7th DIATOM-SYMPOSIUM 1982

An Ontogenetic Approach to Diatom Systematics

by

David G. Mann

Department of Botany, University of Edinburgh, The King's Buildings, Mayfield Road, Edinburgh EH9 3JH, U.K.

With 22 figures

Abstract: The value of frustule ontogeny as a source of taxonomic information and as an aid to the interpretation of structure is demonstrated by reference to a number of examples. Physiognomic aspect s of ontogeny are discussed in relation to the degree to which the sibling protoplasts press upon each other after cytokinesis : two extreme types of division are distinguished and termed interactive and non­interactive division . The origins and nature of the pattern of markings on the valve are analyzed . Varia-tion in these suggests a new theory concerning the evolution of the rap hid diatoms . .

Key Words: Cell division; diatom systematics; evolution; ontogeny; pattern-centres

Introduction

Anyone who has attempted to identify a diatom using one of the standard texts (Peragallo & Peragallo 1897-1908, Hustedt 1927-66, 1930, Cleve-Euler 1951-55, etc.) knows the dependence of diatom systematics on valve characters. This emphasis on the siliceous wall is present in Kiitzing's work (e.g. 1844) and most major studies since. In some ways it is difficult to see why other characters, such as plastid structure, have been ignored, since the classification evolved over the last 140 years is neither easy to use, because it takes an able microscopist to discern the details referred to in keys and descriptions; nor particularly suitable for use by ecologists (the principal users of the classification), since cells can only be identified confidently when dead and cleaned. The policy adopted by generations of diatom taxonomists can to some extent be justified, however, since the classification works, has remained fairly stable for over 50 years, and has predictive value. Thus, for instance, if a new species of Gomphonema Ehrenb. is distinguished using traditional criteria (basically a heteropolar, isovalvar, raphid diatom without fibulae) we can be fairly sure that it will also have a single large, lobed plastid, which lies with its centre against one side of the girdle; that it will have pore fields at the basal pole of each valve; that it will be an attached form, probably living in freshwater; and so on.

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There are a number of reasons, however, for trying to improve the existing system. At the lower taxonomic levels problems are rife . What, for instance, is a diatom 'species'?There are difficult groups, such as Nitzschia sect. Lanceolalae Grun. or the lineolate Navicula Bory species, where there appear to be few distinct boundaries between morphotypes. We don't know to what extent morphotypes interbreed, nor much about clonal variability (but see Geissler 1970a, b) so that at the species level we are building on very shaky foundations indeed. Some genera are clearly hetero­geneous, e.g. Navicula, while between the higher taxa the phenetic and phylogenetic relationships are at best obscure. Furthermore, as dependent studies (physiological, ecological, genetic, biochemical, morphogenetic) become more sophisticated, so too will the taxonomy have to be refined to support them. And not least, we should always be trying to make the classification more convenient for its users.

If we are attempting to produce a classification useful for a variety of purposes -phylogenetic speculation, making predictions about unknown character states, allowing extrapolation from one physiological investigation to another, etc. - then the characters we use must be drawn from every available source. There should be no need to weight any characters if these have been defined carefully and clearly (Davis & Heywood 1963, Sneath & Sokal 1973, Mann 1982c), and this disciplines the taxonomist to investigate and understand each character: is it valid, composite or redundant (because it only restates another character)? Ultimately many of the characters we use now will probably be found to be composite, as we uncover the processes that control their expression. Others will be found to be logically correlated and hence redundant. Here I give some examples of how studies of frustule ontogeny can aid and have aided our understanding of characters that have actual or potential use in diatom taxonomy, together with some speculation about diatom evolution. I make no attempt to cover the cytoplasmic and biochemical events occurring during morphogenesis: these are well reviewed by Schmid el al. (1981) and Volcani (1978).

Material and Methods

Samples were collected from a variety of sites and observed live or cleaned with a mixture of sulphuric and nitric acids. Cambridge S150 and S250 instruments, operated at 30 kV, were used for SEM observations.

Observations and Discussion

Frustule Ontogeny

Frustule ontogeny can be subdivided into girdle ontogeny and valve ontogeny. Neither is yet well known, the first even less than the second. Only since the intro­duction of EM (particularly SEM) techniques has it become possible to make detailed studies of the girdle, and we are still in the exploratory phase of investiga-

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tion. In some species there are two or more different types of band in the girdle (e.g. von Stosch 1975, Fryxell et al. 1981, Mann '1981a) but their functions are unknown. The development of the girdle during the cell cycle may well shed light on this. Different types of band may appear at different times, in relation to cell expansion, the onset of mitosis and the rearrangements of the protoplast that accompany this, the separation of the daughter cells after division, and so on. Bands are sometimes added continuously during the cell cycle, as in'Phaeodactylum tricornutum Bohlin (Borowitzka & Volcani 1978) and Striatella unipunctata (Lyngbye) Ag. (Roth & De Francisco 1977), but elsewhere particular configurations may be maintained for much of interphase, e.g. the 3:2 configuration of Navicula pelliculosa Hilse (Chiappino & Volcani 1977).

In using girdle characters the taxonomist must beware of redundancy. The shape of a girdle element is partly determined by the shape of adjacent elements, which mould it during formation (e.g. Mann 1982a). Areola spacings may not be inde­pendent in valve and girdle, even though they may differ. Analyses of Hantzschia marina (Donkin) Grun. populations (Mann, unpublished) indicate that cells with closely spaced valve striae have closely spaced girdle areolae, although the rela­tionship between these parameters is not simple. Only when we know more about how stria patterns are generated will we be able to assess the full taxonomic signifi­cance of areola spacing.

Valve ontogeny can itself be subdivided. First there is the development of the physiognomy - the development of the overall form of the valve with its humps, bumps and hollows. Second, there is the generation of pattern.

Valve Ontogeny: Physiognomy

As is well known, each valve or girdle element is formed within a membrane-bound sac - a silica deposition vesicle (see Schmid et al. 1981 for references). The valve SDV lies immediately beneath the plasmalemma of the cleavage furrow and so its shape is influenced by

1. the internal form of the parental valves and girdle, 2. the form of the sibling cell, and 3. the cell's internal cytoskeleton.

The relative importance of each of these three sets of factors has yet to be worked out fully for any diatom, although considerable progress has been made with Thalassiosira eccentrica (Ehrenb.) Cleve (Schmid 1984a, b). In part the balance depends upon the position of the diatom along a continuum between two extreme types of division. At one extreme the two sibling protoplasts produced by cell division are quite free from one another within the cylinder formed by the parent frustule and remain so throughout valve formation: at the other extreme the protoplasts are tightly appressed, filling virtually all the space within the parental wall. I term these extremes non-interactive and interactive division respectively (Fig. 1).

