MORPHOGENESIS OF THE LABIATE PROCESS IN THE ARAPHID PENNATE DIATOM DIATOM A VULGARE1

Each valve of the araphid pennate diatom Diatoma has a labiate process (LP) at one end; in a frustule, the LPs are at diagonally opposite ends. After mitosis is over, an elongated dense body detaches from the spindle pole and migrates to one end of the daughter cell, always diagonally opposite the LP of the parental valve. This dense body trails a cone-shaped array of microtubules (MTs). Meanwhile, the new valve has begun to form within the Silica Deposition Vesicle (SDV). Having reached the end of the cell, this dense body moves back slightly and then settles onto the SDV, developing a layered substructure as it does so. Immediately beneath it, the LP of the new daughter valve differentiates. This dense object is clearly the homologue of the fibrous Labiate Process Apparatus (LPA) involved in the differentiation of the LP in several centric diatoms. In a few cases, these LPAs also hair been shown to originate from some component of the spindle pole. Thus, the homologue of the LPA of centric diatoms has now been found in an araphid pennate diatom; in each case, the LPA apparently comes from the pole of the spindle and presumably uses a cytoskeleton of MTs to locate the LP in its correct position. These observations support the possibility that the raphe evolved from the LP.     

VALVE MORPHOGENESIS IN SURIRELLA (BACILLARIOPHYCEAE)1

Valve morphogenesis in two Surirellae (S. ovalis Brebisson and S. robusta Ehrenberg) is described. Mitosis takes place at the broad end of the cell. After cleavage, a new Microtubule Center (MC) arises near each spindle pole and moves to the adjacent plasmalemma. Soon, a specific group of microtubules (MTs) extends from very near the MC around the periphery of the cell. Concurrently, the new tubular Silica Deposition Vesicle (SDV) grows around the periphery of the cell close to these MTs. A double rib of silica is rapidly formed inside the SDV; the space between the ribs becomes the raphe. Mitochondria line up along the MTs, and the SDV may be molded around these to create the canal raphe. Soon, the SDV expands in two directions to create the face and the mantle of the new valve. Meanwhile, each daughter nucleus, accompanied by the MC, moves to its interphase position at the center of the cell; this movement is colchicine-sensitive. As in several other pennate diatoms, an interruption in the raphe of the mature valve coincides with the initial position of the MC. The canal raphe thickens rapidly around the mitochondria; a rudimentary raphe fiber may be associated with the creation of a tiny curvature at the inner raphe fissure. As the SDV expands in the large S. robusta, the daughter cell protoplasts slowly shrink by plasmolysis, thereby creating the complex curved surface of the new valve surmounted by the arching canal raphes which are now quite rigid. In S. ovalis, the daughter cell protoplasts remain appressed and therefore the new valve surface is basically flat. The symmetry of Surirella is quite different from that of other pennate diatoms. However, the cytoplasmic events accompanying valve morphogenesis are similar in all important respects to those described in other raphid pennate diatoms, and clearly supports a naviculoid origin for this genus.     

VALVE MORPHOGENESIS IN THE PENNATE DIATOM ACHNANTHES COARCTATA

Mitosis and valve morphogenesis in the pennate diatom Achnanthes coarctata (Bréb. in W. Sm.) Grun. are described. After cytokinesis, both daughter nuclei and their microtubule centers (MCs) are found near one side of the cell. Each new tubular silica deposition vesicle (SDV) arises centrally, forming a single rib running the length of the cell. Each MC then migrates around its nucleus and positions itself directly adjacent to the new SDV. The enlarging silicalemmas with their associated MCs, nuclei, microtubules (MTs) and microfilaments (MFs) appear in mirror image in the daughter cells. Both SDVs soon generate a second longitudinal rib alongside the first; the gap between the ribs ultimately becomes the future raphe fissure. The MC, MTs and nucleus are associated with each fissure. However, the subsequent behavior of the valve secreting machinery now becomes quite different in the daughter cells. In the cell that will form a raphid valve, the silicalemma, flanked by MFs, expands laterally in both directions over the cleavage furrow. Within the expanding SDV, silica secretion continues, eventually generating the structure of the mature valve, and during this phase the raphe fissure becomes delineated as in other raphid diatoms. In the other daughter cell, however, the MC and its MTs withdraw from the silicalemma, and the SDV moves laterally across the cleavage furrow until the double rib is at the corner of the cell. As silica is secreted into this expanding SDV, the raphe fissure completely fills in. This valve, therefore, lacks a raphe when mature and has a symmetry quite different from that of the valve formed in the other daughter cell. These events are compared with the course of morphogenesis described for other raphid diatoms.     

