On the ultrastructure of differentiating secondary xylem in willow

Summary Studies of differentiating xylem inSalix fragilis L. show the immediate cambial derivatives to be ultrastructurally similar. The Golgi apparatus is important at all stages of wall synthesis, possibly producing (amongst other substances) hemicellulose material which is carried to the wall in vesicles or multivesicular bodies. The endoplasmic reticulum also contributes one or more components to the developing wall: at some stages during differentiation the endoplasmic reticulum produces electron opaque bodies which appear to be guided towards the wall by microtubules. Compact structures formed from concentric membranes (myelin-like bodies) have been found joined to rough endoplasmic reticulum, but their presence is not explained.Two types of plasmalemma elaboration occur: invagination of the plasmalemma itself to form vesicles which may contain cytoplasmic material; and vesicles between the plasmalemma and cell wall which are the result of single vesicles or multivesicular bodies traversing the plasmalemma. Both systems provide a means for transporting cytoplasmic material across the plasmalemma.Microtubules have been seen associated with all vesicles derived from the cytoplasm which appear to be moving towards the wall. The presence of microtubules may generally be explained in terms of orientation of vesicles, even if they also happen coincidentally to parallel the supposed orientation of microfibrils in the wall itself. It is possible to resolve connections between the microtubules and the plasmalemma.     

Ultrastructure ofAspergillus nidulans conidia and conidial lomasomes

Summary Lomasomes in the conidia ofAspergillus nidulans can be divided into at least two distinct structures. The first is a twice double membrane bound core of cytoplasmic origin. The outermost membrane of the lomasome becomes incorporated into the plasmalemma as it migrates to rest next to the cell wall. The second lomasome structure appears to be a triangle shaped series of tubules arranged in a parallel fashion. The wide end next to the cell wall connected to the plasmalemma and the opposite end to an element of the endoplasmic reticulum. The term membranosome has been coined to designate this lomasome structure with its function of plasmalemma extension. Various structures of the conidium such as wall, endoplasmic reticulum and the cytoplasmic matrix undergo changes from the conidial chain stage to the free or resting conidial stage. This suggests that after conidiation and before the resting stage, the conidium continues to mature.     

Zostera capensis setchell: III. Some aspects of wall ingrowth development in leaf blade epidermal cells

Summary The development of wall ingrowths in leaf blade epidermal cells of the marine angiospermZostera capensis was studied by electron microscopy. Prior to the appearance of ingrowths long profiles of endoplasmic reticulum cisternae become arranged peripherally closely following the contours of the walls. The plasmalemma assumes a wavy appearance and in regions where wall ingrowths first start forming (i.e., along the radial, inner tangential and transverse walls) the plasmalemma becomes separated from the walls by an undulating extracytoplasmic space. Small, irregular projections of secondary wall material make their appearance here. Paramural bodies, dictyosomes, endoplasmic reticulum (ER) and possibly also microtubules seem to be closely associated with the initiation and subsequent development of wall projections. As the cells mature, new ingrowths arise in a centrifugal direction along the radial and transverse walls. When wall ingrowths reach a certain stage of their development, mitochondria become strongly polarized towards them and become closely associated with the plasmalemma which ensheaths the ingrowths. There is often also a close association between ER cisternae and the involuted plasmalemma of the wall projections. Initially ingrowths are slender, curved structures, but become more complex as the cells mature. Ingrowths are most extensively developed along the inner tangential and transverse walls. As epidermal cells age there is a loss of wall material from the ingrowths. The probable significance of the formation of wall ingrowths in the epidermal cells is also discussed.     