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1

r l r [

Fig. 1. Diagrams to illustrate non-interactive (a) and interactive (b) division .

b

1 J l 1

In non-interactive division (Fig. la) the mutual influence of the sibling cells is least. The internal shape of the parental thecae also often has little effect on the new valves, except, of course, to determine their outline, and so the dominant influence is the cytoskeleton. Where this is feebly developed the free surface of each daughter cell may be expected to take the shape with minimum surface area, if the cell is turgid. For round-valved diatoms this will be part of a sphere and hence arise the ±hemispherical or evenly curved valves of Melosira nummuloides (Dillwyn) Ag., M. dubia Klitz., M . juergensii Ag., Stephanopyxis Ehrenb., Hyalodiscus Ehrenb., Podosira Ehrenb., Corethron Castracane, etc. (see Hustedt 1927-66). Among elongate diatoms, examples of fairly simple, domed valves are found in Druridgea Donkin and some marine naviculoid diatoms, e.g. Diploneis bombus Ehrenb . and a number of other Diploneis Ehrenb. species, Scoliopleura tumida (Breb.) Rabenh. and Scoliotropis latestriata (Breb .) Cleve. Given an adequate cytoskeleton, how­ever, virtually any valve shape is possible and complex valve shapes are produced in the biddulphioid genera and Chaetoceros Ehrenb . (e.g. von Stosch et al. 1973).

Diatoms exhibiting interactive division (Fig. Ib) can also have complex shapes but are subject to the constraint that the shapes of the sibling cells must be complemen­tary. I dignify this simple rule with the title 'the principle of complementarity', which says merely that for every hummock there must be an equal and opposite hollow (cf. Newton's third law) .

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There are many 'interactive' diatoms, however, in which there are no hummocks (Fig. 1b). The shape is simple, with a mantle around the periphery of the cell and a planar valve face. The valve face corresponds to that part of the plasmalemma of the newly cleaved cell where the protoplasts press against each other, while the mantle forms beneath the plasmalemma where it abuts onto the parental frustule. The abruptness of the junction between valve face and mantle presumably depends on the position of the diatom along the continuum between extreme interaction and non-interaction.

The extent of the moulding of the daughter valve mantles by the parent frustule in interactive diatoms is demonstrated very elegantly by the stepped valves noted by Crawford (1981) in Melosira Ag., Arachnoidiscus Deane, Actinoptychus Ehrenb. and Eunotia Ehrenb., and which also occur in Actinocyclus Ehrenb. (unpublished observations). This moulding suggests that we should be able to predict the shape changes of many pennate diatoms given the shape of the initial cell and certain parameters of the girdle (see also Crawford 1981, Mann 1982b). Even now, over 50 years since the classic treatise on the life cycle of pennate diatoms (Oeitler 1932), which detailed the shape and pattern changes of many species and drew from these useful generalizations about the nature of the changes, many appear not to ap­preciate the full extent and importance of these phenomena. I am convinced, for instance, that Brachysira (formerly Anomoeoneis) styriaca has been greatly under­recorded because the principal diagnostic features - the rhombic-Ianceolate shape, coupled with the elongate central area - are lost during size reduction: the smaller valves are probably usually recorded under B. serians (Breb.) Round & Mann. Similar problems undoubtedly occur elsewhere and emphasize the need for careful studies of shape change and its control. Considerable advances in the numerical description of valve shape have recently been made by Stoermer & Ladewski (1982), using Legendre polynomials, and such methods hold great promise.

The simple valve shape, with planar valve face, will be produced in any interactive diatom where there is no marked cytoskeletal influence and where the two daughter cells remain isotonic during valve formation. But how do simply curved, com­plementary valves arise , such as those in Cocconeis Ehrenb. , Achnanthes Bory or Rhoicosphenia Orun., where one valve is slightly convex and the other slightly concave (Fig. 2a, b)? In Achnanthes coarctata (Breb.) Orun., for instance, the valve face of the rap hid valve is convex in trans apical section, while the pseudoraphe valve is correspondingly concave (superimposed upon this, of course, is the characteristic Achnanthes flexure of the frustule about the median transapical plane). Now when such a cell divides, if there was no anchoring of the plasmalemma to the parent frustule and no cytoskeleton, the appressed surfaces would be planar. It would make no difference if one cell became hypertonic with respect to the other, since inflow of water to one and outflow from the other would restore equilibrium, with the planar area of contact shifted towards the end of the frustule occupied by the initially hypotonic sibling. But if the plasmalemmas of both siblings were anchored to the parent frustule around the cell, near to the junction between the parental thecae - if, for instance, each protoplast was anchored to the more abvalvar girdle bands - then a slight increase in the osmotic potential of the daughter cell inheriting

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the parental pseudoraphe valve, relative to its sibling, could account for the curva­ture of its valve face. This is not the only possible explanation but it is testable, and it points to the need for detailed physiological and anatomical studies during

6

cleavage and the early stages of valve formation . It also indicates a possible reason for differentiation within the girdle, the abvalvar bands being modified to serve as anchorage sites. It is hard to see how diatoms with bent (Achnanthes, Rhoico­sphenia, Gephyria Arnott, etc .) or twisted (Entomoneis Ehrenb., some Surirella Turpin spp. , etc.) frustules could divide equally without this anchoring of the sibling protoplasts. The importance of cell-cell and cell-frustule adhesions is demonstrated well by Schmid's recent studies of Thalassiosira eccentrica (l984a).

Heterovalvar diatoms are not the only ones with a non-planar, 'interactive' junction between sibling protoplasts. For the new valves to be similar, however, the hummocks and hollows of the valve face must either be arranged alternately in radial sectors, as in Actinoptychus spp., or constitute a waveform that is rotational­ly symmetrical about the apical axis, as in some pennate diatoms. Thus, for instance, the eccentric keel of Denticula tenuis Klitz . is complemented by a trough on the other side of the apical axis (Figs 2c, d, 10); the valve faces of Nitzschia pandurijormis Oreg. (Fig. 2e) and N. tryblionella Hantzsch (Fig. 2f) are simple sine wave forms; the marginal ridge of N. sinuata (W. Smith) Orun. is accommodated by a slight step in the proximal mantle (Fig. 2h); etc.