VALVE MORPHOGENESIS IN THE PENNATE DIATOM NAVICULA CUSPIDATA1

The deposition of siliceous valves during asexual reproduction of the pennate diatom, Navicula cuspidata Kütz., is described with emphasis on the cytoplasmic components involved. The events accompanying valve secretion are similar to those already known from other pennate species. After mitosis, the microtubule centre (MC) moves to the center of the cleavage furrow where silica deposition is initiated inside a tubular silicalemma, and it remains associated with the prospective central nodule during valve growth. Microtubules (MTs), emanating from the MC, run parallel to the prospective raphe and together with the raphe fibres, appear to be involved with raphe development. Multiple raphe fibres occupy the maturing raphe fissure, in contrast to the single fibre of Pinnularia viridis, P. maior and Hantzschia amphioxys. The fibers exhibit a periodic substructure and are often opposed to the silicalemma where they may inhibit silica deposition and control the shaping of the raphe fissure. In contrast with the above species, in N. cuspidata MTs are clustered strictly opposite the raphe and lose their association with the MC which degenerates before the valves are mature. The primary role of MTs may be the stabilization of the cytoplasmic region where initial silicification occurs. Mitochondria and endoplasmic reticulum are not involved in molding valve growth in this species. Evidence for vesicle involvement in silica transport and deposition was limited. The possible contributions provided by comparative studies on the ultrastructure of valve morphogenesis towards elucidating the control of valve formation and the taxonomy of diatoms are discussed briefly.     

VALVE MORPHOGENESIS IN THE CENTRIC DIATOM PROBOSCIA ALATA SUNDSTROM1

Valve morphogenesis in Proboscia alata Sundstrom was followed in living cells and during treatment with antiactin and antimicrotubule drugs. Once cleaved, sibling cells rounded up and retracted. Soon, a granular organizing center (OC) appeared adjacent to the stub of the initiated valve. Silicification started within a silicon deposition vesicle (SDV) adjacent to the OC. The elongating valve was initially tubular and sealed at one end, creating the proboscis of the conical valve. The edge of the SDV and thinnest region of the forming valve was lined by a sleeve of bundled microtubules (MTs) that terminated short of the older more rigid part of the valve. The growing proboscis of living cells treated with the anti‐MT drug oryzalin became grossly distorted. EM revealed dense material lining the growing edge of the SDV; immunofluorescence microscopy showed a ring of actin here. Applied to living cells, the antiactin drug cytochalasin D caused the very young proboscis to collapse; in older valves, the base of the proboscis expanded. Thus, valve morphogenesis appeared controlled by the MT cytoskeleton, keeping the proboscis straight while actin molded its conical outline. At the tip of the proboscis was a slit resembling a labiate process. Its morphogenesis involved striated fibers and two MTs, reminiscent of the fibers and MTs associated with raphe morphogenesis. In contrast to spine‐like processes that elongate by tip growth, the tip of the proboscis was formed first, and the consequent “antitip growth” suggests the tip was originally the center of the valve face.     

CELL DIVISION AND MORPHOGENESIS OF THE CENTRIC DIATOM CHAETOCEROS DECIPIENS (BACILLARIOPHYCEAE) II. ELECTRON MICROSCOPY AND A NEW PARADIGM FOR TIP GROWTH

Valve morphogenesis starts when the silica deposition vesicle (SDV) expands across a cleavage furrow covered by an unidentified layer, which may aid in its shaping. A labiate process (LP) is present only in the outer valve of terminal cells in the filament. Before these particular cells form setae, a layered "labiate process apparatus" (LPA) appears on the SDV in the exact center of the forming valve, near the microtubule center arising after cleavage. The LPA thereafter surmounts the lips of the LP as it forms. After the girdle bands separate slightly, two lateral protrusions develop in the corners of the cell. These nascent setae are lined internally by a cylindrical, fibrous band (sleeve), which assembles immediately ahead of the expanding edge of the SDV, very close to the plasmalemma. Then these protrusions, lined by the fibrous band, the SDV, and the forming silica wall, grow through two gaps in the girdle bands. The cytoplasm at the tip of the growing seta is naked. Immediately behind the tip, this fibrous band is adpressed to the plasmalemma and thereby apparently defines the diameter of the seta; it extends to internally ensheath the tipmost edge of the SDV for a short distance, like a tight-fitting inner sleeve. This structure is considered the major organelle involved in seta morphogenesis. Microtubules (MTs), while present, are variable in extent and disposition within the seta. Turgor pressure is considered irrelevant in driving seta growth. Instead, a new paradigm proposed for tip-growing cells generally, may apply to seta morphogenesis, as follows. If, as is suspected, the fibrous band contains actin, cycling of this actin (as in animal cells undergoing ruffling or filopodial extension) could drive seta extension via attachment of the band to the just-formed silica wall. The band is visualized as a molecular treadmill whose support base, the new wall, is being continually extended; extension is controlled and generated strictly at the tip.     