Pattern morphogenesis in cell walls of diatoms and pollen grains: a comparison

Summary Mechanisms acting in pattern morphogenesis in the cell walls of two distant groups of plants, pollen of spermatophytes and diatoms, are compared in order to discriminate common principles from plant group- and wall material-specific features. The exinous wall in pollen is sequentially deposited on the exocellular side of the plasmalemma, while the siliceous wall in diatoms is formed intracellularly within an expanding silica deposition vesicle (SDV) which is attached to the internal face of the plasmalemma. Two levels of patterning occur in diatom and pollen walls: the overall pattern stabilises the wall mechanically and is apparently initiated in both groups by the parent cell, and a microtubule-dependent aperture and portula pattern created by the new mitotic (diatoms) or meiotic (pollen) cells. The parent wall in diatoms, and also the callosic wall in microspores, functions as anchor surfaces for transient, species-specific patterned adhesions of the plasmalemma to these walls, involved in pattern and shape creation. Patterned adhesion and exocytosis is blocked in pollen walls where the plasmalemma is shielded by the endoplasmic reticulum at the sites of the future apertures. In diatoms, wall patterning is uncoupled from the formation of a siliceous wall per se when the SDV and its wall is formed without contact to the the plasmalemma. Conversely, a blue-print pattern laid out in advance along the plasmalemma can be found in several diatoms. This highlights the key function of the plasmalemma and its associated membrane skeleton (fibrous lamina), and its orchestrated co-operation with elements of the radial filamentous cytoskeleton (actin?) in pattern formation. The role of microtubules during generation of the overall pattern may be primarily a transport and stabilizing function. Auxiliary organelles (spacer vesicles, endoplasmic reticulum, mitochondria) involved in diatoms for shaping the SDV, and a mechanism adhering and disconnecting this SDV together with spacer organelles in a species-specifically controlled sequence to and from the plasmalemma, are unnecessary for pollen wall patterning. The precise positioning of the portula pattern in diatom walls is discussed with respect to their role as permanent anchors of the cytoplasm to its wall, and in providing spatial information for nucelar migration and the next cell division, whereas apertures in pollen are for single use only.Abbreviations AF actin filaments - C/Ca callose - CF cleavage furrow - cPL cleavage plasmalemma - DV dense vesicles - ER endoplasmic reticulum - ET epitheca - HT hypotheca - mPL folded plasmalemma - MT microtubules - MTOC microtubule organising centre - PEV primexine (matrix) vesicles - PL plasmalemma - SDV silica deposition vesicle - Si silica - SL SDV-membrane - SPV spacer vesicles Dedicated to Prof. Dr. Dr. h.c. Eberhard Schnepf on the occasion of his retirement     

The cell wall-plasmalemma interface in sieve tubes of barley

Both thick- and thin-walled sieve tubes in leaf-blade veins of Hordeum vulgare L. exhibit a distinct, electron-opaque inner wall layer after fixation in glutaraldehyde-osmium tetroxide and staining with uranyl acetate and lead citrate. This inner wall layer is thickest at the sieve plates and lateral sieve areas where it is permeated by a labyrinth of tubules formed by the plasmalemma. Along the lateral walls between sieve areas the inner wall layer apparently is penetrated by numerous microvilli-like evaginations of the plasmalemma, giving the cell wall-plasmalemma interface the appearance of a brush border. It is suggested that a similar brush-border-like structure may occur at the cell wall-plasmalemma interface of sieve elements in a wide variety of vascular plants.Abbreviation ER endoplasmic reticulum     

Wall structure and bud formation in Rhodotorula glutinis

Summary The first stage in the formation of a bud in Rhodotorula glutinis is the production of a tapered plate of new wall material between the existing wall and the plasmalemma. The parent cell wall is lysed, allowing the bud to emerge enveloped in this new wall. Mucilage is synthesised to surround the developing bud. As the bud grows a septum forms centripetally dividing the two cells. When the daughter cell reaches maximum size the septum cleaves along its axis, producing the bud scar on the parent cell and the birth scar on the daughter cell. The birth scar is obliterated later as the wall of the young cell grows. A system of endoplasmic reticulum and vesicles is found in young buds and is thought to be responsible for the transport of wall material precursors.     

