THE ULTRASTRUCTURE OF CARPOSPOROGENESIS IN THE MARINE HEMIPARASITIC RED ALGA ERYTHROCYSTIS SACCATA1

The ultrastructure of carposporogenesis for Erythrocystis saccata is described. The fusion and gonimoblast cells contain few organelles, and chloroplasts are in a proplastid state, with pit plugs between gonimoblast cells dissolving early in development. Carpospore development may be separated into 3 stages, the first stage being characterized by the appearance of straight-profiled dictyosomes, fibrous vesicles, and an increase of discoid thylakoids within the chloroplasts. During the second, stage the dictyosomes assume a curved profile and striped vesicles are formed by the endoplasmic reticulum. The third stage is initiated by the disappearance of striped vesicles and the appearance of straight-profiled dictyosomes secreting vesicles with cores. Mature carpospores consist of many cored vesicles, fibrous vesicles, and floridean starch grains. A single wall layer surrounds each carpospore since the carposporangial wall becomes incorporated into a mucilaginous matrix surrounding the spores.     

THE ULTRASTRUCTURE OF CARPOSPOROGENESIS IN CALOGLOSSA LEPRIEURII (DELESSERIACEAE,CERAMIALES)

Carposporogenesis in Caloglossa leprieurii is divided into three cytological stages. At stage I, the young spores have few plastids and little starch. Abundant dictyosomes secrete a gelatinous wall layer in scale-like units. At stage II, dictyosomes produce a second fibrillar wall component in addition to the gelatinous constituent. Large fibrillar vesicles accumulate in the cytoplasm. Production of gelatinous material decreases in this stage. By stage III, starch grains and fully developed plastids are abundant. Rough endoplasmic reticulum occupies much of the peripheral cytoplasm. A dense, granular proteinaceous component appears in the wall in association with the fibrillar layer. Arrays of randomly oriented tubules are scattered in the cytoplasm. The mature carpospore is surrounded by an outer gelatinous wall layer and an inner fibrillar layer. Few dictyosomes persist in the mature spore. Carposporogenesis in Caloglossa is compared with that in other red algae.     

ULTRASTRUCTURE OF CARPOSPOROGENESIS IN THE PARASITIC RED ALGA FAUCHEOCOLAX ATTENUATA SETCH. (RHODYMENIALES,RHODYMENIACEAE)

Carpospore differentiation in Faucheocolax attenuata Setch. can be separated into three developmental stages. Immediately after cleaving from the multinucleate gonimoblast cell, young carpospores are embedded within confluent mucilage produced by gonimoblast cells. These carpospores contain a large nucleus, few starch grains, concentric lamellae, as well as proplastids with a peripheral thylakoid and occasionally some internal (photosynthetic) thylakoids. Proplastids also contain concentric lamellar bodies. Mucilage with a reticulate fibrous substructure is formed within cytoplasmic concentric membranes, thus giving rise to mucilage sacs. Subsequently, these mucilage sacs release their contents, forming an initial reticulate deposition of carpospore wall material. Dictyosome vesicles with large, single dark-staining granules also contribute to wall formation and may create a separating layer between the mucilage and carpospore wall. During the latter stages of young carpospores, starch is polymerized in the perinuclear cytoplasmic area and is in close contact with endoplasmic reticulum. Intermediate-aged carpospores continue their starch polymerization. Dictyosomes deposit more wall material, in addition to forming fibrous vacuoles. Proplastids form thylakoids from concentric lamellar bodies. Mature carpospores are surrounded by a two-layered carpospore wall. Cytoplasmic constituents include large floridean starch granules, peripheral fibrous vacuoles, mature chloroplasts and curved dictyosomes that produce cored vesicles which in turn are transformed into adhesive vesicles. Pit connections remain intact between carpospores but begin to degenerate. This degeneration appears to be mediated by microtubules.     