Aspects of morphogenesis and function of diatom cell walls with implications for taxonomy

Summary Aspects of morphogenesis and morphology of diatom cell walls are reviewed to highlight functional correlations between wall structures and three-dimensional cytoplasmic activities during the cell cycle. Morphogenesis of the siliceous valve within the silica deposition vesicle is discussed in the light of the dependency on a precisely orchestrated moulding machinery, involving the cytoskeleton, mitochondria, endoplasmic reticulum, spacer vesicles produced by the Golgi apparatus, and the plasmalemma, in combination with adhesion of the cells to parts of the parental wall and localized plasmolyses. Sensitivity of morphogenetic events to fluctuations of external factors has implications for taxonomy.Abbreviations CF cleavage furrows - cPL cleavage plasmalemma - GB girdle bands - LP labiate process - LPA labiate process apparatus - MC microtubule center - mLP macro labiate process - MT microtubule - MTOC microtubules organizing center - PL plasmalemma - SDV silica deposition vesicle - SL SDV membrane - SpV spacer vesicles Dedicated to Professor Peter Sitte on the occasion of his 65th birthday     

The effect of drugs on diatom valve morphogenesis

Summary The effects of various drugs on cell wall (valve) morphogenesis was investigated in three species of diatoms (Pinnularia spp., Surirella robusta, andHantzschia amphioxys) using light microscopy (LM) and scanning electron microscopy (SEM). Treatment ofSurirella with the microtubule (MT) disrupting agent colchicine during early valve formation results in a characteristic malformation of the valve, whereby part of the normally circumferential raphe canal forms as an abnormal protruding lip on the valve surface, located up to 20 m from the edge of the valve. The position of this malformed lip coincides with the location of a microtubule center (MC) at the time of colchicine addition, suggesting that the MC may play a direct role in positioning the tip of the raphe canal during valve formation. The migration of this MC to the tip of the cell during early valve morphogenesis is reversibly inhibited by the metabolic inhibitor 2-4-dinitrophenol (DNP). The effect of colchicine onPinnularia valve formation is less severe, causing occasional malformation of the raphe, but little if any lateral displacement. InHantzschia, colchicine has no effect on the positioning of the raphe, but prolonged exposure causes fusion of the raphe canal with the valve face. Cochicine treatment also results in the absence of the normal curvature at the central interruption in the raphe, as well as abnormal pore formation in this central area. Addition of cytochalasin D during early valve formation inHantzschia causes the raphe canal to form in the center of the valve face, suggesting that the normal translocation of the raphe canal to the valve edge is actindependent. Comparison of valves from control and cytochalasintreatmentHantzschia suggest that the pore spacing within the valve is determined by the position relative to the raphe, and does not depend on whether to pores form on the side (mantle) or the face of the mature valve.Abbreviations DM diatom medium - DNP dinitrophenol - MT microtubule - MC microtubule center - PSS primary silicification site - SDV silica deposition vesicle     

Aberrant valve formation in a centric diatom,Ditylum brightwellii

Summary Interphase cells of the centric diatom,Ditylum brightwellii (West) Grunow, were treated with a microtubule-inhibitor (amiprophosmethyl, 6×10^–7 M); the cells could proceed to divide, but the spindle apparatus in about 25% of the cells was displaced and their two sibling cells has either two nuclei or none. The cells with two nuclei formed a new valve with two labiate processes, instead of one as in normal cells. Most of the cells lacking a nucleus were unable to form a new valve, and of the 2% that did form new valves, all did so without dividing. The valves with two labiate processes were originally formed in two separate silica deposition vesicles (SDVs) and the two embryonic siliceous valves fused when these two expanding SDVs met. Accordingly, both the pattern of perforations and the shape of the marginal ridges on the new valve vary with the distance between the two initiation sites of the two SDVs. Implications of these observations in the evolution of valves in diatoms are discussed and a hypothesis on multiple origins of the valves is proposed.     

