ULTRASTRUCTURAL STUDIES OF ABSCISSION IN PHASEOLUS: ETHYLENE EFFECTS ON CELL WALLS

This paper reports light and electron microscope observations of changes in the walls of cortical cells in the laminar abscission region of red kidney bean (Phaseolus vulgaris L.). In intact plants two or three rows of cells comprise the abscission zone. Pectic substances are not present in the walls of these cells when wall breaks occur. The separation cavity involves breaks in both radial and longitudinal cell walls. In ethylene-treated explants pectic substances are present in the cell walls when breaking occurs. The separation cavity involves breaks in longitudinal walls only, and breaking is confined to a single row of cortical cells. Prior to cell wall break the plasma membrane frequently invaginates. In intact plants this may be associated with plasmolysis and with the formation of secondary vacuoles. In ethylene-treated explants it may also be related to plasmolysis. At the time of cell wall break many unidentifiable inclusions of varying sizes and shapes are present in the cell wall region. Chloroplasts and mitochondria are structurally altered but recognizable in the cell at the time of wall break. Plasmodesmata are frequently observed in abscission cells and may be structurally elaborate. The observations of the nature of cell wall changes during abscission in ethylene-treated material fail to confirm physiological studies of other workers suggesting that pectin dissolution is necessary and may be sufficient for formation of a separation layer.     

VARIATIONS IN THE CELL WALLS AND PHOTOSYNTHETIC PROPERTIES OF PORPHYRA YEZOENSIS (BANGIALES,RHODOPHYTA) DURING ARCHEOSPORE FORMATION1

The formation of archeospores is characteristic of Porphyra yezoensis Ueda and is important for Porphyra aquaculture. Recently, it has been regarded as a valuable seed source for propagation of thalli in mariculture. Cell wall composition changes are associated with archeospore formation in P. yezoensis. Here, we report changes of cell walls of P. yezoensis during archeospore formation. The surfaces of vegetative cells that were originally smooth became rougher and more protuberant as archeosporangia were formed. Ultimately, the cell walls of archeosporangia ruptured, and archeospores were released from the torn cell walls that were left at distal margins of thalli. With changes in cell walls, both effective quantum yield and maximal quantum yield of the same regions in thalli gradually increased during the transformation of vegetative cells to archeospores, suggesting that the photosynthetic properties of the same regions in thalli gradually increased. Meanwhile, photosynthetic parameters for different sectors of thalli were determined, which included the proximal vegetative cells, archeosporangia, and newly released archeospores. The changes in photosynthetic properties of different sectors of thalli were in accordance with that of the same regions in thalli at different stages. In addition, the photosynthetic responses of archeosporangia to light showed higher saturating irradiance levels than those of vegetative cells. All these results suggest that archeosporangial cell walls were not degraded prior to release but were ruptured via bulging of the archeospore within the sporangium, and ultimately, archeospores were discharged. The accumulation of carbohydrates during archeospore formation in P. yezoensis might be required for the release of archeospores.     

THE ULTRASTRUCTURE OF THE SALT GLANDS OF CYNODON AND DISTICHLIS (POACEAE)

The epidermal salt glands of the grasses Cynodon and Distichlis consist of a small outer cap cell and a large, flask-shaped basal cell. The wall of the basal cell is contiguous with those of the adjacent epidermal cells and underlying mesophyll cells. The basal cell is connected symplastically with all adjoining cells via plasmodesmata. The outer, protruding portion of the glands is covered by a cuticle continuous with that of the adjoining epidermal cells. However, the lateral cell walls of the glands are not incrusted by this cuticle. The cap cell wall has a loose, mottled appearance quite different from the compact striated appearance of the basal cell wall. The cap cell is characterized by dense cytoplasm containing many organelles and a varying number of small vacuoles. The basal cell cytoplasm is distinguished by the presence of an intricate system of paired membranes that are closely associated with mitochondria and microtubules. These membranes are infoldings of the plasmalemma that originate adjacent to the wall separating the cap and basal cells. The space enclosed by the paired membranes, therefore, is an extracellular channel that is open only in the direction of secretory flow. The consistent orientation of this system of paired membranes suggests that it represents a structural specialization which is directly and functionally involved in the secretory process. The close association of mitochondria and microtubules with the paired membranes implies that these structures are also functionally related to the secretory process. Finally, the results of this study indicate that these glands are ultrastructurally similar to those of Spartina and that the glands of these three grasses are structurally distinct from those of dicotyledonous plants.     

