible pollen. Furthermore, in several species there was less compatible pollen on the stigmas than expected on the basis of a random distribution. In none of these species studied for pollen transport was there a shortage of compatible pollen on the stigmas in relation to the potential maximum number of seeds. In other words, the 'pollen transportal' was safe in all these species despite the apparent inefficiency of distyly to increase the compatible pollen flow. The unknown factor is whether the pollen transport would be unsafe without the heterostyly.

The subject of this study, Cratoxylumformosum, is a tropical forest tree which has been recorded as heterostyled by Thistelton-Dyer (quoted by Darwin i877) but of which no detailed study has been made. Cratoxylum is unusual among heterostyled species in having numerous anthers which are arranged on staminal bundles in such a way that more than half the height of the flower is occupied by anthers and the anther positions are partially concurrent in the two types. This unusual anther distribution has provided a situation in which it has been possible to show that without heterostyly, certain novel features and movement of the anthers the pollen transfer to the short-styled form would be disastrously unsafe.

2. MATERIAL AND METHODS

The genus Cratoxylum Blume (Guttiferae) consists of six arborescent species confined to southeast Asia (Gogelein I967). The group of trees of Cratoxylum formosum (Jack) Dyer that was studied in most detail was a roadside plantation of more than 100 trees at Sian Tuan Avenue and Hua Guan Avenue, Singapore District 2158. The trees were on both sides of the street and were about 6 m apart. During the period of study, January-March 1981, 33 trees were in flower and detailed observations were made on six of them. The history of the trees was not



Incompatibility, heterostyly and pollen transport 275

known but they were not grafted clones, and from the segregation of long- and short-styled trees and several vegetative characters there is little doubt that they had arisen from seed. Observations were also made on two trees of C. formosa and a subspecies C. formosa prunifolium growing in the Singapore Botanic Garden as isolated specimens. Material was also collected from six trees growing wild in natural forest at Jason's Bay, Malaysia, 1040 06' E, 1 53/ N, and at Bukit Timah, Singapore Island.

Controlled pollinations for the study of pollen-tube growth were made on branches cut from the trees before flower opening, placed in water and kept in the laboratory at approximately 25 'C. Although flowers continued to open in the early morning of the following 2 days, only the flowers open on the morning of collection were used. Styles were fixed in 3: 1 alcohol: acetic acid 20-24 h after pollination, stained in cotton blue in lactophenol and examined after 24 h of staining. The diameters of pollen grains were measured on fresh pollen placed in cotton blue on a slide without a cover glass. For stigmatic pollen deposits by bees, flowers were collected directly from the trees at Ith 00 and 15h 00 on the day of opening or on the following day. Estimates of the number of the two types of pollen on the body of the bees and on stigmas were made by visual inspection without measuring individual pollen grains because there was no overlap in size. It was found only necessary to resort to a measurement check of a few pollen grains when a more or less pure sample of one type was first encountered. The main natural pollinators were two species of bee, Apis javana and Apis florae, which were kindly identified by Mr D. H. Murphy. A casual unidentified butterfly was a sporadic visitor. Bees of Apis javana were collected singly with forceps while working a flower, and held until unconscious in a vial containing chloroform vapour and stored in the vial until examined. Apisflorae, a much smaller bee, was not so common, and only one specimen was caught. The pollen on the bees was counted by removing the head and thorax and rinsing them separately in a large drop of cotton blue. The corbiculae were sampled by forceps. With numbers of pollen grains below 500, complete counts were made, but with higher numbers, sampling to obtain only the proportion of the two types of pollen was done.

3. FLORAL MEASUREMENTS, POLLEN TUBE GROWTH

AND INHIBITION

The relevant measurements of the floral parts are shown in table 1. The characteristic difference in height of the stigmas of the two types is directly due to the different style lengths, but the heights of the anthers are not so clearly differentiated, particularly in flowers that are opening, as were those measured for table 1. Both long- and short-styled flowers have a spread of anther height, of 5.9 and 5.7 mm respectively, which covers half the height of the flower. The anthers in fully opened long-styled flowers are compressed into a narrow disc, described later, and this produces a clear separation in anther heights between the two types. The stamen lengths do not therefore bear a direct relation to the height of the anthers.

