Structure and development of sieve cells in the primary phloem ofPinus resinosa

Summary The primary phloem consists mostly of sieve cells. Procambial cells and very young sieve cells contain all the components characteristic of young nucleate cells. Increase in wall thickness, which is relatively limited, constitutes the first indication of sieve-cell differentiation. During the period of wall thickening, the plastids develop starch grains and then fibrillar inclusions. Eventually the internal lamellae of the plastids collapse. The plastids do not form crystalline inclusions. As the sieve cell approaches maturity, an extensive network of smooth, tubular endoplasmic reticulum (ER) appears and then becomes mostly parietal in distribution. At maturity, large aggregates of this ER occur at the sieve areas. These aggregates are interconnected longitudinally by the parietal network of ER. In addition to the ER, the mature, plasmalemma-lined primary sieve cell contains a degenerate nucleus, with intact nuclear envelope, plastids, and mitochondria. Dictyosomes, ribosomes, and vacuoles are lacking. P-protein is not present at any stage of development.This work was supported by U.S. National Science Foundation grants GB 8330 and GB 31417 to R. F.Evert.     

STRUCTURE AND DEVELOPMENT OF SIEVE ELEMENTS IN THE LEAF OF ISOETES MURICATA

Leaf tissue of Isoetes muricata Dur. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. The very young sieve elements can be distinguished from contiguous parenchyma cells by their distinctive plastids and the presence of crystalline and fibrillar proteinaceous material in dilated cisternae of the rough ER. During differentiation, the portions of ER enclosing this proteinaceous substance become smooth surfaced and migrate to the cell wall. Along the way they apparently form multivesicular bodies which then fuse with the plasmalemma, discharging their contents to the outside. At maturity, the sieve element contains an elongate nucleus, which consists of dense chromatin material, and remnants of the nuclear envelope. In addition, the mature sieve element is lined by a plasmalemma and a parietal, anastomosing network of smooth ER. Both plastids and mitochondria are present. P-protein is lacking at all stages of development. Tonoplasts are. not discernible in mature sieve elements. The end walls of mature sieve elements contain either plasmodesmata or sieve pores or both, but only plasmodesmata occur in the lateral walls.     

THE LATERAL MERISTEM AND ITS DERIVATIVES IN THE CORM OF ISOETES MURICATA

Corm tissue of Isoetes muricata Dur. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. Very young secondary sieve elements can be distinguished from contiguous cambial cells by their distinctive plastids and by the presence of crystalline and/or fibrillar proteinaceous material in dilated cisternae of rough endoplasmic reticulum (ER). At maturity, the sieve elements are lined by the plasmalemma and a parietal, anastomosing network of smooth ER. Degenerate nuclei persist in all mature sieve elements. In addition, mature sieve elments contain plastids and mitochondria. Sieve-area pores are present in all walls. The lateral meristem of I. muricata consists of 2–3 layers of cells year-round. Judging from numerous collections made between October 1972 and July 1975, new sieve-element differentiation precedes cambial activity by about a month. Early in May, 1–2 cells immediately adjacent to already mature sieve elements differentiate directly into sieve elements without prior division. In early June, at about the time sieve-element differentiation is completed, cambial division begins. Division is sporadic, not uniform throughout the meristem. Dormancy callose accumulates in the secondary sieve elements in late October, and is removed in early May, at about the same time new sieve-element differentiation begins. Cells of the dormant cambium are characterized by the presence of numerous small vacuoles and large quantities of storage materials, including lipid droplets, starch grains, and tannin. By contrast, active cambial cells contain few large vacuoles with little or no tannin, and they have little storage material.     

SIEVE-ELEMENT ONTOGENY IN THE ROOT OF EQUISETUM HYEMALE

Roots of Equisetum hyemale L. var. affine (Engelm.) A. A. Eat. were fixed in glutaraldehyde, postfixed in osmium tetroxide, and sieve elements of various ages were examined with the electron microscope. Young sieve elements are distinguished by their position within the vascular cylinder and by the presence of numerous refractive spherules, which originate within dilated portions of the endoplasmic reticulum (ER). Early in development, the sieve-element walls undergo a substantial increase in thickness. This is followed by the appearance of massive ER aggregates in the cytoplasm and then by a phase involving stacking and sequestering of the remaining ER. Nuclear degeneration is initiated shortly after the appearance of the ER aggregates. The chromatin condenses into masses of variable size along the inner surface of the nuclear envelope. The envelope then ruptures and chromatin is released into the cytoplasm. During the period of nuclear degeneration, mitochondria and plastids undergo structural modification, while components such as dictyosomes, microtubules, and ribosomes degenerate and disappear. The remaining cytoplasmic components assume a parietal position in the cell, leaving the lumen of the cell clear in appearance. At maturity, the plasmalemma-lined sieve element contains plastids, mitochondria, some ER, and refractive spherules. At this time many of the refractive spherules are discharged into the region of the wall. Pores between sieve elements occur largely on the end walls. During pore development, tubules of ER apparently traverse the pores, but because of the presence of massive callose deposits in the material examined, the true condition of mature pores could not be determined. The connections between mature sieve elements and pericycle cells are characterized by the presence of massive wall thickenings on the pericycle-cell side. Plasmodesmata in the wall thickening are matched by pores on the sieve-element side. Ontogenetic and cytoplasmic factors argue against use of the term “companion cell” for the vascular parenchyma cells associated with the sieve elements.     