MUCILAGE-PRODUCING CELLS IN THE SEED COAT OF PLANTAGO OVATA: DEVELOPMENTAL FINE STRUCTURE

As the ovule of Plantago ovata matures into a seed its epidermal cells are transformed from undifferentiated parenchyma to thin-walled containers of almost pure mucilage. During this process the volume of the cells increases 60–80 fold, and the protoplast degenerates to a remnant. Rapid cell expansion begins with pollination and is accompanied by an increase in the size of the nucleus and nucleolus, a change in the random arrangement of ribosomes, a decrease in the thickness of cell walls, and synthesis of starch. Deposition of mucilage inside vacuoles and between the plasma membrane and cell wall accompanies a marked increase in the number and size of Golgi vesicles. Histochemical evidence using the thiocarbohydrazide-osmium vapor method shows polysaccharide to be present within Golgi vesicles while they are still attached to the Golgi apparatus. Mucilage deposition is associated with further cell expansion, separation of the protoplast from the cell wall, fusion of vacuoles and extra protoplasmic space, and the disappearance of starch.     

THE ASSOCIATION OF DRUSE CRYSTALS WITH THE DEVELOPING STOMIUM OF CAPSICUM ANNUUM (SOLANACEAE) ANTHERS

Each of the two stomiums in the anther of Capsicum annuum (sweet pepper) consists of a single layer of cells immediately below the epidermis between two adjacent locules. Each stomium extends the entire length of the anther and splits open at pollen maturity. Many calcium oxalate druse crystals form within the vacuoles of the stomium cells in association with membrane complexes and paracrystalline bodies. These latter structures are reported here for the first time and each is considered to be a nucleation site for druse crystal formation. Prior to the appearance of membrane complexes and crystals within the vacuoles, plasmalemmasomes are visible next to the stomium cell walls and contain vesicles and fibrous material. We propose that these bodies carry wall materials, including calcium ions and possibly oxalate ions, into the vacuoles. Their presence coincides with crystal formation. Two other types of crystals occur in the connective tissue between stomiums and the single vascular strand. These crystals, along with those in the two stomiums, form at precise times during anther development. Contrary to the more numerous suggestions that crystals protect against predators or are metabolic waste products, we believe their formation aids in degradation and weakening of the cell walls between the locules and, thus, contributes to the release mechanism for the pollen.     

Die neue KlasseChlamydophyceae,eine natürliche Gruppe der Grünalgen (Chlorophyta)

On the basis of the present concept of theChlorophyta, a new class, theChlamydophyceae, is established and described. It includes allVolvocales with cell walls, theTetrasporales and thoseChlorococcales with zoospores of theChlamydomonas-type. The diagnostic features of theChlamydophyceae are as follows. Both, flagellates and zoospores have a cell wall with specific ultrastructure which lacks cellulose. The cell wall of the gametes is thrown off before or during fusion. Protoplast divisions are multiple (schizogonic); binary (schizotomic) fissions do not occur. Sporangia and gametangia are formed already on the monadoid level. In asexual resting stages the old cell wall is incorporated into the cyst wall. The polarized structure of theChlamydomonas-like flagellate is ± maintained in non-motile stages. Contractile vacuoles occur in freshwater species, and only sometimes disappear in adult and old non-motile cells; proper central vacuoles are lacking.—From the morphological point of viewChlamydophyceae can be derived from stages in the life cycle ofChlamydomonas. Relationships with theChlorophyceae s. str. and the other Green Algae are discussed.