In these interactive forms the systematist meets a problem. Is he justified in using, for instance, 'one valve convex' and 'other valve concave' as independent characters in Cocconeis placentula Ehrenb. (Fig. 2a); or 'raphe borne on a slight keel' and 'pseudoraphe depressed' in C. pediculus Ehrenb., C. klamathensii Sov.(Holmes et al. 1982) and various of the Cocconeis species with sigmoid raphe systems (Figs 2b, 1O)? Or again, does Nitzschia sinuata (Fig. 2h) differ from its variety delognei (Orun.) Lange-Bertalot (Fig. 2g) in two characters - presence of a marginal ridge and a stepped proximal mantle in N . sinuata sensu stricto and their absence in the variety - or in only one? It may be that the cytoskeleton of one cell keeps its plas­malemma concave, while the cytoskeleton of the other cell keeps its plasmalemma convex during valve formation: in this case the artificial or natural destruction of one daughter might have little effect on the morphology of its sibling. This may occur in Stephanodiscus astraea (Ehrenb.) Orun., since although the forms of the valves are usually complementary in this species, occasionally they are not (Round 1982, fig. 3U». Here there could be no objection to the use of both characters independently. I find it intuitively unlikely, however, that most interactive diatoms are like Stephanodiscus, although this intuition is probably teleological - Stepha­nodiscus seems unnecessarily uneconomical!

If it is not known exactly how the shapes are controlled we may invoke Ockham's Razor and assume that for each pair of complementary features there is only one determining factor. But is complementarity a significant problem? It certainly hasn't been in the past, partly because light microscopy reveals few of the topographical subtleties of the valve. In future, however, taxonomic revision will almost always involve electron microscopy and the details it yields concerning the morphology of

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2

b

c

d

Fig. 2. The 'complementarity principle'. demonstrated by transapical sections through sibling pairs of: a. Cocconeis placentula; b. C. pediculus; c & d. Denticula tenuis (compare c. with Fig. 11); e. Nitzschia pandurijormis (loculate valve); f. N. tryblionella; g. N. sinuata var. delognei; h. N. sinuata.

valve and girdle. Characters will also be drawn from protoplast structure and behaviour (Cox 1981a, b, Mann 1983) and elsewhere, but even so the number of characters usable routinely at the lower taxonomic levels is likely to remain fairly low. Hence in taxonomic analyses carried out either in the traditional 'subjective' manner or by numerical or cladistic methods, it will be important to know whether two characters are independent or logically correlated.

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The principle of complementarity is also relevant to phyletics. Complementarity operates to constrain the evolution of valve shape. In taxa such as Denticula Klitz., Plagiotroj1is Pfitzer or Nitzschia Hass., where there is a straight keel (Fig. 11), the keel can be just as high or higher centrally as anywhere else along its length. The 'penalty', of course, is that there must be an equally deep trough and that the valve profile will be grossly asymmetrical (except in the rotational sense) about the apical plane. Entomoneis and Donkinia Ralfs have evolved a different method of accommodating the keels of sibling valves while maintaining ±interactive division. Here the raphe and the keel that bears it are sigmoid. In the dividing cell there is a 'swop-over' centrally, so that the keel of each daughter lies on one side of the cell towards one pole and on the other side towards the other pole. The valve is thus almost symmetrical bilaterally, though twisted. Because of the central swop-over, however, the keel cannot be as high centrally as it is elsewhere and so, whereas Plagiotropis cells mayor may not be constricted centrally in girdle view, Entomoneis cells invariably are, although Sc;hmid (1979) has reported that in certain (unspecified) circumstances the frustule, and hence presumably the raphe, of E. paludosa can be straight; this would obviate the need for a central constriction. At present it is impossible to say what selective advantage an eccentric, straight keel or a ±central, sigmoid keel may have . I am concerned to show only that, given selection for one of these, certain other features are also necessarily produced or constrained in their form.

Another interesting facet of interactive v. non-interactive division is the general restriction of the latter to marine or brackish waters. Among freshwater diatoms, non-interactive division is rare, occurring in some Surirella species (e.g. see Lauter­born 1896) but apparently nowhere else. It seems that in freshwater the energy cost of maintaining a wall-less plasmalemma in the appropriate configuration through­out valve formation (a process that can take over an hour even in small diatoms such as Navicula bacillum Ehrenb. and N. capitata Ehrenb.) is sufficiently high to offset any selective advantage there might be in the 'exotic' shapes possible with non­interactive division. Marine and brackish diatoms, on the other hand, may have very low turgor during division: some marine algae are known to have low turgor, although generally both freshwater and marine diatoms seem to maintain an osmotic potential 5-8 atmospheres less than that of the surrounding water (Guillard 1962). It may be that the great imbalance between the numbers of marine and freshwater genera noted by Round & Sims (1981) is partly a consequence of the physiological and osmotic contrasts between these ecological groups: generic clas­sification is largely dependent upon valve shape and symmetry, and so anything that restricts the range of shape possible will probably restrict the number of genera we recognize. Perhaps if the classification took more account of other characters, such as plastid form (e.g. Cox 1981a), auxospore formation (e.g. Geitler 1973), etc., the imbalance might be less marked.

Finally, one might predict from the ecological restriction of non-interactive division to saline waters that some euryhaline species may exhibit non-interactive division in part of their range and interactive division elsewhere, with concomitant change in valve shape from a rounded to a squared-off morphology. This does indeed seem to

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be the case in Skeletonema potamos (Weber) Hasle, according to the observations of Hasle & Evensen (1976).

Valve Ontogeny: the Pattern

Pattern-centres and the overall plan

Recently much attention has been given to the development of the valve pattern, particularly by Pickett-Heaps, Schmid, Volcani and their coworkers, and much of this work is summarized in the valuable paper by Schmid et al. (1981) . To a large extent this work has been concerned with the physiology and control of morpho­genesis at the molecular and organelle levels but it has incidentally revealed much that can be used by the taxonomist. The taxonomist can perhaps in some measure recompense the morphogeneticist by pointing to interesting anomalies and cor­relations in the variation pattern, which might repay investigation.

In the organisms studied so far the valve forms from the centre outwards to the margins (Schmid et al. 1981, Schmid & Volcani 1983): reported exceptions to this, in Melosira varians Ag. (Reimann 1960) and germanium-treated Cyclotella nana Hust. (Chiappino et al. 1977) are so far unconvincing. We can thus talk about a pattern-centre not only in the sense of an area or rib at the centre of the pattern, but also in the sense of the initially deposited part of the valve. The earliest stages of development are known with some certainty in very few diatoms, e.g. Navicula pelliculosa (Chiappino & Volcani 1977), Pinnularia major (Klitz.) Rabenh. and P. viridis (Nitzsch) Ehrenb. (Pickett-Heaps et al. 1979). The shape of the silicon deposition vesicle at this time and the time-scale of pattern development are critical in relation to some theories concerning pattern generation (e.g. Lacalli 1981) and require further observation. It appears, however, that, ontogenetically and anatomically, two main types of pattern centre can be distinguished:

a. one in which the principal ribs radiate out from a ring (Fig. 3a), or annulus as it has been termed by von Stosch {I 977), who first recognized the structure in Bellerochea Van Heurck and Streptotheca Shrubsole. This ring is usually central, but not always: in Dactyliosolen Castracane, for instance, it is submarginal (see Hasle 1975).

b. a second in which the pattern centre is a longitudinal rib (Fig . 3b) or sternum, from which subsidiary ribs extend out, usually approximately at right angles (at least near the sternum).

