The fine structure of aleurone cells in the soybean seed coat

Summary The morphology and fine structure of aleurone cells of soybean [Glycine max (L.) Merr.] seed coats were analyzed with transmission electron microscopy for the period of rapid seed fill up to physiological maturity. Thin sections and freeze-fracture replicas were prepared for each stage. The aleurone is a tissue lining the embryo sac and consists of a single layer of cells attached to the aerenchyma of the seed coat proper. During seed fill, aleurone cells contained numerous Golgi-derived vesicles in the basal region of the cytoplasm that were either free or attached to the plasma membrane along the lateral and basal regions of the cell wall. Correspondingly, the Golgi apparatus were well developed with individual dictyosomes having 5 to 8, highly fenestrated stacked cisternae. The degree of fenestration along the periphery of each cisterna increased from the cis to trans region. Rough endoplasmic reticulum (RER) was also abundant, often consisting of up to 30, stacked swollen cisternae which occupied large regions of cytoplasm. Plasmodesmata which connected adjacent aleurone cells was not observed along the dorsal walls of aleurone cells that faced aerenchyma. At physiological maturity, dictyosome cisternae were less fenestrated and had fewer associated secretory vesicles. Stacked lamellae of RER were absent, being replaced by short tubular cisternae and small vesicles. At physiological maturity, the aleurone cells had thick walls, and contained numerous lipid bodies in apposition to the plasma membrane. The cytoplasm appeared densely stained in thin-sections and contained protein bodies and amyloplasts with large starch grains. We conclude that during the period of rapid seed fill aleurone cells produce, package, transport and secrete vesicular contents toward the embryo, that is followed at physiological maturity by the storage of lipid, protein and starch in the same cells. The embryo is the most likely destination for secretory products during the period of rapid seed fill. The fate of the stored food reserves in aleurone cells at physiological maturity may be analogous to that of aleurone tissue of grasses, being utilized during imbibition for processes important to germination.     

A correlated thin-section and freeze-fracture analysis of guinea pig adrenocortical cells

Comparison of the fine structural features of guinea pig adrenocortical cells as seen in thin sections with those revealed by freeze-fracture confirms the structural appearance of steroid-secreting cells as interpreted from thin sections and reveals significant new features of the membranous organelles. Smooth-surfaced endoplasmic reticulum appears as a network of tubules, interwoven or in parallel, and as cisternae, fenestrated and non-fenestrated. These elements are tightly packed in the deeper cortical cells, excluding other organelles from their domain. Tubules and fenestrated cisternae possess randomly distributed intramembranous particles on their PF faces, while closely packed non-fenestrated cisternae possess aggregates of particles interspersed with aparticulate regions on their PF faces. These differences in particle distribution suggest functional specialization among the various forms of reticulum. Mitochondria appear as elongated structures of varying shape. Freeze-fracture reveals that all their cristae have circular origins from the inner membrane. Sinuous tubules, which appear as tubules in section, and straight tubules, which appear as lamellae in section, arise from single sites. Flattened sac-like cristae may have multiple circular origins. Definite contact points seen between inner and outer membranes may facilitate passage of molecules, including steroids, into the mitochondrial compartments. Lysosomes and peroxisomes, which are easily identified in thin sections with the aid of cytochemistry, are difficult to identify with certainty by freeze-fracture. Single membrane-bound granules of slightly smaller diameter than mitochondria may represent lysosomes. Smaller granules interconnected with the tubular reticulum, as well as dilated regions of this organelle, may represent peroxisomes. Plasma membranes show no indication of tight junctions but do have abundant gap junctions which show a zonal differentiation: small gap junctions throughout the cortex, medium-sized regularly shaped gap junctions in zona fasciculata externa, and large irregular gap junctions in zona fasciculata interna and zona reticularis. The large junctions cover planar areas as well as surfaces of projections of one cell into another. Such junctions may allow passage of ions as well as of low-molecular-weight substances between the cells, facilitating or even amplifying the response to trophic hormone stimulation.     