Systematische Bemerkungen zu den Grünalgen I.     

ULTRASTRUCTURE AND DEVELOPMENT OF THE MICROSPORANGIUM OF PSEUDOTSUGA MENZIESII (MIRB.) FRANCO. I. DORMANT STAGES

The dormant (mid-November to mid-February) microsporangia of Pseudotsuga menziesii (Douglas-fir) contain pollen mother cells (PMC's) in diffuse diplotene, surrounded by 1–2 layers of tapetal cells and 3–4 layers of microsporangial wall cells. At the beginning of dormancy, PMC's are large and their walls are lysed. The cell walls contain a thick layer of loosely-arranged fibrils which are produced in large vesicles in the PMC cytoplasm and are secreted across the plasma membrane. PMC's contain several layers of rough ER. The inner tangential and the radial walls of the tapetal cells are lysed. During dormancy the PMC's form many new autophagic vacuoles, the chromatin consists of a network of fine threads comprised of medium-sized granules of uniform size and the nucleoli split. The outer tapetal wall is thick and becomes encrusted by an irregular lipid layer. The tapetal cytoplasm is similar to the PMC cytoplasm but is devoid of amyloplasts. The tapetal cytoplasm shows secretory activity at the beginning of dormancy and again near the end of dormancy. The later secretory activity results in the deposition of a spongy material, especially along the radial and inner walls of the tapetal cells. Tapetal cells contain 1–2 large nuclei which show prominent and irregular clumps of chromatin. Subcellular developmental changes occur in the dormant microsporangia of Pseudotsuga in much the same manner as has been reported for Pinus.     

PHLOEM OF PRIMITIVE ANGIOSPERMS. I. SIEVE-ELEMENT ONTOGENY IN THE PETIOLE OF LIRIODENDRON TULIPIFERA L. (MAGNOLIACEAE)

As part of a continuing study of sieve elements in primitive angiosperms, a study of this cell type was undertaken in Liriodendron tulipifera. A typical ontogenetic sequence was observed in which synthetic processes such as wall thickening are followed in time by cellular lysis of nucleus, ribosomes, microtubules, vacuoles, and dictyosomes. This lysis is selective in that certain cellular components (e.g., the plasmalemma) remain unaffected. Concomitant with lysis is the formation of sieve-area pores from plasmodesmata. Comparison of pore size on end and lateral walls indicates that the use of the term “sieve tube” rather than “sieve cell” to describe these elements is appropriate.     

On the cell wall composition ofSaccharomycopsis guttulata

Summary Cells ofSaccharomycopsis guttulata were ruptured by sonic oscillation and the resulting cell walls were purified by washing and centrifugation. The walls contained 43.7% carbohydrate (expressed as glucose), 39.6% protein and a trace of chitin. Paper chromatography of hydrolyzed cell walls showed that glucose and an unknown reducing compound make up the bulk of the carbohydrate fraction. Mannose and glucosamine were present in small amounts. The cell wall composition ofS. guttulata appears to differ considerably from that ofS. cerevisiae.     

Cytochemical evidence of a fungal cell wall alteration during infection of Eucalyptus roots by the ectomycorrhizal fungus Cenococcum geophilum

When the ectomycorrhizal fungus Cenococcum geophilum changes from a saprophytic to a symbiotic stage, its cell wall structure becomes simplified. The external hyphal wall layer which, in the saprophytic stage, is highly reactive to the Gomori-Swift test becomes poorly reactive and can no longer be distinguished from the internal wall layer in the Hartig net hyphae. The intensely stained external wall layer was also absent from pure cultures of Cenococcum geophilum grown on a medium with a low sugar content. This cell wall alteration could be due to a decrease in the amount of melanin or of melanin plus cystine-containing proteins. This change may be necessary for increased nutrient exchange between symbionts through hyphal walls.     