This is, of course, only a very slight refinement of the traditional dichotomy between centrics and pennates, defined by Schlitt (1896) . The only important difference is that Hustedt (1927-66), for instance, said that in centric diatoms "die Struktur der Schalen ist ... auf einen Punkt gerichtet oder unregelmassig" (and Simonsen makes a similar statement on p. 10 of his 1979 paper), whereas I am stressing that the pattern-centre is not a point but a disc of finite size. The annulus plays an important part in the phylogenetic theories of Round & Crawford (1981), who document its distribution among centric genera and note that the annulus is present in auxospore scales as well as valves.

Whereas organization about a point might imply that centric diatoms initially

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3 a

e

/

b

----li',,'.'!-=== ====111'1. j\',iI-,---

.J ~~,; ... ---

Fig. 3. Pattern-centres in diatoms. In each case the pattern-centre is shown stippled . a. centric, unifacial annulus; b. simple sternum, as in the araphid pennates; c. pseudocentric type; d. bifacial annulus (or double sternum?); e. naviculoid raphe-sternum; f. eunotioid plan, with raphe not integrated into the primary pattern-centre; g. filled-in raphe-sternum (e.g. Cocconeis); h. naviculoid raphe-sternum with reduced raphe system; i. advanced type, with only one raphe slit.

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control rib spacing via 'angular' measurement, organization about a 2-dimensional ring or disc brings the centrics in line with the pennates, since in both the subsidiary ribs are linearly spaced around the edges of the pattern-centre (Fig. 3). It is interesting to note that the centric pattern, with its annulus and radiating ribs, corresponds quite closely to the kind of pattern that can be generated using 'Tyson's Brusselator' (see Lacalli 1981, and especially pp. 559-65), a morphogenetic model of the reaction-diffusion type. However, Tyson's Brusselator cannot model the com­plexities of the central area in diatoms such as Coscinodiscus concinnus W. Smith, in which Brooks' micrographs (1975, plate 8H) show clearly that the annulus directly subtends circa 100 radial ribs: Tyson's Brusselator cannot generate such large numbers of morphogen peaks in a simple whorl.

This basic dichotomy between centric and pennate pattern-centres should not be construed as proof that the diatoms fall naturally into two major groups: other evidence must be used in the delimitation of the higher taxa. Furthermore, within the pennates in particular there are a number of variants on the basic plan, some of which are well-known, others less so. One of the latter is the pattern-centre of Climacosphenia Ehrenb. and Ardissonia de Notaris (see Round 1979), where there seem to be four, not two sets of transapical ribs, one each side of two longitudinal sterna (Fig. 3d). The two centripetally-directed sets of ribs meet in the midline of the valve and create a 'fault line', which mimics the simpler pattern-centre of most other araphid pennate diatoms (Figs 3d, 12, 13). It appears that the two sterna may be continuous around the poles of the diatom, judging by illustrations of 'Synedra' crystallina (Ag.) Klitz. and related taxa (Peragallo & Peragallo 1897-1908, Hustedt 1927-66) and observations of Climacosphenia Ehrenb. (Fig. 12). If so, the pattern­centre of these forms would be an elongate ring, which might be termed a bifacial annulus (Fig. 3d), since the ribs extend out from both its sides, to distinguish it from the unifacial annulus (Fig. 3a) of the centrics . This group of diatoms is fascinating and deserves further investigation.

Another possible variant is to be found in Psammodiscus Round & Mann, recently separated from Coscinodiscus Ehrenb. (Round & Mann 1980). There are many similarities between Psammodiscus, Rhaphoneis Ehrenb. and Delphineis Andrews -e.g. in the type of pore occlusion, the shape of the rimoportule, and the habitat (Round & Mann 1980, Andrews 1981) - and nearly circular Rhaphoneis and Delphineis species are known, e.g. Rh. superba (Janisch) Grun. The principal difference between Psammodiscus and the other two is that they have an obvious axial sternum, whereas in Psammodiscus the pattern radiates from the centre. No trace of an annulus can be seen, at least in the completed valve, and so I suggest that Psammodiscus is in fact a pennate diatom, in which the sternum has become reduced to a small central boss (Fig. 3c). As the sternum diminished in length, the rimoportules, which in the ancestral form were probably placed at the ends of the valve, adjacent to the sternum (as in Delphineis: Andrews 1981, Round & Mann 1980), moved inwards, so that finally they were almost, but not quite central. Loss of one or both rimoportules must then have occurred, since Psammodiscus valves have one rimoportule or none (Round & Mann 1980).

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These two variants involve few species . Another modification of the basic pennate plan is much more widespread and better known. This is the raphe-sternum. This takes se"eral forms and so I will take the naviculoid plan as the standard and describe it first (Fig. 3e). Here the pattern-centre consists of an axial rib or sternum, which is laid down first and forms one side of the raphe system; a central 'nodule', which separates the two raphe slits; and a secondary, composite axial rib, which forms the other side of the raphe system and is produced by silicification from the centre and both poles simultaneously (Geitler 1932, Chiappino & Volcani 1977, Pickett-Heaps et al. 1979, Schmid 1979). The secondary side of the pattern-centre may be distinguished even after valve formation is complete, by the positions of the Voigt discontinuities (Fig. 4a), which are produced where opposing trains of silica deposition meet and complete the secondary rib (Mann 1981b). In passing it may be noted that the existence of these discontinuities implies that the plasmalemma is not directly involved in the generation of the stria pattern.

There is no indication that the primary rib is preceded by the central nodule (see Chiappino & Volcani 1977, Pickett-Heaps et al. 1979) and thus it is unlikely that the central nodule is homologous with the centric annulus, as apparently suggested by Round & Crawford (1981, p. 254). In Eunotia Ehrenb. the raphe slits are not enclosed within the primary pattern-centre but lie oblique to it and at some distance from it, except at the poles (Figs 3f, 14). This arrangement suggests that the navi­culoid plan may have arisen by 'fusion' of two secondary, lateral, raphe-associated pattern-centres with the primary sternum (see below). The pattern-centre of most raphid diatoms must therefore be regarded as composite, and to indicate this I propose the term raphe-sternum, to differentiate it from the simple pattern-centre of the araphid pennates.