Freeze-substitution of dehydrated plant tissues: Artefacts of aqueous fixation revisited

Summary This investigation assessed the extent of rehydration of dehydrated plant tissues during aqueous fixation in comparison with the fine structure revealed by freeze-substitution. Radicles from desiccation-tolerant pea (Pisum sativum L.), desiccation-sensitive jackfruit seeds (Artocarpus heterophyllus Lamk.), and leaves of the resurrection plantEragrostis nindensis Ficalho & Hiern. were selected for their developmentally diverse characteristics. Following freeze-substitution, electron microscopy of dehydrated cells revealed variable wall infolding. Plasmalemmas had a trilaminar appearance and were continuous and closely appressed to cell walls, while the cytoplasm was compacted but ordered. Following aqueous fixation, separation of the plasmalemma and the cell wall, membrane vesiculation and distortion of cellular substructure were evident in all material studied. The sectional area enclosed by the cell wall in cortical cells of dehydrated pea and jackfruit radicles and mesophyll ofE. nindensis increased after aqueous fixation by 55, 20, and 30%, respectively. Separation of the plasmalemma and the cell wall was attributed to the characteristics of aqueous fixatives, which limited the expansion of the plasmalemma and cellular contents but not that of the cell wall. It is proposed that severed plasmodesmatal connections, plasmalemma discontinuities, and membrane vesiculation that frequently accompany separation of walls and protoplasm are artefacts of aqueous fixation and should not be interpreted as evidence of desiccation damage or membrane recycling. Evidence suggests that, unlike aqueous fixation, freeze-substitution facilitates reliable preservation of tissues in the dehydrated state and is therefore essential for ultrastructural studies of desiccation.Abbreviations LM light microscopy - TEM transmission electron microscopy - CF conventional (aqueous) fixation - FS freeze-substitution - ER endoplasmic reticulum     

Ultrastructure of the cell wall regeneration of isolated protoplasts of Skimmia japonica thunb

Isolated protoplasts obtained from leaves and from stem callus cultures of Skimmia japonica were cultivated for 72 h to regenerate a new cell wall. During this process the structural changes in the protoplasts and at the surface of the plasmalemma were studied in ultrathin sections and after freeze-fracturing and deep-etching.The cultured protoplasts show an apparent increase in cell organelles compared to the freshly isolated protoplasts. In particular, mitochondria, endoplasmic reticulum, and ribosomes, many of them appear as polysomes, become numerous. Moreover, special connections between the ER and the plasmalemma are visible. Most important are the fracture faces of the plasmalemma with two different arrangements of membrane-bound particles: (1) particles in hexagonal arrays and (2) rows of ca. 14 particles. Their orientation usually conforms with that of the regenerated microfibrils of the cell wall. According to these results the following model for microfibril synthesis and orientation in higher plants is proposed: While the cytoplasmic activity is involved in the production of cellulose precursors and enzymes, the hexagonal arrays may respresent specialized regions for the outward passage of these cellulose precursors. The rows of membrane-associated particles may function as a linear enzyme complex (matrix) for microfibril biosynthesis and orientation.Abbreviations ER endoplasmic reticulum - IAA -indolylacetic acid - 2,4-D 2,4-dichlorophenoxy acetic acid     

毛竹茎纤维次生壁形成过程的超微结构观察

利用透射电镜观察了毛竹（Ｐｈｙｌｌｏｓｔａｃｈｙｓ ｐｕｂｅｓｃｅｎｓ Ｍａｚｅｌ）茎纤维发育过程中次生壁的形成过程。纤维发育早期,细胞具有较大的细胞核和核仁;细胞质浓稠,具有核糖体、线粒体和高尔基体等细胞器。随着纤维次生壁的形成,细胞壁加厚,细胞质变得稀薄,内质网和高尔基体的数量明显增加,并且两者共同参与了运输小泡的形成;在质膜内侧可观察到大量周质微管分布。随着次生壁的进一步加厚及木质化,细胞壁     

Ultrastructural Study of Secondary Wall Formation in the Stem Fiber of Phyllostachys pubescens

Ultrastructural changes in secondary wall formation of Phyllostachys pubescens Mazel fiber were investigated with transmission electron microscopy. Fiber developed initially with the elongation of cells containing ribosomes, mitochondria and Golgi bodies in the dense cytoplasm. During the wall thickening, the number of rough endoplasmic reticulum and Golgi bodies increased apparently. There were two kinds of Golgi vesicles, together with the ones from endoplasmic reticulum formed transport vesicles. Many microtubules were arranged parallel to the long axis of the cell adjacent to the plasmalemma. Along with the further development of fiber, polylamellate structure of the secondary wall appeared, with concurrent agglutination of chromatin in the nucleus, swelling and disintegration of organelles, while cortical microtubules were still arranged neatly against the inner side of plasmalemma. Lomasomes could be observed between the wall and plasmalemma. The results indicated that the organelles, such as Golgi bodies together with small vesicles, rough endoplasmic reticulum and lomasomes, played the key role in the thickening and lignification of the secondary wall of bamboo fiber, though cortical microtubules were correlative with the process as well.     