Studies on the biochemistry and fine structure of silica shell formation in diatoms

Summary The morphogenesis of the different types of labiate processes is compared among three species of centric diatoms [Stephanopyxis turns (Greville) Ralfs,Odontella sinensis (Greville) Grunow, andOdontella aurita (Lyngbye) Agardh]. In all species, a cytoplasmic structure,i.e., the labiate process apparatus, situated close to the developing labiate process, appears before the labiate process begins to form and disappears upon its maturation. The possibility that the labiate process apparatus is implicated in the phylogeny of the labiate process is discussed.     

Valve morphogenesis in an araphid diatom Rhaphoneis amphiceros (Rhaphoneidaceae,Bacillariophyta)

The pattern centre in valve morphogenesis is an annulus in centric diatoms or a sternum in pennate diatoms. The genus Rhaphoneis is currently placed within a lineage that diverges at the root of the pennate diatom clade in most molecular phylogenies, and its valves have a unique pattern to their striae, i.e. radiating from both apices, giving the impression that a pattern centre exists at both ends of the valve and virgae (ribs) formation proceeds centripetally. The present study, however, shows that the pattern centre is actually a linear sternum and the formation of virgae proceeds centrifugally, a pattern centre that is commonly found in most araphid diatoms. Thus, the hypothesis that valve morphogenesis based on a linear sternum and perpendicular virgae is a synapomorphy of pennate diatoms is supported. Our study also demonstrates that the pattern of valve formation can be observed by light microscopy with a direct mounting method when the specimen is relatively large, i.e. exceeding approximately 50 µm in valve length. An important advantage of the use of the direct mounting method is that it requires no repeated centrifugation steps for dehydration, steps necessary for observation by a scanning electron microscope, causing the loss and/or collapse of the specimen, particularly with fragile valves in the early stages of development.     

THE EVOLUTION OF ELONGATE SHAPE IN DIATOMS1

Diatoms have been classified historically as either centric or pennate based on a number of features, cell outline foremost among them. The consensus among nearly every estimate of the diatom phylogeny is that the traditional pennate diatoms (Pennales) constitute a well‐supported clade, whereas centric diatoms do not. The problem with the centric–pennate classification was highlighted by some recent analyses concerning the phylogenetic position of Toxarium, whereby it was concluded that this “centric” diatom independently evolved several pennate‐like characters including an elongate, pennate‐like cell outline. We performed several phylogenetic analyses to test the hypothesis that Toxarium evolved its elongate shape independently from Pennales. First, we reanalyzed the original data set used to infer the phylogenetic position of Toxarium and found that a more thorough heuristic search was necessary to find the optimal tree. Second, we aligned 181 diatom and eight outgroup SSU rDNA sequences to maximize the juxtapositioning of similar primary and secondary structure of the 18S rRNA molecule over a much broader sampling of diatoms. We then performed a number of phylogenetic analyses purposely based on disparate sets of assumptions and found that none of these analyses supported the conclusion that Toxarium acquired its pennate‐like outline independently from Pennales. Our results suggest that elongate outline is congruent with SSU rDNA data and may be synapomorphic for a larger, more inclusive clade than the traditional Pennales.     