In some diatoms the raphe slits are very short, yet otherwise the valve plan is naviculoid, with the raphe well integrated into the pattern-centre (Fig. 3h) . In Berkeleya rutilans (Trentepohl) Grun., however, the essentially lateral position of the raphe is well displayed and the elongate 'central nodule' (Hendey 1964) or area (Cox 1975) seems to represent an unmodified part of the araphid sternum (this contrasts with Hustedt's (1935) interpretation), although I do not mean to imply by this that the B. rutilans raphe system is primitive. Short raphe systems are also found in the convex valve of Rhoicosphenia and in Peronia; here and in Berkeleya the raphe is secondarily reduced.

Loss of the raphe has occurred in one valve of each frustule of Cocconeis and Achnanthes, but the mechanism of this seems to be quite different from that operat­ing during the evolution of Peronia Bn!b., Rhoicosphenia and Berkeleya Grev. Cocconeis and Achnanthes have no raphe in the fully formed pseudoraphe valve (Fig. 3g) because, at least in some species, it is filled in during development (Mann 1982a). Ontogeny here provides the vital clue to the interpretation of the ghost raphes noted by a number of authors (e.g. Simonsen 1979, Holmes et al. 1982), and also shows that no such filling in occurs during valve formation in Rhoicosphenia or Berkeleya (Mann 1982a and unpublished).

The last variation on the raphid plan is shown by those diatoms in which there are

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4 v

v

a

v

b c d

Fig. 4. The interpretation of the advanced type of pattern-centre (see Fig. 3): a. the standard naviculoid raphe-sternum, with central nodule and two Voigt discontinuities (v); b, c & d., interpretations 1, 2 and 3 (see text).

no central raphe endings and the raphe runs unbroken from pole to pole (Figs 3i, 11, 16, 17). This occurs only where the raphe is subtended by fibulae, probably because the valve would be extremely weak if it lacked both these and a central nodule (Mann 1982c, and see Pickett-Heaps & Kowalski 1981). Three explanations could be advanced to reconcile this variant with the standard naviculoid sequence described by Chiappino & Volcani (1977):

1. Sequence similar to the standard, but the central part of the valve remaining unsilicified, creating a connection between the two slits (Fig. 4b).

2. Two closely-spaced axial sterna laid down initially (cf. Climacosphenia and Ardissonia), delimiting a single longitudinal raphe slit (Fig . 4c) .

3. Primary rib formed as usual but the secondary side produced by centripetal growth from both poles, with no 'contribution' from the centre (Fig. 4d).

Unfortunately my series of developing valves in Denticula tenuis and Nitzschia sinuata, both taxa without central raphe endings, do not extend back early enough to enable me to judge between these three directly. The thin-section studies of Pickett-Heaps (1983) rule out hypothesis 2, since in the earliest stages of valve

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formation only one rib is present, but they do not give information on 1 and 3. However, these hypotheses make different predictions about the positions of the Voigt disoontinuities. Hypothesis 1 predicts two Voigt discontinuities in the normal places (i.e. approximately halfway between centre and pole in each half of the valve: Fig. 4b), while hypothesis 3 predicts only one, central discontinuity (Fig. 4d). Careful examination of Denticula tenuis and N. sinuata, in which the stria pattern near the raphe is coarse enough to reveal discontinuities, demonstrates a single, central discontinuity, supporting hypothesis 3 (Figs 11, 16, 17).

Cryptic asymmetry in rap hid diatoms

Every raphid diatom, then, has valves that are bilaterally asymmetrical in their ontogeny (Fig. 4a, d). Not only this, they are also usually asymmetrical with respect to other features, themselves correlated with the asymmetry of deposition, such as the curvature of the terminal raphe fissures and· the shape of the raphe in cross section (see Mann 1983 for a summary of the relationships between them). From this it follows that two types of frustule, cis and trans, are theoretically possible, which differ in the orientations of the valves with respect to each other (Fig. 5a, b). In cis cells the valves have the same orientation; in trans cells, the opposite orienta­tion (Mann I982a).

Some species form only cis frustules, while others produce cis and trans in various proportions. In the former all the valves in a dividing cell must have the same orientation (Fig. 5e). However, even in species producing trans frustules the sibling valves always have the same orientation as each other (see Mann 1983): the sibling pair is mirror symmetrical about the median valvar plane, as noted also by Chiappino & Volcani (1977) for Navicula pelliculosa, which can produce trans cells (see Coombs et al. 1968, fig. 13a) and by Pickett-Heaps et al. (1979) for Pinnularia Ehrenb., which produces cis and trans in a 1:2 ratio. In taxa producing both types of cell, the ratio of cis to trans is usually either 1:2 or 1: 1, depending on the types of division occurring (Fig. 5d, e). Since in many rap hid diatoms the primary halves of the new valves are produced on the side of the cell where the nucleus divided, the production of cis and trans cells and their proportions may depend upon the movement of the nucleus during the cell cycle. In Navicula bacillum, for instance, the nucleus divides first against one side of the girdle and then against the other, the protoplast rotating through 180 0 in each cell cycle. As a result cis and trans cells are produced in a 1:2 ratio (Mann,in preparation). Other naviculoid taxa with this ratio may behave similarly. The groupings of taxa indicated by cis and trans analysis correlate encouragingly well with the groupings indicated by other evidence, e.g. pore occlusions (Mann I98Id), raphe structure and girdle structure, but in several cases disagree with the traditional classification, suggesting a need for revision.

Valve dimorphism in Nitzschia and Denticula

In most species of Nitzschia and Denticula the raphe system is eccentrically placed and often it is borne on a keel (Fig . 11), producing the complementarity effects

126

. ~ ~

J J ,

trans CIS

trans trans

l d

1 J ,

cis trans

cis cis

cis trans

'-1 1...../

, trans cis 1

e .. trans trans

l l cis

'-1 Fig. 5. Cis (a) and trans (b) symmetry and cell division (c) in raphid diatoms. In a & b. the arrowheads show the conventions (pointing towards the primary side of the valve) used to indicate valve polarity in d & e. Note that sibling valves always have the same orientation (c, d & e) . According to the divison types occurring, rap hid diatoms producing both cis and trans cells usually produce them in either a I : I (d) or 1:2 (e) ratio.