Ultrastructure of post-fertilization development in the red alga Scinaia articulata (Galaxauraceae,Nemaliales, Rhodophyta)

The ultrastructure of the carposporophyte and carposporogenesis is described for the red alga Scinaia articulata Setch. After fertilization, the trichogyne disappears, and the pericarp develops to form a thick protective tissue that surrounds the carposporophyte. The hypogynous cell cuts off both one-celled and two-celled sterile branches. Patches of chromatin are frequently observed in evaginations of the nuclear envelope, which appear to produce vesicles in the cytoplasm of the cell of the sterile branch. Large gonimoblast lobes extend from the carpogonium and cleave to form gonimoblast initials. Subsequently, a fusion cell is formed from fusions of the carpogonium, the hypogynous cell and the basal cell of the carpogonial branch. The mature carposporophyte comprises the fusion cell that is connected to the sterile branch cells, gonimoblast cells and carpospores and is surrounded by extensive mucilage. Young carpospores possess a large nucleus and proplastids with a peripheral thylakoid, but they have few dictyosomes and starch granules and are indistinguishable from gonimoblast cells. Subsequently, dictyosomes are formed, which produce vesicles with an electron-dense granule, which indicates an initiation of wall deposition. Thylakoid formation coincides with incipient starch granule deposition. The nuclear envelope produces fibrous vacuoles and concentric membrane bodies. Carpospores are interconnected by pit connections with two cap layers. Dictyosome activity increases, resulting in the production of vesicles, which either continue to deposit wall material or coalesce to form fibrous vacuoles. The final stage of carposporogenesis is characterized by the massive production of cored vesicles from curved dictyosomes. Mature carpospores are uninucleate and contain fully developed chloroplasts, numerous cored vesicles, numerous starch granules and fibrous vacuoles. The mature carpospore is surrounded by a wall layer and a separating layer, but a carposporangial wall is lacking.     

ULTRASTRUCTURE OF CARPOSPOROPHYTE DEVELOPMENT IN THE RED ALGA GLOIOSIPHONIA VERTICILLARIS (CRYPTONEMIALES,GLOIOSIPHONIACEAE)

The ultrastructure of carposporophyte development is described for the red alga Gloiosiphonia verticillaris Farl. The auxiliary cell produces gonimoblast initials, which divide to produce two types of gonimoblast cells—the nondividing vacuolate cells and terminal generative gonimoblast cells. The generative gonimoblast cells form clusters of carpospore initials, which eventually differentiate into carpospores. After gonimoblast filaments are formed, the auxiliary cell undergoes autolysis, causing degeneration of septal plugs between the auxiliary cell and adjacent cells, thus forming a fusion cell. Since this cell lacks starch and appears degenerate throughout carposporophyte development, a nutritive function cannot be ascribed to the fusion cell. Carpospore differentiation is simple and proceeds through three developmental stages. Young carpospores structurally resemble gonimoblast cells, because they contain undeveloped plastids, large quantities of floridean starch, and are surrounded by extensive mucilage instead of a distinct wall. In addition, dictyosomes form and begin to produce vesicles with fibrous contents representing carpospore wall material. During the intermediate stage, dictyosomes continue to produce vesicles that contribute additional carpospore wall material, thereby compressing the mucilage and creating a darker-staining layer outside the carpospore wall. Plastids form internal thylakoids by invaginations of the inner membrane of the peripheral thylakoid. The endoplasmic reticulum forms large granular vacuoles that appear to be degraded during subsequent stages of development. Mature carpospores form cored vesicles. They also contain mature chloroplasts, large amounts of floridean starch, and occasionally granular vacuoles. During this stage, interconnecting carpospore-carpospore and carpospore-gonimoblast cell septal plugs begin to undergo degeneration. This process may be mediated by tubular structures.     

The Ultrastructure of Tetrasporogenesis in the Marine Red Alga Chondria tenuissima (Good, et Woodw.) (Ceramiales, Rhodomelaceae)