ULTRASTRUCTURE OF GLANDULAR TRICHOMES OF LEAVES OF NICOTIANA TABACUM L., cv XANTHI

Electron microscopy confirms previous light microscope observations that tobacco leaf trichomes are glandular and that there are two different types. Both the tall trichome (multicellular stalk, unicellular or multicellular head) and the short trichome (unicellular stalk; multicellular head) exhibit characteristics common to gland cells—a dense cytoplasm, numerous mitochondria, and little vacuolation. The tall trichome contains structurally well developed chloroplasts and an elaborate network of endoplasmic reticulum. The short trichome contains undifferentiated plastids and endoplasmic reticulum which parallels the nucleus and plasmalemma. Few dictyosomes are seen either in the short trichome or in the tall trichome. The short trichome appears to undergo structural changes concurrently with the appearance of secretory product within the cells. The most noticeable change is the formation of the extraplasmic space between the cell wall and the plasmalemma. Electron dense secretory product is observed between the plasmalemma and the cell wall and within the intercellular spaces.     

SIEVE-ELEMENT ULTRASTRUCTURE IN PLATYCERIUM BIFURCATUM AND SOME OTHER POLYPODIACEOUS FERNS: THE NACREOUS WALL THICKENING AND MATURATION OF THE PROTOPLAST

Sieve elements of various ages were examined in petioles and midribs of Platycerium bifurcatum (Cav.) C. Chr. and Phlebodium aureum (L.) J. Sm., only older ones in similar parts of leaves of Polypodium schraderi Mett. and Microgramma lycopodioides (L.) Copel. Nacreous walls apparently are formed by most, if not all, protophloem and metaphloem sieve elements in all four species. In Platycerium and Phlebodium nacreous wall formation is closely correlated with the appearance of numerous membranes or vesicles in the region of the wall. These extracytoplasmic membranes apparently are derived from protrusions of the plasmalemma. After the nacreous layer is fully thickened, many endoplasmic reticulum (ER) membranes apparently end up outside the plasmalemma of Platycerium, where they degenerate and gradually intergrade in appearance with the fibrillar material comprising the nacreous thickening. In Phlebodium, Polypodium, and Microgramma the ER forms multivesicular bodies. As the cells approach maturity, the membranes delimiting the multivesicular bodies fuse with the plasmalemma and their vesicular contents, which are not discharged into the region of the wall, disappear. Gradually, the nacreous layer decreases in thickness and disappears. At maturity the enucleate sieve-element protoplasts of all four species are essentially similar. They are lined by a plasmalemma and a parietal, anastomosing network of ER and contain both plastids and mitochondria. The plastids in Polypodium and Microgramma are chloroplasts, but those in Platycerium and Phlebodium lack grana and intergrana lamellae.     

Morphology ofEntomophthora muscae protoplasts grownin vitro

Summary Entomophthora muscae (C.) Fres. can be grownin vitro as protoplasts. Light and electron microscopical studies of thein vitro developed protoplasts have demonstrated the absence of an organized wall over the protoplasmic Con A-positive membrane at all stages of growth. The cytological organization is typical of the Entomophthorales with condensed chromatin in the interphase nuclei and small eccentric metaphase spindles. Long strands of endoplasmic reticulum, microubules and vesicles surrounding the plasmalemma may be involved in maintaining the precise shape ofE. muscae protoplast. Starvation of the fungus induces the formation of hyphal bodies after deposition of Con A- and WGA-positive wall material at the plasmalemma surface.Abbreviations Con A concanavalin A - DH Drosophila cell culture medium - FITC fluorescein isothiocyanate - GLEN glucose-lactal-bumin-yeast extract-NaCl culture medium for protoplasts - HBL hyphal body-like protoplasts - MM Mitsuhashi and Maramorosch' insect cell culture medium - PATAg periodic acid-thiocarbohydrazide-silver proteinate technique - PBN phosphate buffer with NaCl - S spherical protoplasts - WGA wheat germ agglutinin     