127

noted earlier (Fig. 2c-h). There it was implied that in such taxa the sibling valves are identical, but this is not strictly true. As a result of the ontogenetic asymmetry of the raphe-st~rnum, these genera exhibit a cryptic dimorphism, which is most obvious in relation to the curvature of the terminal fissures. In one type of valve both terminal fissures point towards the proximal (sensu Mann 1977) margin; in the other, towards the distal margin (Figs 16, 17 and Mann 1980, 1981c). I did not at first realize the significance of this phenomenon. However, given that in these genera the raphe systems of sibling cells lie on opposite sides of the parent cell, while the primary halves of the raphe-sterna are produced on the same side of the parent cell, it is obvious that one sibling must have a narrow primary side and terminal fissures that point towards the distal margin, while the other has a wide primary side and terminal fissures that point towards the proximal margin (Fig. 6). Since I spoke about this at the Symposium I have had the opportunity to read a paper by Pickett­Heaps (1983), in which he comes to a similar conclusion based on observations of the development of the fibulae, which grow out from the narrow side in one sibling (and see Fig. 19) and the broad side in the other. So far , however, it is not clear whether the fibulae grow out from the primary side or the secondary.

The cryptic dimorphism of Denticu/a and Nitzschia is thus an automatic consequence of the general property of raphid diatoms that the primary part of the raphe-sternum forms on the same side in both siblings (Figs 5c-e, 6), coupled with the specific property of these genera that the raphe systems come to be on opposite sides of the cell (Figs 2d-h, 6) . Hence the dimorphism and the diagonal symmetry of the sibling pair with respect to the positions of the keels are not independent: there is only one character here separating Hantzschia from Nitzschia or Denticu/a. It may be predicted that other diatoms with eccentric, straight keels, e.g. P/agiotropis spp. will also be found to exhibit cryptic dimorphism.

The Classification of Cymbella

Within Cymbe//a Ag. two groups of species may be distinguished, according to whether the terminal fissures are ventrally or dorsally directed (Fig. 7). Those with ventrally directed fissures often form mucilage tubes and were for this reason originally classified in a separate genus, Encyonema Klitz. Recently a group of this name and including some of the species with ventrally directed fissures has been recognized by Krammer (1982) as a subgenus of Cymbe//a. The curvature of the terminal fissures and correlated features (e.g. the Voigt discontinuities) indicate that the dorsal side of the valve in species with ventrally directed fissures (e.g. C. minuta Hilse) is homologous with the ventral side in species with dorsally directed fis­sures (e.g. C. /anceo/ata Ag.) and represents the primary side of the valve (Mann 1981b): the orientation of the valve is correlated throughout with the positions of plastid and nucleus (Oeitler 1981). This raises interesting questions concern­ing the evolution of the genus. Most workers would probably consider that Cymbe//a arose from a bilaterally symmetrical, naviculoid ancestor, although

128

6

distal curvature

proximal cur-.clure

Fig. 6 The production of valve dimorphism in Nitzschia and Denticula. Sibling valves have the same orientstion with respect to their raphe-sterna (top): however, because of the displacement of their raphes towards opposite sites of the cell (bottom left), at each division one valve is formed with distally directed terminal fissures, and one with proximally directed fissures.

it is difficult to find a clear statement of this; Cleve (1894-5) certainly implies such a derivation. If so, the near naviculoid shape exhibited by some extant Cymbel/ae, e.g. C. angustata (W. Smith) Cleve, must have given way in some lines to the more strongly asymmetrical cymbelloid shapes of other extant species, e.g. C. minuta and C. lanceolala. Did the two cymbelloid groups, which we can call the ventral (e.g. C. minula) and dorsal (e.g. C. lanceolata) groups re­spectively, develop independently from naviculoid ancestors, as in pathway a. in Fig. 7? Or did the cymbelloid shape arise only once, the ventral and dorsal curvatures arising by polarity reversals, in which the protoplast, and hence the orientation of the valve secreting machinery 'swopped sides'? This might not be a difficult transition, since considerable rearrangement of the protoplast routinely occurs during auxospore formation and it might be that small genetic changes could produce the dramatic reversal postulated .

129

1 •

- c • ..... b·

Fig. 7. Evolutionary possibilities in Cymbel/a (see text).

Perhaps, however, we should not rule out the possibility that Cymbella may not have evolved from a naviculoid ancestor. Perhaps the first Cymbellae were dorsal types, the naviculoid and ventral groups arising gradually, as in pathway b. (Fig. 7), or by a mixture of 'gradualism' and 'punctuation', the latter involving polarity reversal. Or again, perhaps pathway c. operated.

These possibilities lead to different predictions concerning the variation pattern, predictions that can be tested by further observations. For instance, if polarity reversal is frequent one would expect to find a reticulate pattern of variation, the ventral-dorsal dichotomy cutting across groupings based upon other characters. The gradual change of pathways a., b. and c., on the other hand, should produce more consistent trends. Although there is still insufficient evidence to be sure, Krammer's (1982) data suggests that polarity reversal is at best rare.

The Interpretation of Striae and Costae

Ontogenetic studies (Schmid et al. 1981 list the relevant works) suggest that the diatom valve is essentially a rib construction and that there is no 'basal siliceous layer', variously ornamented, perforated and moulded, as described by recent terminologies (Anonymous 1975, Ross et al. 1979, Cox & Ross 1981). The first elements deposited are ribs and plain areas appear to arise by controlled filling-in between them. Thus, for instance, the extensive 'hyaline' axial area of Caloneis amphisbaena (Bory) Cleve is lacking during the early stages of valve formation: the transapical ribs extend right up to the raphe system (Mann 1983, fig. 3c).

130

8 b u' , ' u v v uv

o 0 0 ' 00 00 o G l1J ' 0 0 00 o r 0, 0000 '0 ,0 00000 o iri 1Q:J 0 0 0 0 o Ir '00000 (\ r ki l(\{\(\(\

u v u v u

a

c

Fig. 8. Denticula. a the structure of the valve, with raphe-sternum (r) subtending transapical ribs and rows of pores. Some of the ribs bear septa (s), which are continuous beneath the raphe (cf. Fig. 15). b. Interpretation I (realized in D. tenuis): each transapical rib is a primary outgrowth from the raphe­sternum, some ribs becoming secondarily thickened. c. Interpretation 2 (realized in D. rainierensis): only the septa are primary, the striae being truly multiseriate. (In b & c the primary elements are shown in black).