The pollen sizes show the difference in size characteristic of heterostyled plants:



276 D. Lewis

the pollen of the long-styled flower is 19.7 gim in diameter and that of the short-styled flower 25.4 gm. Controlled pollinations on flowering shoots collected from Cratoxylum formosum growing in Sian Tuan Avenue and kept with their ends in water in the laboratory and examined after fixing and staining 20 h after pollination, gave a clear and classical picture of inhibited pollen tubes in the selfed long- and short-styles and fully compatible tubes at the base of the style in the two legitimate crosses, long x short and short x long. Most of the inhibited tubes

TABLE 1. HEIGHTS OF STIGMAS AND ANTHERS IN MILLIMETRES, MEASURED FROM THE BASE OF THE OVARY IN LONG- AND SHORT-STYLED FLOWERS THAT HAVE JUST OPENED

(The heights in brackets are from long-styled flowers 30 min after opening, after the stamens have reflexed to form a disc of anthers.)

long-styled short-styled difference

height of stigma 10.4 4.4 6.0 height of anthers

highest 8.2 (3.2) 11.6 3.4 (7.4) lowest 2.3 (2.2) 5.9 3.5 (3.7) difference 5.9 (1.1) 5.7

in the long-styled flowers selfed were 0.5 mm long and growing over the surface of the stigma but not penetrating into the stigma; a minority had penetrated about 0.3 mm into the stigma. Most of the selfed pollen on the short styles penetrated the stigma and was inhibited at 0.5 mm.

Controlled pollinations were examined on two trees growing in the Singapore Botanic Garden. One was a short-styled tree of C. formo8um and the other a long-styled tree of C. prunifolium. From cut branches in the laboratory, both trees when selfed had short tubes in the stigma. The cross between long-styled C. prunifolium as female and the short-styled C.formo8um as male produces compatible pollen tubes at the base of the style. The reciprocal cross, short-styled plant of C. formo8um x long-styled plant of C. prun -,folium, had inhibited pollen tubes in the stigma. The short-styled form of C. prunifolium was not available, and this incomplete study of the interspecific relations is given here because it is essential to the interpretation of observations made after natural pollination by the bee Api8 javana on these trees, to be described later.

4. FLOWER OPENING AND STAMEN MOVEMENT

The sequence of events in flower opening in the early morning and closing in the afternoon was recorded in five trees, two lonLg-styled and three short-styled, which were in full flower in Sian Tuan Avenue. Recordings were made from 6h 30 to 15h 00 on 14 January 1981, which was a sunny dry day with temperatures about 27 'C. The sequence is summarized in table 2. The two long-styled plants had a similar timing, and only differed in the time of start of flower opening: one was approximately 1 h later than the other. The unique feature for any heterostyled



Proc. R. Soc. Lond. B, volume 214 Lewis, plate I

Wia n ther

coe _ 9stamens

[_ z~~~~~~~~~~~~~~~~~~~~~~Fcn p. 277




plant is the bending of the stamens in the long-styled flower. This reflexing of the stamens follows the opening of the petals and coincides with the dehiscing of the anthers. The reflexing continues until the 144 anthers have bent through 90? to form a nearly solid disc at approximately 3 mm height, which matches the height of the stigmas of the short-styled flowers. At 13h 00 the stamens and anthers have returned to the upright position, the petals drop and the flower does not open again (figure 1). The three short-styled plants were very similar in their time and

TABLE 2. SEQUENCE OF STAGES IN THE OPENING, BEE VISITATION AND FADING

OF THE FLOWERS OF CRATOXYLUM FORMOSUM

time stages in flower opening

long-styled 6h 30 bud ready to open 6h 45 starting to open, slight movement of petals 7h 12 petals half open at an angle of 450, stamens upright 7h 53 petals fully open, styles diverging, stamens reflexing, anthers

dehiscing, bees visiting 8h 30 stamens reflexed to 90? to produce a disc of anthers 2 mm thick at