SILICON DEPOSITION IN DIATOMS: CONTROL BY THE pH INSIDE THE SILICON DEPOSITION VESICLE

To test the hypothesis that silicification occurs under acid conditions in the silicon deposition vesicle (SDV), the acidity of the SDV of the pennate diatoms Navicula pelliculosa (Brébisson et Kützing) Hilse, N. salinarum (Grunow) Hustedt, and Nitzschia sigma (Kützing) Smith was determined during development of new frustule valves. Cells were incubated with the weak base 3-(2,4-dinitroanilino)-3′-amino-N-methylpropylamine (DAMP) followed by immunocytochemical localization in whole cells and on ultrathin sections. After resupplying silicate to cells synchronized by silicon depletion, the uptake of this nutrient from the medium was the same with or without DAMP; new valves developed without morphological aberrations that could conceivably have been caused by the probe. DAMP was found in cellular compartments known to be acidic, such as vacuoles active as lysosomes, the lumen of thylakoids, and microbodies. In the nucleus and mitochondria, which are circumneutral and basic compartments, the probe did not appear. Besides its presence in acidic compartments, DAMP was specifically accumulated within the SDV during formation of new valves; during the process of valve maturation, the SDV seemed to become increasingly acidic. In control experiments using the ionophores chloroquine, valinomycin, and nigericin, the compartmental location of DAMP was clearly disturbed, resulting in a random intracellular distribution. Accumulation of the fluorescent probe rhodamine 123, which can be translocated over membranes by a reducing potential, confirmed that the SDV can translocate weak bases. The results with DAMP suggest that the pH of the SDV is important in the silicification of diatoms: It facilitates a fast nucleation and aggregation of silica particles, thus increasing the rate of formation of the mature frustules. In addition, the acidic environment might protect the newly formed valves against dissolution before completion and coverage by the organic casing prior to their secretion.     

Supramolecular organization of fucoxanthin–chlorophyll proteins in centric and pennate diatoms

Fucoxanthin–chlorophyll proteins (FCP) are the major light-harvesting proteins of diatom algae, a major contributor to marine carbon fixation. FCP complexes from representatives of centric (Cyclotella meneghiniana) and pennate (Phaeodactylum tricornutum) diatoms were prepared by sucrose gradient centrifugation and studied by means of electron microscopy followed by single particle analysis. The oligomeric FCP from a centric diatom were observed to take the form of unusual chain-like or circular shapes, a very unique supramolecular assembly for such antennas. The existence of the often disputed oligomeric form of FCP in pennate diatoms has been confirmed. Contrary to the centric diatom FCP, pennate diatom FCP oligomers are very similar to oligomeric antennas from related heterokont (Stramenopila) algae. Evolutionary aspects of the presence of novel light-harvesting protein arrangement in centric diatoms are discussed.     

Comparing radiolarian and diatom diversity and abundance from the northeast Pacific

Particle trap samples (VERTEX) from 4 locations in the northeast Pacific were used to compare centric and pennate diatoms with nassellarian and spumellarian radiolarians. The proportion of pennates and nassellarians were highest at VERTEX 3 from the northeast tropical oceans, off the west coast of Mexico. Colonial radiolarians were characteristic of warm, high saline waters from the southern edge of the north Pacific central gyre, north of Hawaii (VERTEX 4), while pennate diatoms were abundant. The subtropic oceanic Pacific (VERTEX 5A) had more pennate diatoms, while VERTEX (5C), in a coastal cyclonic eddy located west of California, had an even proportion of centric and pennate diatoms. Comparison of major faunas and flora at various oceanic sites of present siliceous sedimentation in conjunction with atmospheric, hydrographic and chemical data, can be helpful in reconstructing paleoceanographic and paleoenvironmental conditions of the past oceans throughout the Phanerozoic.     

AN INVESTIGATION OF THE CELL WALL COMPONENTS OF ACTINOCYCLUS SUBTILIS (BACILLARIOPHYCEAE)1

The frustule of Actinocyclus subtilis (Greg.) Ralfs has been examined in detail with light and electron microscopy. The complex valve structure can only he appreciated fully by means of thin-sections. The loculate areola is closed to the inside by a basket of organic material, which is supported by a system of radiating, lightly silicified ribs. The basket is easily destroyed by acid cleaning, thus leaving an open hole (foramen). The loculus is connected to the outside by a system of interconnecting channels, whose external openings are too small to be resolved by the SEM. Between the locules of the valve face is a complex system of bullulae. The labiate processes are slightly tilted from the vertical axis vis à vis the valve mantle. Fibrillar material and mucilage are present within each labiate process, observations not reported previously. Histochemistry indicates that acid mucopolysaccharides are present within the lumen of the labiate process. We propose that the secretion of mucilage is related to the rotational, but directed movement noted in this centric diatom. The pseudonodulus is a solid silica plate subtended on the inside by a disc of organic material external to the cytoplasm. The relationship between Actinocyclus, Hemidiscus, Roperia, Charcotia and Azpeitia is discussed based on these new findings of valve morphology. Also the relationship between the families Hemidiscaceae and Coscinodiscaceae is discussed.     