The striae of diatoms sometimes consist of single rows of areolae, sometimes of several rows. Cox & Ross (1981) have aptly named these uniseriate and multi seriate striae. One explanation for biseriate or multi seriate striae is that the structure is basically uniseriate, but that some of the primary ribs become more strongly sili­cified. Alternatively the striae may be truly multiseriate . No instances of the first have yet been proved, although it is interesting that in the convex valve of Rhoico­sphenia curvata Orun., the sternum sometimes bears a small projection between each pair of transapical ribs in the early stages of valve formation, suggesting perhaps that alternate ribs have been suppressed (Mann 1982a); in the mucilage secreting area the striae do have approximately double the spacing found elsewhere on the valve, and biseriate striae are not infrequent (Lange-Bertalot 1980, Mann 1982a). By contrast, in several cases it seems clear that the striae are truly multi­seriate. In Hantzschia marina, the principal transverse elements visible in the mature valve - the prominent 'hoops' (Mann 1977) - are the first and only structures to grow out from the raphe-sternum and the last part of the stria to be silicified is its midline (Mann 1978).

131

Schmid & Volcani (1983) suggest that in loculate diatoms valve ontogeny follows a directional, rather than a structural sequence: in Thalassiosira Cleve and Cosci­nodiscus, the valve interior forms first, then the exterior, in spite of the opposite

• positioning of cribra and foramina in the two genera. In view of this it is not obvious what ontogeny tells us about homology between striae of different types . On the other hand it may be noted that in both Thalassiosira and Coscinodiscus the first formed layer has the same basic construction, namely an annulus with radiating ribs (compare Hasle & Fryxell 1977, fig . lla with Brooks 1975, plate 7H) so that ontogeny does in some ways follow a structural sequence.

Elaboration of the valve does not always occur from inside out. Fibulae, for example, are late, internal developments (Pickett-Heaps & Kowalski 1981, Pickett­Heaps 1983; and see Figs 15, 19). In all Denticula species the fibulae are extended into massive bars or septa, which traverse the valve from margin to margin, but this is an instance where similar end-products arise by different ontogenetic pathways. D. rainierensis Sov. (Figs 20,21) and D. elegans Klitz. or tenuis (Fig. 15) valves look quite similar: the prominent septa are separated by several rows of small poroids (Fig . 8a), although the rows are somewhat irregular in D. rainierensis. Ontogeneti­cally the first transapical structures to appear in D. tenuis or elegans are the ribs between the lines of pores (Figs 18, 19); the fibulae arise later and thicken to produce the septa (Fig. 8b). In D. rainierensis, however, the prospective septa are the first structures laid down after the raphe-sternum, and the lines of areolae are delimited successively, as silicification extends out laterally from the septa (Fig. 22). Thus, in D. tenuis and elegans we may say that the striae are uniseriate and that only a few of the primary transapical ribs bear fibulae (septa) (Fig. 8b), whereas in D. rainierensis the striae are multiseriate and every primary transapical rib bears a fibula (Fig. 8c). The details of this will be considered elsewhere.

The Evolution of the Raphid Diatoms

The complex, asymmetrical ontogeny of the naviculoid raphe invites speculation about its phylogeny. Since the primary axial rib seems to appear before or at the same time as the central nodule (see above for references), and since some rap hid diatoms (e.g. those fibulate species without central raphe endings) appear to lack all trace of a central nodule, I do not believe that the rap hid diatoms can be derived directly from a centric ancestor, by modification of the annulus and the ribs it bears. The ancestor must have had a pennate organization. Hasle (1973) has suggested that raphe slits represent modified rimoportules, and this seems quite reasonable. Both are unoccluded holes in the valve, whereas areolae usually have some kind of velum; furthermore, in araphid pennates rimoportules are few in number and occupy well-defined positions in the valve - again, properties one might expect of a raphe-precursor. Li & Volcani's unpublished work, reported at this Symposium, may clarify the relationship between rimoportule and raphe consider­ably; for the moment the rimoportule is the best candidate among extant structures for having been the raphe-precursor.

132

9

a c

v

v

Fig. 9. The evolution of the raphe. a. The araphid ancestor, with rimoportules at each pole. b-d. Dif­ferentiation of the raphe from one rimoportule at each pole. Note the finer striae associated with the raphe because of its essentially transapical orientation; the rotation of the raphe towards the centre; and the parallel movement of the undifferentiated rimoportule to an apical position. e & f. Abutting of the raphe onto the sternum and final integration into the primary pattern-centre. (Throughout, the patter­centre is shown stippled, and the raphe slits are outlined heavily in black.)

Thus I support Simonsen's view (1979) that the raphid diatoms must be derived from an araphid pennate ancestor. Simonsen stated that Hasle (1973) had disproved his earlier contention that the raphe might have evolved by fusion of rows of rimoportules, such as are found in Pseudohimantidium Hust. & Krasske (Simonsen 1970). I do not believe this possibility can yet be ruled out, although it is probably simpler to assume that each raphe slit arose from a single rimoportule, rather than by some ontogenetic and phylogenetic fusion of several rimoportules to form a single slit. In any case, the araphid ancestor is likely to have had several rimopor­tules at each pole, since in the Eunotiaceae (sensu Simonsen 1979) there are often one or two rimoportules per valve as well as two raphe slits (Hasie 1973, Kobayasi et al. 1981).

In araphid diatoms the rimoportules are lateral to the sternum. Their long axes are usually at right angles to the sternum and parallel to the striae. Thus, for instance, the single rimoportule of Tabellaria Ehrenb . lies some distance from the sternum in

133

Hustedt's scheme suffers from its inability to explain the ontogenetic asymmetry of the raphe-sternum, and from the improbability of an essentially lateral structure, the rimoportule, elongating into the centre of the axial sternum, splitting it in two. Furthermore, the "rlicklaufende Spalten" do not penetrate the valve (see below) .. and thus cannot be considered equivalent to the raphe of Peronia or the naviculoids.

Geissler & Gerloff (1963) derive the eunotioid plan from an araphid diatom resembling Falcula Voigt and, as in the present scheme, central raphe systems are considered to be derived from eccentric ones. Geissler & Gerloff do not give details of how they believe the naviculoid raphe system developed, but seem to imply that backward pointing fissures similar to those in several extant Eunotia species (e.g. E. Jlexuosa (Breb.) Klitz. and E. pseudopectinalis Hust.: see Hustedt 1927-66, figs 778, 779) lengthened and that it was these, rather than the ventral parts of the raphe, that gave rise to the central raphe of Peronia and the naviculoids. They state

"Den Anfang des zweiten Entwicklungszweiges [the shifting of the raphe system to a central position in the solitary eunotioid ancestors of Peronia and presumably the naviculoidsJ wtirden vielleicht Arten gebildet haben, bei denen der rticklaufende Rhaphespalt fast median in der Valvaflache liegt , wie bei Eunotia flexuosa oder E. pseudopectinalis ... Sie k6nnten tiber Typen wie E. synedraeformis und E. con versa zu Peronia weiterieiten; denn bei E. synedraeformis liegen nach Hustedt (1952) die Rhaphenaste "v6l1ig in der Valvaflache und besitzen von den nur kleinen Endknoten ausgehende, fast in der Mittel­linie liegende und ziemlich lange rticklaufige Spalten." Bei E. con versa ist diese mediane Verschiebung der Rhaphe und der Pseudorhaphe noch deutlicher ausgepragt."