2.5-4.5 mm height 9h 00 anther slightly curved below the horizontal

15h 00 anthers returned to upright position, petals upright or fallen from flower

short-styled 6h 30 buds with small aperture at top of petals, bees visiting 7h 40 starting to open, anthers dehiscing 8h 36 petals fully open, anthers upright or at most 150 from upright

15h 00 petals upright or fallen from flower

sequence of opening. The stamens and anthers remained upright or with only a slight divergence and at all times presented the anthers in a cylindrical or slightly conical shape occupying the heights 5.9-11.6 mm as given in table 1.

Both the long- and short-styled flowers have at the inner basal surface of the petal a ligule or squamule as described by Thistelton-Dyer. This is a solid structure, which is in loose contact with the stamens. In the long-styled flower these ligules appear to act as a fulcrum on which the stamens bend (figure 1 a), although this is probably not the only function of the ligule because the ligules in the short-styled flower are as large and well developed as those in the long-styled flower.

5. POLLEN FLOW

The stigmas of short- and long-styled trees were sampled at different times of the day from trees in Sian Tuan and Hua Guan avenues, Singapore, (populations 1, 2) and those of short-styled trees in a wild population at Jason's Bay, Malaysia. The results are given in table 3. The numbers given for short stigmas are total counts; those for long stigmas are from random areas of the stigma and an estimated total of 400 pollen grains per stigma. The proportion of the two types of pollen is an accurate figure but the estimated total is probably an underestimate.



278 D. Lewis

The greatest significance is found in the total number of pollen grains on 'short' and 'long' stigmas. The counts of samples of flowers that were fully open gave a value of 1200 per flower for the long-style and 64.7 for the short-style. Furthermore, there is a highly significant difference in the percentage of compatible (legitimate) pollen on the two types of stigma, 45 % for the long-style and 90 % for the short-style. A further significant difference was found in the recently opened flowers sampled at 10Oh 00: the short-style had only 22 pollen grains per stigma, of which 44 0 were compatible, in contrast to 64.7 per stigma, for the long-style, of which 90 0 were

TABLE 3. NUMBERS OF POLLEN GRAINS ON NATURALLY POLLINATED STIGMAS

(For 'short' stigmas the numbers are total counts, but for 'long' stigmas the numbers are from samples.)

'short' stigmas 'long' stigmas

population ... 1 2 3

time of sampling 10h 00 I th 00 i5h 30 12h 30 10h 00-12h 00 number of stigmas 9 19 25 13 31 number of grains of

'long' pollen 28 387 448 275 1116 'short' pollen 38 41 43 35 942

compatible pollen (On) 43.7 90.4 91.2 88.9 45.7 number of grains of compatible 9.3 61.1 53.7 63.4 548.4t pollen per flower (3 stigmas)

t The number of compatible pollen grains per flower for the long-styled is derived from the percentage (45.7) and an estimated total of 1200 pollen grains per flower.

compatible. This low number of pollen grains and low percentage of compatible pollen in the early morning precedes the reflexing of the stamens described earlier. This difference with time was not observed with long-styled flowers. The small differences in pollen per flower in the three 'short' samples, population 1 1 lh 00, 15h 30 and population 2 12h 30, are not significant and are due to abnormally high values for a single flower in each of two samples.

Stigmas and styles were examined of naturally pollinated 1 day old flowers from the isolated trees of long-styled C. prunifolium in the Singapore Botanic Garden. The nearest neighbour was a single tree of C. formosum, short-styled in flower, and both trees were worked by two species of bees, Apis javana and Apis ftorae.

Out of the six flowers of C. prunifolium examined only one flower had any pollen from the neighbouring tree (120 m) and all the six flowers had 400-500 self-type pollen on each stigma. The one that received neighbouring pollen had the following.

style neighbouring no. pollen grains pollen tubes at base of style 1 5 3 2 2 0 3 1 0




All the 18 stigmas of the six flowers had many self-pollen tubes inhibited at 0.75 mm and all styles contained three or four pollen tubes with swollen ends inhibited half way down the style at 2.8-4.0 mm. These longer inhibited tubes were not observed in the controlled self pollinations in the flowers pollinated in the laboratory.