Ultrastructure of Aerial Hyphae in Linderina pennispora

Ultrathin sections of the aerial hyphae of Linderina pennispora,fixed in glutaraldehyde, show the wall to be composed of anouter electron-dense layer and an inner less-dense one. Thefully developed septum is plugged by electrondense material.A similar septum exists at the base of the pseudophialide. Cytoplasmiccontinuity on both sides of the septum is maintained by theplasmalemma which passes through the septal pore around theperiphery of the plug. Membrane-lined vesicles may occur inthe wall, between the wall and the plasmalemma and internalto the plasmalemma. Ribosome-like particles are numerous inthe cytoplasm and endoplasmic reticulum is virtually absent.     

A new interpretation of plasmodesmatal ultrastructure

Summary It is shown that simple, unbranched, plasmodesmata between young xylem ray cells of willow have no direct intercellular continuity apart from the plasmalemma which limits the cytoplasm and lines the plasmodesmatal canal. Each plasmodesma is traversed by a 200 Å diameter tubule (the desmotubule) which has a wall with probably 11 subunits arranged around a central cavity through which runs a 40 Å diameter rod. This rod is connected to the inside of the tubule wall, by fine filaments. At the ends of each plasmodesma the plasmalemma and cell wall are closely appressed to the tubule, thus precluding direct continuity between the cytoplasm of adjacent cells. Through the central part of the plasmodesmata the tubule is separated from the plasmalemma by a 90–100 Å wide gap. Cytoplasmic microtubules in the same tissue have a diameter of approximately 250 Å and a wall probably composed of 13 subunits: both desmotubules and cytoplasmic microtubules therefore have a centre-to-centre subunit spacing of about 47 Å. It is suggested that the desmotubules are not microtubules but may be nuclear spindle fibres which become trapped in the wall during cell plate formation. The endoplasmic reticulum, while closely approaching the plasmodesmata, is not continuous across them. It is thought most unlikely that the endoplasmic reticulum traverses plasmodesmata, as the dimensions of the central tubule — found here as well as by other workers — are smaller than those which would be expected to allow a stable molecular configuration in a unit membrane. The plasmalemma, where it lines the plasmodesmatal canal, appears to have particulate subunits in the outer opaque layers and the presence of these subunits may be attributable to the need for stability in membranes arranged about so small a radius.     

ULTRASTRUCTURE AND INITIATION OF WALL PATTERN IN PEDIASTRUM BORYANUM

The reticulate pattern in the wall of Pediastrum boryanum emerges rapidly during wall formation following aggregation of the swarming zoospores to form the coenobium. Electron micrographs during colony formation show that microtubules, present during the motile phase and aggregation, are gone prior to wall formation and probably do not participate in wall pattern regulation. A single dictyosome lies adjacent to the nucleus and from blebs of the nuclear membrane receives vesicles at its forming face. Vesicles formed at the maturing face have not been observed to contribute to the cell wall. Electron-lucent patches occur in the plasma membrane prior to wall formation. The first indication of a reticulate pattern in wall development is the deposition on the plasma membrane of interconnected plaques of outer wall material at the corners of hexagons. The sites of the plaques may correspond to clusters of ribosomes on endoplasmic reticulum underlying the plasmalemma. Following completion of the outer wall the thicker inner wall layer is deposited and within it the reticulate pattern of ridges is soon evident in tangential sections as strips of greater electron density. It is suggested that the pattern of the wall is templated by the plasma membrane.     

ELECTRON MICROSCOPIC OBSERVATIONS OF CULTURED CELLS AND PROTOPLASTS OF AMMI VISNAGA

Protoplasts were prepared from cultured cells of Ammi visnaga (Umbelliferae) by enzymatic digestion of the cell walls and examined microscopically. Staining of fresh protoplasts with Calcofluor and silver hexamine demonstrated the apparent absence of wall material. Protoplasts contained more cell organelles than the whole cells, particularly endoplasmic reticulum and associated polysomes. The plasmalemma of most protoplasts appeared smooth; some protoplasts were connected by structures resembling plasmodesmata. Multinucleates resulting from fusion were frequently observed.     