Tetraspore development has been studied in Chondria tenuissimausing light and electron microscopy. The transformation of tetrasporangialmother cells into mature tetrasporangia involves a series ofstructural changes, especially of dictyosomes and of the nucleus.The youngest stage of tetrasporogenesis consists of a uninucleatetetraspore mother cell with synaptonemal complexes present duringearly prophase of meiosis I. Mitochondria are aggregated aroundthe nucleus, dictyosome activity is low, and proplastids occurin the peripheral cytoplasm. The cleavage furrows are initiatedalmost concomitantly with commencement of meiosis. When thecleavage furrows are initiated, spherical bodies bounded bytwo membranes are found within the cytoplasm; they develop intovacuoles with fibrillar contents (fv₁), which increase in sizeduring tetraspore development by fusing with each other andwith Golgi vesicles. The Golgi vesicles and the vacuoles withfibrillar contents (fv₁) contribute material to the developingtetraspore wall. During the middle stage of tetraspore formationthe vacuoles with fibrillar contents (fv₁) are dominant, dictyosomeactivity increases, as well as the number of plastids and mitochondria;starch formation also increases. Stacked cisternae of the endoplasmicreticulum are found within the peripheral part of the nucleus.The same nuclear structures are also observed in tetrasporangiaof the marine red alga Gastroclonium clavalum. The final stageis characterized by the disappearance of vacuoles with fibrillarcontents (fv₁) and of the stacked ER within the nucleus, presenceof straight, large dictyosomes which produce cored vesicles,an abundance of starch grains and by the formation of fullydeveloped chlorqplasts. The cored vesicles contain Thiéry-positivematerial and contribute to the formation of vacuoles with fibrouscontents (fv₂) as they are dominant in the tetraspores beforetheir liberation. Rhodophlyla, Chondria, tetrasporogenesis, ultrastructure, Golgi apparatus     

Characterization of fibrous compound of Golgi vesicles in tapetal cells ofDelphinium

Summary It was shown that Golgi structures abundantly appearing in tapetal cells ofDelphinium Ajacis L. developing anthers, prior to meiocytes meiosis, show a fine fibrous material within their vesicles. At the time of the formation of tapetal cell wall this fibrous component, released by an exocytotic process, is incorporated into the cell wall. The membrane of dictyosomes derived vesicles participates in the development of plasma membrane. Fibrous material appears to be morphologically similar to the fibrils of tapetal cell wall; this cell wall gives a positive reaction for cellulose and pectins, as visible in the light microscope. Moreover, the fibrous and pectinase resistant compound of dictyosomes derived vesicles and the fibrils of cell wall disappear partly after cellulase digestion which proves their cellulosic character. On the other hand pectinase treatment as well as ruthenium red staining suggest associated with cellulose pectins within Golgi vesicles.     

Ultrastructure of the differentiating zygotospores of Porphyra leucosticta (Rhodophyta)

The ultrastructure of zygotosporogenesis is described for the red alga Porphyra leucosticta Thuret. Packets of eight zygotosporangia, each packet derived from a single carpogonium are interspersed among vegetative cells. Zygotospore differentiation in Porphyra can be separated into three developmental stages. (i) Young zygotospores exhibit a nucleus and a large centrally located, lobed plastid with pyrenoid. Mucilage is produced within concentric membrane structures during their dilation, thus resulting in the formation of mucilage sacs. Subsequently, these sacs release their contents, initiating the zygotospore wall formation. Straight‐profiled dictyosomes produce vesicles that also provide wall material. During the later stages of young zygotospores, starch polymerization commences, (ii) Medium‐aged zygotospores are characterized by the presence of fibrous vacuoles. These are formed from the ‘fibrous vacuole associated organelles’. The fibrous vacuoles finally discharge their contents. (iii) Mature zygotospores are recognized by the presence of numerous cored vesicles produced by dictyosomes. Cored vesicles either discharge their contents or are incorporated into the fibrous vacuoles. There is a gradual reduction of starch granules during zygotospore differentiation. Mature zygotospores are surrounded by a fibrous wall, have a large chloroplast with pyrenoid and well‐depicted phycobilisomes but are devoid of starch granules.     

Ultrastructure of trichoblasts in the red alga Osmundea spectabilis var. spectabilis (Rhodomelaceae,Ceramiales)

The apex of the tetrasporangial branches of Osmundea spectabilis var. spectabilis (= Laurencia spectabilis var. spectabilis) exhibits cavities in which tufts of multicellular trichoblasts occur. Trichoblast development in Osmundea spectabilis var. spectabilis begins with the differentiation of an epidermal cell within the crypt. This cell differentiates into a trichoblast mother cell (TMC). The TMC divides to form a two-celled incipient trichoblast. Successive periclinal divisions of the apical cell of the young trichoblast result in the formation of a multicellular developing trichoblast. With the exception of the apical cell all trichoblast cells are at the same developmental stage. They possess a large nucleus, abundant plastids with peripheral and some internal thylakoids and dictyosomes. Daughter chloroplasts result from one constriction or multiple fission of a single chloroplast. Dictyosomal cisternae and mucilage sacs contribute material to wall formation. Each differentiating trichoblast cell is surrounded by a bi-layered wall. The outer wall layer represents the trichoblast mother cell wall and the inner wall layer is the trichoblast cell wall. Mature trichoblast cells have thin walls, probably as a consequence of mucilage extrusion, the most likely function of trichoblasts in Osmundea.     