TABLE 4. POLLEN LOADS OF INDIVIDUALS OF Apis JAVANA CAPTURED ON FLOWERS OF CRATOXYLUM FORMOSUM

(The total numbers of pollen grains on the thorax and corbiculae were large and sampled only and provided reliable percentages; the numbers on the head were significantly less and were total counts. Figures are percentages except in total columns where they refer to number of pollen grains on the head only.)

short-styled tree long-styled tree

head thorax corbiculae head thorax corbiculae

total S L S L S L total S L S L S L 147 100 0 100 0 100 0 76 0 100 0 100 0 100 19 84 16 100 0 100 0 394 0 100 0 100 0 100 71 79 21 100 0 100 0 161 1 99 0 100 0 100

428 19 81 100 0 100 0 367 1 99 0 100 0 100 866 10 90 84 16 92 8 67 0 100 0 0 0 0 119 10 90 49 51 82 18 121 0 100 0 0 0 0 67 65 35 89 11 74 26 56 0 100 1 99 0 0 26 100 0 60 40 70 30 375 0 100 7 94 0 0

116 44 56 57 43 66 34 114 9 91 0 100 15 85 56 98 2 0 100 4 96

161 1 99 0 100 3 97 149 0 100 0 100 1 99 57 2 98 1 99 1 99

The observations clearly show the almost complete pollen isolation produced by 100 m separation; they also confirm the compatibility of C. formosum pollen on the C. prunifolium style. The tree of C. formosum carried a few mature capsules of seed from a previous flowering, which from these experiments we can conclude will contain hybrid seed. The tree of C. formo8um had no capsules and this is in agreement with the controlled pollination test showing that C.formo8um short-style is incompatible with the 'long' pollen of C. prunifolium.

6. BEE POLLEN LOADS

The pollen loads of 22 individuals of Api8 javana were counted, nine of them caught on short-styled trees and 13 on long-styled trees in Sian Tuan and Hua Guan avenues, Singapore, on the same day as the stigmatic pollen buds were sampled. The data are given in table 4 to show the degree of variation from bee to bee and the highly significant differences found according to the type of tree on which the bee was collected and on the part of the bee's body examined. The pollen loads



280 D. Lewis

flower long-styled (L) short-styled (S) 130) (00 106000)( (55.0) (45.0) I\

stigmas L S L S 652 548 58 6

(54.3) (45.7) (90.5) (9.5)

bee thorax head L S L S

(58.7) (41.3) (83.7) (14.3)

FIGURE 2. Pollen production per flower, pollen transport to the stigmas, and on the head and thorax of the transporting insect Apis javana. Heavy arrows represent compatible legitimate pollen flow, light arrows compatible pollen flow. Numbers in brackets are percentages, others are numbers of grains of pollen.

TABLE 5. NUMBERS OF POLLEN GRAINS OF SHORT-STYLED (S) AND LONG-STYLED

(L) PLANTS, EXPRESSED AS PERCENTAGES, ON THE BEES COLLECTED FROM

LONG- AND SHORT-STYLED TREES, COMPARED WITH THE STIGMATIC DEPOSIT

AND THE POLLEN PRODUCTION OF THE TWO TYPES OF TREES

head thorax corbiculae

S L S L S L bees captured on

short-styled tree 27.6 72.4 81.7 18.3 87.1 12.9 long-styled tree 2.9 94.1 0.9 99.1 5.9 94.1

mean 15.25 84.75 41.3 58.7 46.5 53.5

short-styled long-styled

S L S L S L

stigmatic deposits 9.5 90.5 45.7 54.3 - -

pollen production - - 45.0 55.0

were remarkably pure Cratoxylum formosum; six pollen grains of foreign species were found on the bees and only two on Cratoxylum stigmas.