A checklist of algal species found in the East Siberian Sea in May 1987

Summary A checklist of microalgae found in sea-ice in the East Siberian Sea in May 1987 has been compiled. A total of 122 taxa have been identified, consisting of 101 taxa in 18 genera of pennate diatoms, 14 taxa in 8 genera of centric diatoms, 4 taxa in 3 genera of dinoflagellates (including Ebria sp.), and 2 taxa in 1 genus of chrysophytes (silicoflagellates); choanoflagellates (Craspedomonadales) were found in one sample.     

AUXOSPORE FORMATION AND THE MORPHOLOGY OF THE INITIAL CELL OF THE MARINE ARAPHID DIATOM GEPHYRIA MEDIA (BACILLARIOPHYCEAE)1

Recent studies have led to a rapid increase in knowledge of auxospore formation in diatoms. However, these studies have been limited to centric and raphid pennate diatoms, and there is still very little information for the araphid pennate diatoms. Using LM and SEM, we studied the development of the auxospore and the initial cell of the marine epiphytic diatom Gephyria media Arnott. Auxospores were bipolar and curved in side view, as in many other pennate diatoms. SEM revealed many transverse perizonial bands, all of which were incomplete rings. There was an elongate, sprawling, silicified structure beneath the ventral suture of the transverse perizonial bands. This structure is presumably equivalent to the longitudinal perizonial band in other pennate diatoms, although we could not determine the homologous relationship between the two features. Scales were found both in the inner wall of the perizonium and around the primary perizonial bands. The presence or absence of scales may be of phylogenetic significance in diatoms, only during the final stages of auxospore formation because scales are found in early spherical stages. The distinctive finger‐like structures observed throughout all stage of G. media have not been observed before in the other diatom taxa.     

The use of outdoor phytoplankton continuous cultures to analyse factors influencing species selection

Natural phytoplankton populations have been grown in outdoor continuous cultures at three dilution rates (D = 0.5, 0.25, and 0.1 · day^?1) under nitrogen (N) or silicon (Si) limitation and two light intensities. At a high specific nutrient flux (high dilution rate) under N limitation an assemblage of primarily small, fast growing centric diatoms such as Skeletonema costatum (Grev.) Cleve and Chaetoceros spp. dominated with a low percentage of flagellates. At a low specific nutrient flux, a mixture of larger, slower growing centric diatoms, small flagellates, and pennate diatoms was obtained. Similar trends were observed under silicate limitation. Decreasing the light intensity at the lowest dilution rate selected for an assemblage similar to that observed at the high dilution rate and high light intensity.The results of these competition experiments suggest that specific nutrient flux (dilution rate) is an important factor in determining between group dominance (e.g., centric and pennate diatoms and small flagellates). Successful competitors representing broad phytoplankton groups can be arranged along a resource gradient of specific nutrient flux (dilution rate), with groups such as centric and pennate diatoms, represented as high and medium flux species, respectively.     

PHYLOGENETIC POSITION OF TOXARIUM,A PENNATE‐LIKE LINEAGE WITHIN CENTRIC DIATOMS (BACILLARIOPHYCEAE)1

The diatom genus Toxarium Bailey has been treated as a pennate because of its elongate shape and benthic lifestyle (it grows attached to solid substrata in the marine sublittoral). Yet its valve face lacks all structures that would ally it with the pennates, such as apical labiate processes, a midrib (sternum) subtending secondary ribs and rows of pores extending perpendicularly out from the midrib, or a raphe system. Instead, pores are scattered irregularly over the valve face and only form two distinct rows along the perimeter of the valve face. In our nuclear small subunit rDNA phylogenies, Toxarium groups with bi‐ and multipolar centrics, as sister to Lampriscus A. Schmidt. Thus, the genus acquired a pennate‐like shape and lifestyle independently from that of the true pennates. The two species known, T. hennedyanum Grunow and T. undulatum Bailey, differ only in a single feature: the valve perimeter of the former shows only a central expansion, whereas that of the latter possesses in addition a regular undulation. Yet both forms were observed in our monoclonal cultures, indicating that the two taxa represent extremes in a plasticity range. Toxarium resembles another elongate and supposedly araphid diatom, Ardissonea De Notaris, in being motile. Cells can move at speeds of up to 4 μm·s ^{? 1} 1 Received 7 June 2002. Accepted 4 October 2002. through secretion of mucilage from the cell poles or they remain stationary for longer periods, when they form short polysaccharide stalks. Division during longer periods of quiescence leads to the formation of small colonies of linked or radiating cells.