ULTRASTRUCTURE OF SPERMATIUM LIBERATION IN THE MARINE RED ALGA PTILOTA DENSA1

The differentiation of male gametes of the marine red alga Ptilota densa was studied by electron microscopy. Mature primary spermatangia are enveloped by a single cell wall and possess a clearly polar subcellular organization. The nucleus is situated apical to large, striated, fibrous vacuoles which are apparently formed by the repeated fusion of dictyosome vesicles. The transformation and liberation of spermatia from spermatangia involve both the secretion of the fibrous vacuoles at the base of the cell and the subsequent rupturing of the spermatangial cell wall. Liberated spermatia are coated with a thin mucilage layer and contain numerous small vesicles and several mitochondria and dictyosomes. The nucleus is cup-shaped and generally lacks a limiting envelope. These findings are discussed in relation to other light and electron microscopic studies of differentiating spermatangia in red algae.     

ULTRASTRUCTURE OF TETRASPOROGENESIS IN PALMARIA PALMATA (RHODOPHYTA)1

The tetrasporangial initial in Palmaria palmata (L.) O. Kuntze (formerly Rhodymenia palmata (L.) Greville) arises from a cortex cell which enlarges and deposits a protein-rich wall layer. This cell undergoes mitosis to form a tetrasporocyte and a stalk cell. Synaptonemal complexes are formed in the sporocyte nucleus while in the cytoplasm floridean starch is deposited in association with ER or with particles presumed to be ribosomes. Microbody-like structures become numerous between the nuclear envelope and perinuclear ER, and clusters of non-membranous, spherical structures also are associated with the nucleus. Chromatin condensation is reversed following pachytene and a prolonged diffuse stage ensues, when dictyosomes and ER produce vesicles which deposit mucilage rich in sulfated and acidic polysaccharides around the tetrasporocyte. A conspicuous lenticular thickening of the mucilage sheath develops at the apical end of the sporangium. Dictyosomes are frequently associated with mitochondria which may be associated with chloroplasts. Following nuclear divisions the tetrasporocyte is cleaved into four spores by sequentially initiated, but simultaneously completed periclinal and anticlinal furrows. When mucilage deposition ceases, the dictyosomes begin to produce vesicles with glycoprotein-rich contents. These vesicles are abundant in released tetraspores, and they probably contain adhesive material aiding in the attachment of the liberated spores.     

ULTRASTRUCTURE OF TETRASPOROGENESIS IN THE CORALLINE ALGA HALIPTILON CUVIERI(RHODOPHYTA)1

Tetrasporogenesis begins with the formation of the tetra-sporocyte, an elongate, apparently wall-less, cell containing few organelles. The tetrasporocyte rapidly elongates and a distinctive cell wall forms before the onset of meiosis. During this elongation phase there is also an increase in the number of plastids and mitochondria. The meiotic tetrasporocyte is characterized by extensive development of perinuclear endoplasmic reticulum (PNER) and peripheral endoplasmic reticulum (PER) and during the latter stages of sporogenesis by internuclear endoplasmic reticulum. Immediately next to the nuclear envelope the inter-cisternal spaces of the PNER are filled with very electron dense material and the PNER cisternae are quite narrow, while further away from the nucleus the PNER cisternae dilate. Throughout meiosis there is continued replication of plastids and mitochondria as well as synthesis of starch and the formation of Golgi-derived vesicles with very osmiophilic contents. Cytokinesis begins with the formation of striated thickenings on the inside of the tetrasporocyte wall, at the sites where the cleavage furrow, produced by infurrowing of the plasmalemma, will be formed. Early in cytokinesis the PER disappears and is replaced by osmiophilic vesicles and mitochondria. Tubular plasmalemma invaginations of 27–30 nm width also appear during the early stages of tetraspore wall formation. The ultra-structure of the early stages of tetraspore germination is also described.     