In table 5 the pollen loads expressed as percentages of 'short' (S) and 'long' (L) are compared. The thorax and corbiculae have excess pollen from the tree on which the bee was captured, and this does not correspond to the stigmatic deposits of styles from the same type of tree. If the thorax loads of bees from both long- and short-styled trees are summed and the two types of pollen expressed as percentages




of the total there is remarkable agreement with the stigmatic loads of long-styles and also with pollen production of the two types of tree. The same is true for corbicular loads.

The head of the bee has a great excess of 'long' pollen even on bees collected from a short-styled tree. The percentages of the two types of pollen on the bee's head and on the 'short' stigmas agree closely. A balance sheet of pollen production and transport is given in figure 2, which shows general agreement but with one obscure point, that is the agreement of 'long' stigmatic with mean thorax loads but not with the thorax load of the bees captured on the long-styled tree. This needs clarification and is discussed later.

7. DISCUSSION

The morphological floral differences that accompany heteromorphic incompatibility are not essential to the self incompatibility and can be dissociated by genetic recombination (Lewis I954; Ernst 1955; Dowrick 1956). These morphological features are remarkably similar in the many heteromorphic species which range right across the phylogenetic classification of the flowering plants (Vuilleumier I967; Ganders 1979). Other morphological floral features, such as the symmetry of the flower or the shape of the corolla, which are adaptations to a particular type of insect behaviour, are not constant in heterostyled species although symmetrical tubular sympetalous flowers are the most common.

It has been argued, calculated and generally accepted that the self incompatibility of heterostyled species evolved first and the morphological differences later (Crowe I964; Charlesworth & Charlesworth 1979). It must be concluded from this and from the wide distribution of heterostyly in genera with flowers of different basic morphologies that the morphological features of heterostyly are polyphyletic in origin. This indicates that a common biological function and strong selection must have been present to produce the uniformity of the morphological features. The obvious biological function of the characters affecting the relative positions of stigmas and anthers is to encourage legitimate (compatible) pollination by the pollinating insect (Darwin I877).

Pioneer studies of stigmatic pollen loads on 13 species, made by Ornduff (I 970, I97I, I975, 1979) and Ganders (I 974) and reviewed by Ganders (I 979), have failed to demonstrate any marked differential transport of compatible pollen, and the validity of this function has been brought into question (Olesen I979). The published studies show that compatible pollen deposits on the stigmas of long-styles is random in three species and deficient in ten species, on the stigmas of short-styles the compatible pollen is random on three, deficient in six, and in excess in four species. Two common features of these pollen-load studies are: (i) the long-styled flower produces more pollen than the short-styled flower; (ii) the total number of pollen grains, of both types, deposited on stigmas is less on short- than on long-styles. These two features indicate that the legitimate pollination of short stigmas is a more acute problem than the pollination of long stigmas, and the fact that better than random pollination has been found only on the short-styled flower indicates



282 D. Lewis

that the pollination of the short-styled flower may have been a crucial factor in the evolution of heterostyly. The pollen flow in Cratoxylum shows that the pollination of the short-styled flower has little margin of safety.

C.formosum differs from other heterostyled species in having many anthers (144) instead of the usual low numbers, 2, 4, 6 and 12, in other heterostyled species. The only other heterostyled species with a similar large anther number is Hypericum aegypticum (Ornduff I975), which is in the same family. Both C. formosum and H. aegypticum have stigmatic pollen loads and pollen productions that are typical of the heterostyled species given above, except that C. formosum is extreme in two respects. The total number of pollen grains deposited naturally by the visiting bee is approximately 1200 per long-styled flower and only 64 per short-styled flower, giving a stigmatic pollen ratio L/S of 18.7, whereas other species show ratios varying from 1.2 to 4.6 (Ganders I979). Another difference is the high percentage of compatible pollen deposited on the short stigmas, 90.5 0 instead of the 55.1 0%

expected on the basis of a random distribution; this is quite different from the greatest excess of compatible pollen found in other species, 75.0 0 compatible pollen with 69.5 0 as the random expectation (for Jepsonia heteranda (Ornduff 197 I)). H. aegypticum, with its similar flower structure, does not show these extremes; its total stigmatic load ratios L/S is 1.5 and it has a below-random percentage of compatible pollen on the 'short' stigma, 46.8 0 with 63.0 0 expected on the basis of random transfer.