Development and ultrastructure of the primary phloem in the shoot of Ephedra viridis (Ephedraceae)

Sieve cell differentiation in the primary phloem of Ephedra viridis is first indicated by an increase in thickness of the wall, which begins in the corners of the cell, and next by the proliferation of smooth tubular endoplasmic reticulum (ER). As differentiation proceeds, cisternae of rough ER form stacks along the wall, losing their ribosomes in the process. Concomitantly, all of the mitochondria, plastids, and ER become parietal in distribution, and the vacuoles collapse. Nuclear degeneration is pycnotic and accompanied by the formation of tubular invaginations of the nuclear envelope into the peripheral chromatin. At maturity, an anastomosing network of smooth ER borders the plasmalemma, interconnecting aggregates of smooth tubular ER located primarily opposite the sieve areas. In addition to ER, the mature sieve cell contains mitochondria, plastids, and remnants of the degenerate nucleus, all of which are parietal in distribution. P-protein is lacking at all stages of sieve cell development. Sieve pore and compound median cavity development is similar to that reported for the sieve cells of conifers. Albuminous cells are associated with the sieve cells of the metaphloem throughout the shoot but with sieve cells of the protophloem only in the node. Among their cytoplasmic components are broad bundles of microfilaments spatially associated with a complex system of rough and smooth ER.     

Sieve-Element Ontogeny in the Aerial Shoot of Equisetum hyemale L.

The aerial shoots of Equisetum hyemale L. var. affine (Engelm.)A. A. Eat. were examined with the electron microscope as partof a continuing study of sieveelement development in the lowervascular plants. Young E. hyemale sieve elements are distinguishablefrom all other cell types within the vascular system by thepresence of refractive spherules, proteinaceous bodies whichdevelop within dilated portions of the endoplasmic reticulum(ER). Details of cell wall thickening differ between protophloemand metaphloem sieve elements. Following cell wall thickeningthe ER increases in quantity and aggregates into stacks. Shortlythereafter, nuclear degeneration is initiated. During the periodof nuclear degeneration some cytoplasmic components-dictyosomes,microtubules and ribosomes-degenerate and disappear, while organellessuch as mitochondria and plastids persist. The latter undergostructural modifications and become parietal in distribution.Eventually the massive quantities of ER are reduced, leavingthe lumen of the cell clear in appearance. At maturity the plasmalemma-linedsieve element contains a parietal network of tubular ER, aswell as mitochondria, plastids, and refractive sphemh At thistime many of the spherules are discharged into the region ofthe wall. Sieveelement pores occur in both lateral and end walls.At maturity many pores are traversed by large numbers of ERmembranes. The metaphloem sieve elements of the mid-internodalregions apparently are sieve-tube members. The connections betweenmature protophloem sieve elements and pericycle cells are associatedwith massive wall thickenings on the pericyclecell side.     

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.     

STRUCTURE AND DEVELOPMENT OF THE SIEVE ELEMENT IN THE STEM OF LYCOPODIUM LUCIDULUM

Stem tissue of Lycopodium lucidulum Michx. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. Although their protoplasts contain similar components, immature sieve elements can be distinguished from parenchymatous elements of the phloem at an early stage by their thick walls and correspondingly high population of dictyosomes and dictyosome vesicles. Late in maturation the sieve-element walls undergo a reduction in thickness, apparently due to an “erosion” or hydrolysis of wall material. At maturity, the plasmalemma-lined sieve elements contain plastids with a system of much convoluted inner membranes, mitochondria, and remnants of nuclei. Although the endoplasmic reticulum (ER) in most mature sieve elements was vesiculate, in the better preserved ones the ER formed a tubular network closely appressed to the plasmalemma. The sieve elements lack refractive spherules and P-protein. The protoplasts of contiguous sieve elements are connected with one another by pores of variable diameter, aggregated in sieve areas. As there is no consistent difference between pore size in end and lateral walls these elements are considered as sieve cells.     