Ca2+-binding sites and phosphatase activities in sieve element reticulum and P-protein of chick-pea phloem. A cytochemical and X-ray microanalysis survey

Summary In stem ofCicer arietinum, the loss of ribosomes attached to the rough ER cisternae during sieve element ontogeny results in the formation of sieve element reticulum (SER). By enhancing contrast of the SER, the OsFeCN postfixation/staining of material prefixed in glutaraldehyde in presence of calcium enables a good visualization of this membrane system. The pattern of staining in the SER is slightly lower when Mg²⁺ is substituted for Ca²⁺. These results support the view that the OsFeCN staining requires divalent cations and that the SER can accumulate Ca²⁺. The detection of Ca²⁺ by means of the pyroantimonate method in conjunction with X-ray microanalysis and the cytochemical localization of Ca²⁺ -ATPase in the SER cisternae provides evidence for Ca²⁺ sequestration by the SER. On the other hand, Ca²⁺-binding sites and ATPase activity are localized in P-protein. The ability to bind Ca²⁺ seems to enable the SER to function as an effective Ca²⁺ sink which may participate—with the sieve tube plasma membrane and mitochondria—in the maintenance of low Ca²⁺ concentration in phloem sap. In addition, the close association between P-protein and SER membranes exhibiting Ca²⁺-binding capabilities suggests that a Ca²⁺-mediated functional relationship may exist between the two structures. It is postulated that the SER may play a role in putative Ca²⁺ control of P-protein organization.Abbreviations SER sieve element reticulum - ER endoplasmic reticulum - P protein, phloem protein - OsFeCN method, osmium tetroxide-ferricyanide method - EDTA ethylenediamine tetraacetic acid - EGTA ethyleneglycol-bis-(-aminoethyl ether) N,N-tetraacetic acid - ATP adenosine 5-triphosphoric acid - ATPase adenosine triphosphatase - PCMB p chloromercuribenzoate - IDP inosine diphosphate     

Ultrastructural features of well-preserved and injured sieve elements: Minute clamps keep the phloem transport conduits free for mass flow

Summary After chemical fixation following two different preparation procedures, the ultrastructure of mature sieve elements (SEs) was systematically compared in the transport phloem ofVicia faba leaves andLycopersicon esculentum internodes. The SEs in samples obtained by gentle preparation were well preserved, while those in conventionally prepared samples were generally injured. (1) In well-preserved SEs, parietal P-proteins were associated with cisternae of the SE endoplasmic reticulum (ER). Additionally, theV. faba SEs had crystalline P-proteins, and a homogeneous network of filamentous P-proteins occurred in the lumen of theL. esculentum SEs. In injured SEs, all P-proteins were dispersed. (2) In well-preserved SEs, stacked ER cisternae associated with P-proteins lay also on the sieve-plate walls, but passages were kept free in front of the sieve pores. Injured SEs lacked these orderly arranged deposits. Instead, irregular filamentous and membranous materials occluded the sieve pores. (3) In well-preserved SEs, the sieve-pore lumen was free of obstructions, apart from small, lateral coatings of P-proteins. Sieve pores in injured SEs were always occluded. (4) The SE organelles and, in tomato SEs, also the parietal ER located at the longitudinal walls were firmly attached in the SE periphery and stayed in place after injury. The stable parietal attachment is likely exerted by minute, clamplike structures which link the outer membranes of the SE components with one another or to the SE plasma membrane. Single, straight clamps with a length of about 7 nm anchored the SE components directly to the SE plasma membrane. The connections between adjacent SE organelles and/or parietal ER cisternae were mostly twice as long (about 15 nm) and often were branched. Presumably, the long, branched clamps were constituted by the interaction of opposite short clamps. The ultrastructural results are discussed with respect to SE functioning.     

Microtubule-associated smooth endoplasmic reticulum in the frog's brain

Summary Electron microscopic studies of neural processes in the cerebellum, optic tectum, and cerebral hemisphere of the frog reveal a distinctive system of SER cisternae lying at intervals (commonly 1–2 m apart) perpendicular to the long axis of axons and dendrites, interconnected by tubular, longitudinally orientated SER elements, and in direct continuity with the outer membrane of mitochondria. The transverse cisternae are fenestrated, with a single mierotubule (or rarely, two) passing through the centre of each 50–75 nm fenestration. Extensions of the SER-microtubule complex may be located parasynaptically in axon terminals and dendrites. The SER of dendritic spines also appears to be continuous with the fenestrated cisternae.Possible roles for the specialized SER (particularly of the parasynaptic extensions), such as calcium ion sequestration and ATP or monoamine oxidase transport, are discussed.Thanks are due to Profs. E. G. Gray and J. Z. Young for helpful discussion and to Mrs. N. Morgan and Mr. R. Boddy for technical assistance.     