LIGHT AND ELECTRON MICROSCOPIC STUDIES OF THE FUSION CELL IN ASTEROCOLAX GARDNERI SETCH. (RHODOPHYTA,CERAMIALES)12

The fusion cell in Asterocolax gardneri Setch, is a large, multinucleate, irregularly-shaped cell resulting from cytoplasmic fusions of haploid and diploid cells. Subsequent enlargement takes place by incorporating adjacent gonimoblast cells. The resultant cell consists of two parts—a central portion of isolated cytoplasm, surrounded by an electron dense cytoplasmic barrier, and the main component of the fusion cell cytoplasm surrounding the isolated cytoplasm. The fusion cell contains many nuclei, large quantities of floridean starch, endoplasmic reticulum, and vesicles, but few mitochondria, plastids and dictyosomes. The endoplasmic reticulum forms vesicles that apparently secrete large quantities of extracellular mucilage which surrounds the entire carposporophyte. The isolated cytoplasm also is multinucleate but lacks starch and a plasma membrane. Few plastids, ribosomes and mitochondria are found in this cytoplasm. However, numerous endoplasmic reticulum cisternae occur near the cytoplasmic barrier and they appear to secrete material for the barrier. In mature carposporophytes, all organelles in the isolated cytoplasm have degenerated.     

CELL DIVISION AND FILAMENT FORMATION IN THE DESMID BAMBUSINA BREBISSONII (CHLOROPHYTA)1

Cell division and semicell expansion in the filamentous desmid Bambusina brebissonii Kütz. were investigated using transmission and scanning electron microscopy. Interphase cells are typical of desmids, containing a full complement of organelles and a cell wall penetrated by complex pores, but the cells lack a well-defined median constriction. Cell division involves an open spindle and the centripetal growth of a primary septum formed by the fusion of small, dark-staining vesicles probably derived from dictyosomes. Telophase nuclei are separated by a system of interzonal microtubules and numerous large, lighter-staining vesicles also derived from the dictyosomes. Following cell division, an elaborate replicate cross wall is formed which consists of both primary and secondary wall layers. During semicell expansion, a portion of the primary wall splits apart as the new semicells evaginate and expand to their full size. The primary wall stops splitting at a thick ring of secondary wall material leaving the cells united by the remaining common layer of primary wall. When semicell expansion is completed, the primary wall is not shed from the lateral walls of the new semicells, and pores through both primary and secondary wall layers begin to produce sheath material. However, pores in the end walls of cells do not function unless the filament is broken. The intact primary wall between cells and the absence of sheath production between cells comprise the mechanism serving to hold the cells of Bambusina brebissonii together in long filaments.     

ROLE OF DICTYOSOMES IN WALL FORMATION DURING CELL DIVISION OF CHLORELLA VULGARIS

The behavior of dictyosomes in wall formation during cell division of Chlorella vulgaris follows a definite pattern. During formation of the partition membrane they migrate into the equatorial plane and pair. There is a close spatial relationship between the dictyosomes and the partition membrane which, itself, may be derived from the fusion of dictyosomal vesicles. Dictyosomes also may participate significantly in the deposition of new wall material.     

Transformation of the Golgi apparatus in the cell cycle,especially at the resting and earliest developmental stages of a green alga,Micrasterias americana

Summary Transformation of the Golgi apparatus inMicrasterias americana at various stages after full growth and at the earliest stage of cell growth was investigated using an electron microscope. Silver-hexamine staining and the acid phosphatase (ACPase) test were also carried out. In cells cultured for two days after full growth, dictyosomes began to produce hypertrophied vesicles (HVs) along their five peripheral reagions. The HVs contained fibrous material, which was stained by silver-hexamine, and small granules which reacted with ACPase. The HVs were pinched off the dictyosomes and fused with each other and with the vacuoles. In the earliest stage of cell growth, the cisternae of the dictyosomes were stretched in one direction, which modified the shape from circular to elliptical and the dictyosomes curved along the long axis of the ellipse. These curved dictyosomes which produced middle sized vesicles (MVs) from the distal networks, divided into two identical parts along the short axis of the ellipse.     