Inhibition of lysosomal fusion with symbiont-containing vacuoles in Paramecium bursaria

Paramecium bursaria harbors several hundred intracellular Chlorella symbionts which remain undigested at the same time that the host cell phagocytizes and digests other organisms. Using electron microscopy and thorotrast labelling, we have shown that secondary lysosomes fuse with food vacuoles, but do not fuse with vacuoles containing symbiotic algae. From these and other data we suggest that the symbiotic algae alter the membrane of the vacuole which surrounds them, thus inhibiting fusion with secondary lysosomes.     

Reduction in vacuolar volume in the tapetal cells coincides with conclusion of the tetrad stage in Arabidopsis thaliana

Although the tapetum is known for its role in the removal of the tetrad wall, a morphological change in the tapetum correlating with such a role has not been described. Here we report that in two ecotypes of Arabidopsis thaliana, Landsberg erecta and Columbia, the vacuoles in tapetal cells underwent progressive enlargement prior to the separation of tetrads but became drastically reduced when tetrads just separated from one another. Such a drastic change in vacuolar volume was not observed in later anther development. We also observed that the walls of associated tetrads were much less stained with Toluidine Blue O than the walls of separate tetrads, indicating that the tetrad walls underwent an alteration during the tetrad stage. Furthermore, we identified the N-terminal propeptide signals for sorting vacuolar proteins in 15 β-1,3-glucanases, five polygalacturonases, and two endocellulases that are expressed in Arabidopsis young floral buds; all three types of the enzymes are known to participate in degradation of the tetrad wall. These results suggest that the tapetal vacuoles might be a storage site for these enzymes prior to their secretion to the anther locule.     

ULTRASTRUCTURAL OBSERVATIONS ON THE TRICHOBLASTS OF EQUISETUM

Equisetum trichoblasts are densely cytoplasmic, containing numerous starch-containing plastids, mitochondria, and concentrations of rough endoplasmic reticulum with attached polysomes. Numerous vesicles of Golgi origin are present, containing a lightly staining fibrillar material; these vesicles appear to fuse with the wall. The outer tangential and radial walls become thickened while the inner tangential wall remains thin with numerous plasmodesmata. As the trichoblasts develop into root hairs, vacuolation occurs, resulting in large vacuoles. This may represent autolytic vacuolation. The cytoplasm of the root hairs is similar to that of the trichoblasts.     

ADVENTITIOUS ROOT DEVELOPMENT FROM THE COLEOPTILAR NODE IN ZEA MAYS L.

Department of Botany and Bacteriology, University of Arkansas, Fayetteville, Arkansas 72701 Zea mays L. root development from the coleoptilar node was observed by light and electron microscopy. Roots developed opposite collateral vascular bundles in the coleoptilar nodal region. Three distinct histogens (stelar, cortical-protoderm, and root cap) became evident in early development. In median sections of the young roots, root cap and cortical regions formed a “hat” configuration over the stelar region. As the root matured, this “hat” developed centripetally to encapsulate the stelar region. Central core cells of the root cap were characterized by having numerous dictyosomes, amyloplasts, vacuoles, and thin cell walls. As these cells matured into outer or peripheral cap cells, the Golgi vesicles became hypertrophied. These hypertrophied vesicles contained a granular PAS-positive material which accumulated between the plasma membrane and the cell wall and formed a thick layer. As the PAS-positive material passed through the cell wall, it changed to a fibrillar texture. A PAS-positive material similar to that in the outer root cap cells was found adjacent to the outer walls of the protodermal cells. In median sections, PAS-positive material was not present in the promeristem region. Root cap cells as well as parent cortical cells were crushed as the young root forced its way through the parent tissue.     