Cell-specific expression of two arabinogalactan protein epitopes recognized by monoclonal antobodies JIM8 and JIM13 in maize roots

Summary The cell-specific expression of two arabinogalactan protein (AGP) epitopes recognized by monoclonal antibodies JIM8 and JIM13 is reported in maize roots. Employing immunofluorescence and immunogold electron microscopy, the JIM8 antibody was shown to label exclusively protophloem sieve elements, while the JIM13 antibody labelled sieve elements very strongly and adjacent pericycle and companion cells, as well as sloughing root cap cells less strongly. Since the labelling of sieve elements with JIM8 antibody was specific and did not spread to other cell types during root development, it is concluded that this AGP epitope can serve as a specific marker of these specialized cells within the maize root. In the case of the AGP epitope recognized by JIM13 antibody, part of the immunofluorescence label was also found to be associated with cytoplasmic strands in the pericycle and sloughing root cap cells. Immunogold-labelling of sieve elements revealed the association of both AGP epitopes (JIM8 and JIM13) with cortical sieve element reticulum and plasma membranes. Labelling of sieve element reticulum was prominent at its domains of adhesion to the plasma membrane, P-type plastids, and mitochondria. Based on our subcellular studies, we propose a new function of AGP epitopes in endomembrane recognition and adhesion within the sieve elements of maize roots.Abbreviations AGP arabinogalactan protein - SER sieve element reticulum     

Fine structure of the sensory epithelium of guinea-pig organ of Corti: Subsurface cisternae and lamellar bodies in the outer hair cells

The fine structure of subsurface cisternae and lamellar bodies in the outer hair cells of the guinea-pig organ of Corti was studied with thin sections and freeze-fracture replicas. Subsurface cisternae in the outer hair cells consist of multilayers along the lateral plasma membrane of the cell. The outermost layer is a flattened cistern in the upper part of the supranuclear region, but comprises a series of tubules in the lower part. Deeper layers are fenestrated cisternae in which disc-like areas are found in the upper part of the supranuclear region. Lamellar bodies consist of concentric layers of fenestrated cisternae and are located in the apical cytoplasm beneath the cuticular plate. They are continuous with the subsurface cisternae. In the supranuclear cytoplasm, bulges of the subsurface cisternae and the lamellar bodies are found. Dilated cisternae are also present. Some dilated cisternae contain many small vesicles, which display acid phosphatase activity. The dilated cisternae are considered as forms of the bulges undergoing transformation into multivesicular bodies. The possible role of the lamellar bodies, and the origin and fate of the subsurface cisternae are discussed.     

Ca2+-sequestering smooth endoplasmic reticulum in an invertebrate photoreceptor. I. Intracellular topography as revealed by OsFeCN staining and in situ Ca accumulation

Two ultrastructural approaches were used in photoreceptor cells of the leech, Hirudo medicinalis, to (a) investigate the intracellular topography of the smooth endoplasmic reticulum (SER) and (b) identify among the various subregions of the SER those which might function as Ca-sequestering sites. When the cells are prefixed with CaCl2-containing glutaraldehyde and postfixed with osmium tetroxide-ferricyanide (OsFeCN), only a part of the total SER is specifically stained. The stained SER cisternae include the submicrovillar cisternae (SMC), subsurface cisternae (SSC), the nuclear envelope, Golgi-associated SER, paracrystalline SER, and SER associated with glycogen areas. An extensive tubular SER cisternal system always remains unstained. When the cells are permeabilized by saponin and subsequently incubated with Ca2+, MgATP, and oxalate, the SMC (Walz, 1979, Eur. J. Cell Biol. 20:83-91), the SSC and the nuclear envelope contain electron-opaque Ca-oxalate precipitates indicating their ability to function as an effective Ca2+ sink. The results show that the very elaborate SER in this photoreceptor cell includes many functionally heterogeneous subregions. Of special physiological significance are those components (SMC and SSC) which are effective in Ca2+-buffering in the immediate vicinity of the plasma membrane.     