Ultrastructure of the developing pigment strand of rice (Oryza sativa L.) in relation to its role in solute transport

Summary The developing pigment strand of rice (Oryza sativa L.) was studied by conventional electron microscopy and also by use of thick sections post-fixed with zinc iodide and osmium (ZIO).When the rice caryopsis achieves maximum length, a suberised adcrusting wall layer is laid down over the original primary walls of the pigment strand. Concomitant with suberin deposition a proliferation of tubular endoplasmic reticulum occurs in the cytoplasm giving rise to numerous interconnected vesicles which bear ribosomes. The vesicles in the general cytoplasm retain their ribosomes while those close to the wall become smooth and contain an electron-opaque granular material which is eventually deposited to the outside of the plasmalemma. This granular material may be the precursor(s) from which suberin is polymerised. The suberised wall attains about six times the width of the original primary wall and plasmodesmata, which traverse both primary wall and suberised wall layers, become greatly elongated.Lipid bodies increase in both size and frequency during development, eventually coalescing to form a complete plug across the pigment strand and occluding the symplast of this tissue. The significance of these ultrastructural observations is discussed in relation to the previously demonstrated role of the pigment strand as a translocation pathway for water and assimilates during grain filling.Abbreviations ER endoplasmic reticulum - ZIO zinc iodide-osmium fixation     

ULTRASTRUCTURAL EVIDENCE FOR SECRETORY PHASES IN THE LIFE HISTORY OF STIGMATIC HAIRS OF COTTON: A DRY TYPE STIGMA

Stigmatic hairs of the cotton flower were studied through their developmental stages up to anthesis. Stigmatic hairs invariably develop from a densely straining band of epidermal cells opposite the transmitting tissue cells. At anthesis, these are single cell structures measuring up to 300 μm long. At the 5-mm stage of stylar length (7–10 days before anthesis), some stigmatic hair cells begin to accumulate an osmiophilic substance between the plasmalemma and the cell wall, possibly synthesized in the endoplasmic reticulum. This material is apparently never secreted outside the cell wall. Immediately following this secretory phase in some stigmatic hair cells a second secretory phase starts. A dense osmiophilic substance, different in appearance from the previous phase, accumulates in the vacuoles of each hair cell. Concomitantly, dimorphism develops in the cytoplasmic densities of stigmatic hair. Some stigmatic hair cytoplasm appears very dense and shows signs of degeneration while other cytoplasm appears normal. A third secretory phase, which begins at anthesis, occurs in the normal hair cells. This phase is characterized by enhanced activity in the cytoplasm of the endoplasmic reticulum and Golgi apparatus. Large vesicles containing granular material are seen fusing with the plasmalemma. Coincident with this activity there is dissolution of the middle layers of the cell wall and the cuticle is ruptured at various points. The dense osmiophilic substance that had accumulated in the vacuole breaks down into fine granular material. Significance of these changes is discussed in relation to the pollen germination mechanism on the dry type stigma of cotton.     

Fine Structure of Botrytis fabae Sardina Conidia

The fine structure of Botrytis fabae conidia was studied usinga variety of electron-microscope techniques. The spore walllacks conspicuous ornamentation and consists of microfibrilsembedded in a granular matrix. The two distinct wall layersseen in chemically fixed sections cannot be detected in cross-fracturedreplicas; the two layers are probably structurally similar.The outer surface of the plasmalemma is covered with branchedinvaginations and two kinds of particles. Three distinct typesof particles are present on the inner surface of the plasmalemma.In freeze-etched replicas nuclei, vacuoles, and other organellesalways appear smoothly rounded. Small vesicles pass throughthe plasmalemma into the cell wall. Particles approximately10 nm in diameter occur in compact rows on the cristae of cross-fracturedmitochondria: dense spherical particles, probably of calciumphosphate, are present in chemically fixed mitochondria. Prevacuolesand vesicles with membranous inclusions can be seen in bothcross-fractured replicas and chemically fixed sections. In cross-fracturedreplicas vacuoles and lipid bodies are frequently joined bystrands of endoplasmic reticulum.