Changes in cell wall composition of deformedras1 − cells ofSchizosaccharomyces pombe

Disruption of theSchizosaccharomyces pombe ras1 gene results in a morphological transformation to large spheres, in contrast to wild-type cells which grow as rods. Chemical analysis of isolated cell walls showed no significant changes in saccharide content but an increase in protein and phosphate contents inras1 ⁻ walls relative to parent walls. Polymers tightly bound to the cell wall were solubilized by SDS treatment. Several compounds with molar mass ranging from 22 to 130 kDa and more were resolved by gel filtration and SDS-PAGE. Among low-molar-mass species, a component moving as a band at 31 kDa was conspicuous inras1 ⁻ cell walls. It was solubilized by heating in Tris-HCl buffer and shown to have a β-1,3-glucanase activity against laminarin. The level of the enzyme was by 30% higher in theras1 ⁻ cell wall than in the wild-type cell wall. This enzyme may participate in the remodelling of the rigid glucan network and account (at least partially) for the aberrant cell shape. Theras1 ⁻ cell wall contained a high level of charged polymers, especially phosphoproteins, raising the appealing possibility thatras1 ⁻ is involved in a putative kinase cascade required to sense and respond to external stimuli destined for the cell wall. Although the present study shows thatras1 loss of function and altered cell wall composition are closely linked defects, it has still to be shown that theras1 protein is directly involved in alterations found in the mutant cell walls.     

Ultrastructural features of the salt gland of Tamarix aphylla L.

Summary The salt gland in Tamarix is a complex of eight cells composed of two inner, vacuolate, collecting cells and six outer, densely cytoplasmic, secretory cells. The secretory cells are completely enclosed by a cuticular layer except along part of the walls between the collecting cells and the inner secretory cell. This non-cuticularized wall region is termed the transfusion are (Ruhland, 1915) and numerous plasmodesmata connect the inner secretory cells with the collecting cells in this area. Plasmodesmata also connect the collecting cells with the adjacent mesophyll cells.There are numerous mitochondria in the secretory cells and in different glands they show wide variation in form. In some glands wall protuberances extend into the secretory cells forming a labyrinth-like structure; however, in other glands the protuberances are not extensively developed. Numerous small vacuoles are found in some glands and these generally are distributed around the periphery of the secretory cells in association with the wall protuberances. Further, an unusual structure or interfacial apparatus is located along the anticlinal walls of the inner secretory cells. The general structure of the gland including the cuticular encasement, connecting plasmodesmata, interfacial apparatus, and variations in mitochondria, vacuoles, and wall structures are discussed in relation to general glandular function.     

STRUCTURE AND DEVELOPMENT OF THE SIEVE ELEMENTS IN PSILOTUM NUDUM

Shoot tissue of Psilotum nudum (L.) Griseb. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. Young sieve elements can be distinguished from contiguous parenchyma cells by their distinctive plastids, the presence of refractive spherules, and the overall dense appearance of their protoplast. The refractive spherules apparently originate in the intracisternal spaces of the endoplasmic reticulum (ER). With increasing age the sieve-element wall undergoes a marked increase in thickness. Concomitantly, a marked increase occurs in the production of dictyosome vesicles, many of which can be seen in varying degrees of fusion with the plasmalemma. Other fibril- and vesicle-containing vacuoles also are found in the cytoplasm. In many instances the delimiting membrane of these vacuoles was continuous with the plasmalemma. Vesicles and fibrillar materials similar to those of the vacuoles were found in the younger portions of the wall. At maturity the plasmalemma-lined sieve element contains a parietal network of ER, plastids, mitochondria, and remnants of nuclei. The protoplasts of contiguous sieve elements are connected by solitary pores on lateral walls and pores aggregated into sieve areas on end walls. All pores are lined by the plasmalemma and filled with numerous ER membranes which arise selectively at developing pore sites, independently of the ER elsewhere in the cell. P-protein and callose are lacking at all stages of development.     