Cytochemical localization of adenosine triphosphatase in the phloem of Pisum sativum and its relation to the function of transfer cells

The cytochemical localization of ATPase in differentiating and mature phloem cells of Pisum sativum L. has been studied using a lead precipitation technique. Phloem transfer cells at early stages of differentiation exhibit strong enzyme activity in the endoplasmic reticulum (ER) and some reaction product is deposited on the vacuolar and plasma membranes. As the phloem transfer cells mature and develop their characteristic wall structures, strong enzyme activity can be observed in association with the plasma membranes and nuclear envelopes. Mature phloem transfer cells with elaborate cell-wall ingrowths show ATPase activity evenly distributed on plasma-membrane surfaces. Differentiating sieve elements show little or no enzyme activity. When sieve elements are fully mature they have reaction product in the parietal and stacked cisternae of the ER. There is no ATPase activity associated with P-protein at any stage of sieve-element differentiation or with the sieve-element plasma membranes. It is suggested that the intensive ATPase activity on the plasma membranes of the transfer cells is evidence for a transport system involved in the active movement of photosynthetic products through these cells.Key to labeling in the figures ER endoplasmic reticulum - P parenchyma cell - PP P-protein - SE sieve element - SPP sieve-plate pore - TC transfer cell     

Ultrastructural localization of calsequestrin in rat skeletal muscle by immunoferritin labeling of ultrathin frozen sections

The ultrastructural localization of calsequestrin in rat skeletal muscle (gracilis) was determined by indirect immunoferritin labeling of ultrathin frozen sections. Calsequestrin was found in the lumen of transversely and longitudinally oriented terminal cisternae but was absent from most of the longitudinal sarcotubules and the fenestrated sarcoplasmic reticulum. Calsequestrin was occasionally observed in vesicular structures found in the central region of the I band. Since calsequestrin is believed to provide the major site of Ca2+ sequestration in the sarcoplasmic reticulum, the present results support the view that Ca2+, transported to the lumen of the sarcoplasmic reticulum, is preferentially sequestered in the terminal cisternae, but they also suggest that additional Ca2+ sequestration may occur near the center of the I band.     

Changes in the Nucleus and the Endoplasmic Reticulum during Differentiation of a Sieve Element in Mimosa pudica L.

The differentiating sieve element in stem and leaf phloem ofMimosa pudica L. was examined with the electron microscope.The nucleus of a sieve element at first resembles that of acompanion cell, except that its nucleolus is smaller. The firstindication of approaching disintegration is a change in theappearance of the nuclear envelope. With the fixatives used,its profile becomes wider and clearer, giving the impressionof having been dilated. Subsequently, the nuclear contents disintegrateand the envelope breaks apart. No nuclear remnants are discerniblein mature sieve elements. The endoplasmic reticulum (ER) ofyoung sieve elements consists of cisternae-bearing ribosomes.At the beginning of nuclear disintegration ER cisternae becomestacked, with electron-dense material accumulating between thecisternae. The stacking assumes various forms. Some of the stackedER is independent of cell organelles and occurs next to theplasmalemma along the cell wall. Part of the ER becomes stackedagainst the plastids or the nucleus. There is also unstackedER which forms an anastomosing system along the plasmalemma.All of the modified ER bears no ribosomes. The anastomosingER appears to persist longer than the stacked ER.     

Preparation and morphology of sarcoplasmic reticulum terminal cisternae from rabbit skeletal muscle

We have developed a procedure to isolate, from skeletal muscle, enriched terminal cisternae of sarcoplasmic reticulum (SR), which retain morphologically intact junctional "feet" structures similar to those observed in situ. The fraction is largely devoid of transverse tubule, plasma membrane, mitochondria, triads (transverse tubules junctionally associated with terminal cisternae), and longitudinal cisternae, as shown by thin-section electron microscopy of representative samples. The terminal cisternae vesicles have distinctive morphological characteristics that differ from the isolated longitudinal cisternae (light SR) obtained from the same gradient. The terminal cisternae consist of two distinct types of membranes, i.e., the junctional face membrane and the Ca2+ pump protein-containing membrane, whereas the longitudinal cisternae contain only the Ca2+ pump protein-containing membrane. The junctional face membrane of the terminal cisternae contains feet structures that extend approximately 12 nm from the membrane surface and can be clearly visualized in thin section through using tannic acid enhancement, by negative staining and by freeze-fracture electron microscopy. Sections of the terminal cisternae, cut tangential to and intersecting the plane of the junctional face, reveal a checkerboardlike lattice of alternating, square-shaped feet structures and spaces each 20 nm square. Structures characteristic of the Ca2+ pump protein are not observed between the feet at the junctional face membrane, either in thin section or by negative staining, even though the Ca2+ pump protein is observed in the nonjunctional membrane on the remainder of the same vesicle. Likewise, freeze-fracture replicas reveal regions of the P face containing ropelike strands instead of the high density of the 7-8-nm particles referable to the Ca2+ pump protein. The intravesicular content of the terminal cisternae, mostly Ca2+-binding protein (calsequestrin), is organized in the form of strands, sometimes appearing paracrystalline, and attached to the inner face of the membrane in the vicinity of the junctional feet. The terminal cisternae preparation is distinct from previously described heavy SR fractions in that it contains the highest percentage of junctional face membrane with morphologically well-preserved junctional feet structures.     