I presume that the Peronia raphe system is then supposed to have developed by incorporation of the backward pointing fissures into the sternum (pseudoraphe). This I find unlikely because the fissures are blind grooves (Hasle 1973, Kobayasi et al. 1981) and are surely equivalent to the terminal fissures of the naviculoids. The Geissler-Gerloff scheme as I understand it predicts that the primary side of the raphe-sternum is equivalent to the ventral side of the pattern-centre in Eunotia, while mine predicts it to be equivalent to the dorsal side. It may be possible to refute one of these by examining the relationship between protoplast and valve orientation in Eunotia, Peronia and the naviculoid diatoms, and this matter is under investiga­tion.

Simonsen has recently (1979) provided a comprehensive account of diatom classi­fication and phylogeny, and apparently supports a diphyletic origin of the raphid diatoms (his fig . 3). In this case we might expect to find marked differences between the raphidioid diatoms and other rap hid forms in valve ontogeny, raphe structure and raphe function. I do not believe that present evidence supports such a view (the raphes in both groups, for instance, are similar in having helictoglossae and ter­minal fissures) but more information is clearly necessary: that raphe-like structures may have developed independently in a number of lines is suggested by the curious form with a single 'raphe slit' described by Norris (1973) as Nanoneis hasleae. Simonsen cites the apparently younger age of the Eunotiaceae relative to the Naviculaceae as evidence against the transitional position of Eunotia and its allies, but the fossil evidence is difficult to evaluate. It may well be that earlier deposits have been lost through diagenesis, as suggested by Round & Crawford (1981).

136

Postlude

Data is usable for taxonomy only if it exhibits variation between the taxa in question; it will be used only if it is economic to gather. The examples I have given demonstrate that ontogeny can reveal hitherto unknown differences between diatoms and that it can aid interpretation of many features of the imago. The evidence available suggests that ontogeny, like protoplast structure and auxospore formation, has considerable taxonomic potential, especially in relation to the delimitation of genera and higher taxa. A full evaluation cannot yet be made but further investigation is certainly worthwhile.

Acknowledgements

I am grateful to the S.E.R.C. (OR B/09636) and the Royal Society for financial and material assistance; Dr. A.-M.M. Schmid for valuable discussion; Professor M.M. Yeoman for his continued support; my wife for help with the preparation of the text; and Professor F.E. Round for encouragement while I held a S.R.C. student­ship at Bristol University (1974-7): many of the ideas contained in this paper had their beginnings then.

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MANN, D.O. (l98Ic) - A new species of sigmoid Nitzschia (Bacillariophyta) . Israel J. Bot. 30, 1-10.

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MANN, D.O. (l982a) - Structure, life history and systematics of Rhoicosphenia (Bacillariophyta) I. The vegetative cell of Rh. curvata. J. Phycol. 18, 162-76.

MANN, D.O. (1982b) - Structure, life history and systematics of Rhoicosphenia (Bacillariophyta) II . Auxospore formation and perizonium structure of Rh. curvata. J . Phycol. 18,264-74.

MANN, D.O. (l982c) - The use of the central raphe endings as a taxonomic character. PI. Syst. Evol. 141,143-52.

MANN, D.O. (1983) - Symmetry and cell division in raphid diatoms . Ann. Bot. (in press) .

NORRIS, R.E . (1973) - A new planktonic diatom, Nanoneis hasleae gen. et sp. nov. Norw. J . Bot. 20, 321-5.

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PICKETT-HEAPS, J.D., D.H. TIPPIT & J .A. ANDREOZZI (1979) - Cell division in the pennate diatom Pinnularia IV . Valve morphogenesis. BioI. Cellulaire 35, 199-203.

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141

Explanation of Plates

Plate 1

All SEM, angle of tilt approximately 45 ° unless stated otherwise. - Fig. 10. Cocconeis sp., raphid valve. Note the concave valve face and the raphe rai"sed on a slight keel. x 4, 700.-Fig. 11. Denticula tenuis valve, tilted 55°, showing the undulate proximal (p) and distal (d) halves of the valve face, and the distally curved terminal fissure (tf). Note also the central VOigt discontinuity (arrow) Qn the distal side. x 7,300.-Fig. 12. Climacosphenia, broad pole. The two sterna (s) subtending the subsidiary ribs are apparently continuous with each other. x 2,500. - Fig. 13. Climacosphenia, part of v<\lve face, showing the two sterna (s; dashed) and the four sets of transapical ribs. Note the fault line (arrow) where the two centri­petal sets intersect. x 4,600. - Fig. 14. Eunotia sp., showing the eccentric sternum (dashed) at the junction of valve face and mantle, and the ribs it subtends. Note how the ribs are interrupted by the raphe slit, which is adjacent to the sternum only at the pole, and also the finer striae associated with the raphe. x 4,600. - Fig. 15 Denticula tenuis, interior: late stage in valve formation, sqowing the rows of poroids and prominent (as yet incomplete) septa. x 8,500.

Plate 2

All SEM, angle of tilt approximately 45 ° unless stated otherwise. Figs 16-19. Denticula tenuis. - Fig. 16. Valve, 0° tilt, showing distally (see Fig. 11) directed terminal fissures, and ceIttral Voigt discontinuity (v) on the distal side. x 5,700. - Fig. 17. Valve, 0° tilt, showing proximally (se~ Fig. 11) directed terminal fissures, and central Voigt discontinuity (v) on the proximal side. x 5,800. - Fig. 18. Early stage in deposition, distal side of valve: note equal ribs, lack of fibulae and absence Qf a central nodule (arrow). x 11,650. - Fig. 19. Slightly later stage; beginning of fibula formation. Note the unilateral development of the fibulae (arrows) and the equality of all ribs and striae. x 10,400. - Figs 20-22. Denticula rainieren­sis. - Fig. 20. Mature valve, interior. Note several rows of poroids between adjacent prominent septa (cf. Fig. 15). x 10,400. - Fig. 21. Detail, showing irregular rows of poroids. x 13,600. - Fig. 22. Incomplete valve, exterior. Note that the valve is formed in blocks, each block centre<:l around the position of a septum. The rows of poroids are delimited successively, as silicification proceeds out from the septa, and culminates in fusion along the midline between adjacent septa (arrows). x 16,()()().

142

MANNI

143

MANN2

144