One of the significant differences is the unique bending of the anthers in the long-styled flower of Cratoxylum, forming the disc of anthers at the right height for the 'short' stigmas and in a position to cover the bee's head; this is confirmed by the similar percentages of the two types of pollen on the bee's head and the ' short' stigma. The random distribution of pollen on the 'long' stigmas corresponds to the pollen distribution on the bee's thorax.

The point in the evolution of the system where natural selection would operate most effectively is that where the number of compatible pollen grains on a stigma falls below the number of potential seeds. None of the 13 species studied are at risk in this respect and only C. formosum is critical in the short-styled flower; with 36 seeds and 58 compatible pollen grains per flower the pollen load/seed ratio is only 1.6. If the percentage of compatible pollen was not the exceptional 90 0 and was the more usual random at 55 0, the ratio would be less than one. C. formosum therefore has one part of its pollination mechanism in a critical situation and saved by an extreme form of anther stigmatic positioning by stamen movement.

The estimation made of the pollen transport lacks an important factor, i.e. the duration of a bee's activity on a single tree and the frequency of flights to other trees in one forage. The trees had tens of thousands of flowers in bloom at the same time and the fact that pure pollen loads were found on some bees indicate that the flights from tree to tree were infrequent. It is remarkable therefore that the stigmatic loads of the long-style corresponded to the mean thorax load of the bees and not to the thorax load of the bees caught on the long-styled tree.

The stigmatic and bee pollen loads would agree if it is assumed that about 50 O of the pollination of the 'long' stigmas was from bees that had recently come from a short-styled tree. Alternatively, the small and less common Apisflorae might play



Incompatibility, heterostyly and pollen transport 283 a major part in pollinating the 'long' stigmas, but the one specimen caught on a long-styled tree had pollen loads typical of Apis javana. There appeared to be no other significant pollinator on the trees examined, because during the period of study the only other insects seen were four unidentified butterflies, which made a short visit to the tree and did not work the tree systematically.

The biological function of the ligule at the base of the petal in Cratoxylum has received no satisfactory explanation (Robson I98I). A more systematic study of this organ in relation to anther movement in other species is needed before its general use as a fulcrum for the stamen bending shown in C. formosum can be postulated.

Sincere thanks are due to Professor A. N. Rao who made me aware of the problem, to Professor Hsuan Keng for help with identification of the species Cratoxylum, to Mr D. H. Murphy for identifying the pollinating bees and to the University of Singapore and the Inter-University Council for financial support.

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Crowe, L. K. I 964 The evolution of outbreeding in plants. 1. The angiosperms. Heredity, Lond. 19, 435-457.

Darwin, C. I 877 The differentforms offlowers on plants of the same species. London: John Murray. Dowrick, V. P. J. I956 Heterostyly and homostyly in Primula obconica. Heredity, Lond. 10,

219-236. Ernst, A. I955 Self fertility in monomorphic primulas. Genetica 27, 91-148. Ganders, F. R. I 974 Disassortative pollination in the distylous plant Jep8onia heterandra. Can.

J. Bot. 52, 2401-2406. Ganders, F. R. I979 The biology of heterostyly. N. Z. Ji Bot. 17, 607-635. Gogelein, A. J. F. I967 A revision of the genus Cratoxylum Bl. Guttifer8e Blumea 15, 453-474. Lewis, D. 1954 Comparative incompatibility in angiosperms and fungi. Adv. Genet. 6, 235-285. Olesen, J. M. 1979 Floral morphology and pollen flow in the heterostylous species Pulmonaria

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Documents

Incompatibility, Stamen Movement and Pollen Economy in a Heterostyled Tropical Forest Tree, Cratoxylum formosum (Guttiferae)