A tubulo-cisternal endoplasmic reticulum system in the potassium transporting marginal cells of the stria vascularis and effects of the ototoxic diuretic ethacrynic acid

Summary Sections of metal impregnated tissue and freeze-fracture have been used to examine intracellular membrane systems in marginal cells of the stria vascularis in mammalian cochleae. A continuous network of elements of the smooth endoplasmic reticulum was revealed. Notable features of this system were a series of flattened cisternae just inside and parallel with the lateral plasma membrane in continuity with an apical network of tubules, cisternae and sheets oriented in parallel with the luminal membrane. The whole system was closely associated with mitochondria. These characteristics suggest that the potassium transporting marginal cells possess a tubulo-cisternal endoplasmic reticulum (TER) like that found in many sodium transporting epithelial cells. The lateral elements of the TER dilated, appearing like vacuoles, and opened to the lateral extracellular space in response to the effects of ethacrynic acid. This diuretic impairs ion transport in the stria vascularis. It is suggested that the TER in marginal cells is involved in the transport of ions and fluid from the cell to the intercellular space when ion balance is disturbed and may play a role in cell volume regulation.This work was supported by the Medical Research CouncilPart of this work was presented at the 18th Workshop on Inner Ear Biology, Montpellier, September, 1981     

A special pattern of the smooth endoplasmic reticulum in the kidney of the snail Cryptomphalus aspersa.

A special structural pattern of the smooth endoplasmic reticulum (SER) has been observed in the kidney of the snail Cryptomphalus aspersa. Two types of cells (clear and dark) cover the foldings of the renal sac; the dark cells are by far the most numerous. A cisterna of SER enveloping the nucleus appears invariably in both types of cells, with no disruptions, or small ones (from 50 to 90 nm) along its profile. The layer of cytoplasm lodged between the external nuclear membrane and this cisterna is found invariably to be from 0-20 to 0-25 mum in width. Glycogen is abundant in the cytoplasm as alpha particles, and also in the nucleus, but as beta particles. It is noteworthy that absolutely no glycogen is present in the layer of cytoplasm lodged between the nuclear membrane and the surrounding SER envelope. Long profiles of SER are also observed closely approaching and parallel to the plasma membrane of the dark cells. Considering the role of SER in glycogen metabolism in the kidney of the snail, the possible function of these cisternae as a support system ofr enzymes involved in the metabolism of glucides is discussed.     

黄瓜韧皮部的类血影蛋白

以黄瓜 (CucumissativusL .)叶柄为实验材料 ,应用胶体金免疫电镜技术证明类血影蛋白存在于韧皮部的筛管_伴胞复合体中 ,广泛分布于筛分子中的韧皮蛋白纤丝以及筛分子网络结构上 ,并且分布在伴胞的细胞质和线粒体膜以及筛分子与伴胞之间的分支状胞间连丝上 ,表明该蛋白可能由伴胞合成并经由二者之间的胞间连丝运输到筛分子中。用免疫印迹技术证明 ,黄瓜韧皮部汁液蛋白中存在类血影蛋白 ,其分子量约为 2 6 0kD ,与动物细胞中血影蛋白的分子量接近     

Ultrastructural immunolocalization of apolipoprotein B within human jejunal absorptive cells

Apolipoprotein B (apoB) was localized by electron microscopy within absorptive cells of human jejunal biopsy specimens taken fasting and after micellar fat infusion. Nakane's double antibody immunoperoxidase technique was used to label apoB near open cut surfaces of 60-Micrometers fixed tissue slices sectioned by a Ralph knife in a Vibratome. In fasting tissue, apoB label was found within structurally intact peri-mitochondrial rough endoplasmic reticulum (RER) and within Golgi cisternae of absorptive cells covering the tips of jejunal villi. After fat infusion, apoB label was found adjacent to very low density lipoproteins (VLDL) and chylomicrons within apical smooth endoplasmic reticulum (SER). Less label was seen within RER than in fasting absorptive cells, and RER-SER connections containing apoB label were occasionally seen. Expanded Golgi vesicles and cisternae contained VLDL, chylomicrons, and apoB label. Vesicles containing chylomicrons and apoB label were occasionally visualized bordering the lateral plasma membrane in a configuration suggesting exocytosis. Specific apoB label was regularly seen within intercellular spaces and capillaries, but the in vivo significance of this Localization was problematical. These observations suggest that apoB is synthesized in RER, transfers to SER where it is incorporated into new VLDL and chylomicrons, and moves to Golgi cisternae and vesicles to be prepared for exocytosis through the plasma membrane.     