FINE STRUCTURE OF THE ZOOSPORE OF OEDOCLADIUM CAROLINIANUM (CHLOROPHYTA) WITH SPECIAL REFERENCE TO THE FLAGELLAR APPARATUS1, 2

Ultrastructure of the motile zoospore has been investigated in Oedocladium catolinianum & Hoffman. An unwalled zoospore is usually produced from the contents of a terminal vegetative cell and consists of two principal regions: a small anterior dome and a larger body region; a ring of flagella marks the juncture of these two areas. Chloroplast inclusions consist of thylakoids, mature and incipient pyrenoids, starch and striated microtubules; no eyespot has been observed. Zoospores appear to possess permanent contractile vacuoles with numerous accessory vacuoles, coated vesicles and occasionally coated tubules. The cytoplasm of the dome contains numerous mitochondria ER and golgi bodies, as well as two distinct types of vesicles. The first contains an electron-dense; granular core and is surrounded by a loose, sinuate membrane. The second vesicle is electron-opaque and is found at the apex of the dome: it contains mucopolysaccharides employed during zoospore adhesion. A complex flagellar apparatus encircles the lower region of the dome. It consists of ca. 30–65 flagella, a ring-shaped fibrous band, flagella roots and additional supporting material. The flagella and roots alternate with one another beneath the fibrous band. The compound flagellar roots consist of two superimposed components: an outer ribbon-like unit composed of three microtubular elements and a single striated inner component. A band of support material lies beneath the proximal end of the basal bodies. It is a continuous fibrous band, although it often appears as three distinct, repetitive units.     

Cytochemical localization of cellulase activity in pollen mother cells of david lily during meiotic prophase I and its relation to secondary formation of plasmodesmata

Summary Cellulase activity was localized at the ultrastructural level in pollen mother cells (PMCs) of David lily [Lilium davidii var.willmottiae (Wilson) Roffill] at different stages of meiotic prophase I. The enzyme was observed to appear at the early leptotene stage and reached its highest level at the subsequent zygotene stage, and its subcellular distribution revealed by the presence of electron-dense deposits of reaction product was found to be restricted exclusively to the endoplasmic reticulum (ER), the vesicles derived from that, and the cell wall, especially at the sites of secondary plasmodesmata and cytoplasmic channels where the wall was being digested. Other cytoplasmic organelles, such as dictyosomes and Golgi vesicles, lacked such deposits of reaction product. After zygotene the enzyme activity decreased abruptly, and at the pachytene stage only very few deposits could be observed in the cell wall. Our results indicate that cellulase is synthesized on rough ER and secreted directly via the smooth ER and ER-derived vesicles into the cell wall by exocytosis, where it brings about local wall breakdown, leading to the secondary formation of plasmodesmata and cytoplasmic channels.     

ULTRASTRUCTURE OF SPERMATIAL DEVELOPMENT IN THE PARASITIC RED ALGAE LEVRINGIELLA GARDNERI AND ERYTHROCYSTIS SACCATA12

Morphologically, the development of spermatia in Levringiella gardneri and Erythrocystis saccata is identical, although cytologically several differences are evident. Mature spermatia contain 1 or 2 large spermatial vesicles that contain fibrous material, several small mitochondria, some proplastids, and are surrounded by a wall, either single-layered as in Erythrocystis or triple-layered as in Levringiella. Spermatial vesicles are formed by aggregations of endoplasmic reticulum in Levringiella, whereas concentric membrane bodies and dictyosomes may be involved in Erythrocystis. In addition to being fibrillar, the contents of the vesicle assume a convoluted appearance in Levringiella. Several spermatia are formed per mother cell and are connected by small pit connections which rupture to allow spermatial release from the spermatangial branch.     

CELL SHAPE AND WALL PATTERN IN RELATION TO CYTOPLASMIC ORGANIZATION IN PEDIASTRUM SIMPLEX

P. simplex is a single-pronged, fenestrated species of Pediastrum. Comparison is made in regard to cell differentiation and structure with P. boryanum, a 2-pronged, unfenestrated species, with emphasis on the origin of cell wall pattern and the regulation of cell shape. The characteristic wall pattern is initiated with the deposition of plaques of wall material of the outer wall layer when zoospores have assembled in the colony. The pattern is postulated to be templated in the plasma membrane. The inner, thicker wall layer is fibrillar and deposited from vesicles derived from the golgi apparatus. In P. simplex 2–4 dictyosomes are present in contrast to the single dictyosome of P. boryanum. The dictyosomes lie at the concave inner face of the nucleus. Blebs of its ribosome-free outer membrane are contributed to the forming face of the golgi apparatus. Parallel microtubules underlie the plasma membrane in the aggregating zoospores and disappear after the initiation of wall formation. The possible role of microtubules and other organelles in the determination of cell shape in Pediastrum is discussed.