CELL WALL CHEMISTRY AND ULTRASTRUCTURE OF CHLOROCOCCUM OLEOFACIENS (CHLOROPHYCEAE)1,2

Cell walls of Chlorococcum oleofadens Trainor & Bold were examined ultrastructurally and chemically. The wall of zoospores has a uniform 30 nm width and a regular lamellar pattern. Zoospores and young vegetative cell walk exhibit periodicities, consisting of 20 nm ridges on the outer layer. Vegetative cell walls have a variable thickness of Up to 800 nm and are composed of multiple layers of electron dense material. Further, vegetative walk contain a microfibrillar material composed predominantly of glucose and presumed to be cellulose. Except for this cellulose, vegetative cell wall chemistry is very similar to that of Chlamydomemas being composed of glycoprotein rich in hydroxyproline. The hydroxyproline in Chlorococcum walls is linked glycosidically to a mixture of hetrooligosaccharides composed of arabinose and galactose, and in one instance, an unknown 6-deoxyhexose. Altogether, the glycoprotein complex accounts for at least 52% of the wall. The amino acid composition of the walls is stikingly similar to those of widely different plant species. Indirect evidence indicates zoospore cell walls are also chemically similar to those of Chlamydomonas, and like them, are cellulose free. Thus a major chemical difference between zoospore and vegetative cell walk of Chlorococcum is the presence of cellulose in the latter. The contribution of this microfibrillar cellulose to the physical properties of the vegetative wall is discussed.     

CHEMICAL COMPOSITION AND STRUCTURE OF THE CELL WALLS OF THE CONCHOCELIS AND THALLUS PHASES OF PORPHYRA TENERA (RHODOPHYCEAE)1,2

Purified cell walls were prepared from both the conchocelis and thallus phases of Porphyra tenera (Kjellm.). The nitrogen content of cell walls from the conchocelis was significantly greater than that for the thallus cell walls, being 3.35 ± 0.26% and 2.39± 0.03%, respectively. Amino acid analysis revealed important differences. The conchocelis cell wall hydrolyzates were richer in aspartic acid, glutamic acid, methionine, and basic amino acids. The thallus cell wall hydrolyzates, however, contained much more glycine and alanine than did those of the conchocelis. Hydroxyproline was not detected in cell walls of either phase. The neutral sugar content of cell wall hydrolyzates from the thallus was more than double that from the conchocelis being 83.6% and 34.5%, respectively. The former contained predominantly mannose which accounted for 72.2% of the neutral sugars while the latter was principally galactose (49.9%) and glucose (36.4%). Methylation analysis confirmed the presence of cellulose microfibrils in the conchocelis in contrast to xylan microfibrils in the thallus. The results establish that the conchocelis and thallus phases of P. tenera differ markedly in the structure and composition of the cell walls.     

DEVELOPMENTAL ANATOMY OF THE STEM OF WELFIA GEORGII,IRIARTEA GIGANTEA,AND OTHER ARBORESCENT PALMS: IMPLICATIONS FOR MECHANICAL SUPPORT

Changes in stem anatomy with radial position and height were studied for the arborescent palms Welfia georgii, Iriartea gigantea, Socratea durissima, Euterpe macrospadix, Prestoea decurrens, and Cryosophila albida. Vascular bundles are concentrated toward the stem periphery and peripheral bundles contain more fibers than central bundles. Expansion and cell wall thickening of fibers within vascular bundles continues throughout the life of a palm, even in the oldest tissue. Within individual vascular bundles, the fibers nearest the phloem expand first and fiber cell walls become heavily thickened. A front of expanding fibers moves outward from the phloem until all fibers within a vascular bundle are fully expanded and have thick cell walls. Peripheral vascular bundles differentiate first and inner bundles later. In the stem beneath the crown, vascular bundles and ground tissue cells show little or no size increase, but marked cell wall thickening during development for Welfia georgii. Beneath the crown, diameters of peripheral vascular bundles increase more than twofold for Iriartea gigantea, while diameters of central bundles do not increase. In Iriartea stems, ground tissue cells at the periphery elongate to accommodate expanding vascular bundles and cell walls become thickened to a lesser degree than in fibers; central ground tissue cells elongate markedly, but cell walls do not become thickened; and large lacunae form between central parenchyma cells. For Iriartea, Socratea, and Euterpe, sustained cell expansion results in limited, but significant increases in stem diameter. For all species, sustained cell wall thickening results in dramatic increases in stem stiffness and strength.