Immunocytochemical localization of hepatic legandin and Z protein utilizing frozen sections for light and electron microscopy.

Ligandin (glutathione-s-transferase) and Z protein are soluble hepatocellular proteins that are involved in the transfer of organic ions, including bilirubin and some hormones and carcinogens from the plasma to the liver. The intracellular distribution of ligandin and Z protein was studied by applying the peroxidase-antiperoxidase procedure of L. A. Sternberger (Immunocytochemistry, Prentice Hall Inc., 1974) to paraffin sections and free-floating 10-micrometers frozen sections that were processed for both light and electron microscopy. Ligandin and Z protein were localized to the cytosol of hepatocytes in association with smooth endoplasmic reticulum (SER), but no reaction product was present between cisternae of rough endoplasmic reticulum. Penetration of reagents was enhanced in 10-micrometers frozen sections and the preservation of subcellular structures was equivalent to thicker, unfrozen sections.     

Differentiation and Ultrastructure of the Protophloem Sieve Elements of Minor Veins in the Maize Leaves: Changes in the Protoplast

Protophloem sieve element differentiation in the minor veins of the maize ( Zea mays L. ) leaves was first evidenced as an increase of the wall thickness, which began in the comers of the cell and then extended to other parts of the wall, and the appearance of long rough endoplasmic reticulum cistemae distributed throughout the cytoplasm, and then the presence of characteristic crystalloid inclusions within the plastids. As differentiation progressed, long cisternae of rough endoplasmic reticulum appeared to transform into shorter forms and eventually aggregated into small stacks, losing their ribosomes during the process. The nuclei degenerated, although frequently persisted until very late in differentiation the stages of maturation, as darkly stained amorphous aggregates surrounded by double nuclear envelope or only inner membrane of nuclear envelope. Subsequently, the nuclear envelope collapsed and became discontinuous. At the beginning of nuclear degeneration the perinuclear spaces were partly dilated and sometimes the outer nuclear envelope in the dilated portions then ruptured, and was accompanied by the disappearance of the cytoplasmic portion near it. During the peried of nuclear degeneration, in addition to the endoplasmic reticulum, plastids and mitochondria underwent structural modification, while components such as ribosomes, cytoplasmic ground substances, vacuoles and dictyosomes disintegrated and disappeared. At maturity, the surviving protoplasmic components, including plasmalemma, mitochondria, small stacked smooth endoplasmic reticulum and P-type plastids with crystalloids, became parietal in position. As differentiation of adjacent metaphloem sieve elements proceeded, the protoplasmic components of the mature protophloem sieve elements progresively degenerated and finally obliterated.     

Structure of functional soybean sieve elements

Soybean (Glycine max cv. Bragg) petiolar tissue containing translocated ¹⁴C-sucrose was quick frozen, freeze-substituted in acetone or propylene oxide and embedded in Epon. This procedure allowed cytological observations on sieve elements whose functional condition could be verified by microautoradiography. Sieve elements and companion cells were essentially free of ice damage. Aside from a P-protein crystal, the central portion of the sieve tube lumen was devoid of stainable content except in the vicinity of sieve plates. Various sized clumps of stacked endoplasmic reticulum (ER) lined the wall. Superficially, the ER “membranes” seemed to consist of parallel arrays of 100 Å protein fibrils. Although that possibility could not be excluded, it seemed more likely that the fibrils were actually between ER cisternae and that the lipoprotein ER membrane could not be detected readily due to the loss of lipids during tissue preparation. The amount and distribution of proteinaceous material in the vicinity of sieve plates was variable but, when present, still consisted almost entirely of 100 Å fibrils organized into membrane-like arrays. Stacks of ER in various degrees of disorganization and a few 100 Å fibrils were found near sieve plates, with some fibrils extending through the pores. However, most (70%) of the sieve plate pores were essentially free from obstruction. The observations favor an osmotically generated pressure flow mechanism of translocation in soybean.