ULTRASTRUCTURE OF THE SECONDARY PHLOEM OF TILIA AMERICANA

Summer and winter (July and January) samples of secondary phloem of Tilia americana were studied with the electron microscope. Parenchyma cells contain: nuclei, endoplasmic reticulum, ribosomes, plastids, mitochondria and occasional dictyosomes. Well-defined tonoplasts separate vacuoles from cytoplasmic ground substance. Vacuoles often contain tannins. Lipid droplets are common in cytoplasm. Endoplasmic reticulum–connected plasmodesmata are aggregated in primary pit fields. Companion cells differ from parenchyma cells in having numerous sieve-element connections, possibly slime, and in lacking plastids. Mature, enucleate sieve elements possess 1–4 extruded nucleoli. Numerous vesicles occupy a mostly parietal position in association with plasmalemma. The mature sieve element lacks endoplasmic reticulum, organelles (except for few mitochondria) and tonoplast. In OsO_4– and glutaraldehyde-fixed elements, slime has a fine, fibrillar appearance. Normally, these fine fibrils are organized into coarser ones which form strands that traverse the cell and the plasmalemma-lined pores of sieve plates and lateral sieve areas.     

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.     

THE ULTRASTRUCTURE OF DEVELOPING LATEX DUCTS IN MAMMILLARIA HEYDERI (CACTACEAE)

Electron microscopy was used to investigate early development of latex ducts in Mammillaria heyderi (Cactaceae). Numerous vesicles (secondary vacuoles) form from invaginations of the plasmalemma near sites of wall thinning, from endoplasmic reticulum (ER), and from vesiculate grana of degenerate plastids. Dictyosomes, though they occur in young duct cells, do not seem to be responsible for the formation of vesicles. Cytoplasmic vesicles may contain fibrillar, globular, or crystalline materials, or may be devoid of any type of particulate matter. They may be responsible for storage of numerous laticiferous components. Lysosomal materials could be stored in some vesicles and contribute to the degradation of the protoplast. Some nuclei contain condensed chromatin and are subject to deformation and collapse. Mitochondria and lipid bodies are common in young duct cells but ER is rare. When ducts form in young tissues, plastids in the lumen do not produce starch grains or extensive membranous networks. The plastids eventually degenerate to become a part of latex. If ducts form in older, established tissues having mature plastids, the plastids undergo extreme modification.     

STRUCTURE OF THE VASCULAR PARENCHYMA IN THE STEM OF LYCOPODIUM LUCIDULUM

At maturity the vascular cylinder of the stem of Lycopodium lucidulum contains two distinct types of parenchyma cells, one which is always associated with sieve cells, the other with tracheids. The remaining parenchyma cells have characteristics intermediate between the two extremes. The most conspicuous feature of the sieve cell-associated parenchyma cell is the very dense appearance of its protoplast, due to a high ribosome population and absence of large vacuoles. The large, ramifying nuclei of these cells have numerous connections with the endoplasmic reticulum (ER). The tracheid-associated parenchyma cells, which are light in appearance, contain many small vacuoles and a relatively small ribosome population. These cells also contain relatively small nuclei and considerable ER cisternae. The parenchymatous elements which have characteristics intermediate between sieve cell- and tracheid-associated parenchyma may or may not be contiguous to the sieve cells or tracheids. An intergradation in wall thickness occurs among parenchyma cells of the vascular cylinder, the thicker-walled cells being adjacent to the sieve cells, the thinner-walled ones next to the tracheids. An intergradation also occurs in the frequency of plasmodesmata between the various parenchyma cells. The closer parenchyma cells are to the sieve cells the greater the number of connections between them. No plasmodesmata were found between the tracheid-associated parenchyma cells.     

LIGHT MICROSCOPE INVESTIGATION OF SIEVE-ELEMENT ONTOGENY AND STRUCTURE IN ULMUS AMERICANA

At maturity the sieve elements of Ulmus americana L. contain a parietal network of very fine strands of slime which is continuous from one sieve element to the next through the sieve-plate pores. Upon injury this parietal network, which is derived from the slime bodies of immature sieve elements, sometimes becomes distorted into longitudinally oriented strands. Some of these strands frequently extend the length of the cells and often are continuous from one sieve element to the next through the sieve-plate pores. At times past such strands have erroneously been interpreted as normal constituents of the mature sieve-element protoplast. Many mature sieve elements of U. americana contain nuclei, which apparently persist for the life of the sieve elements. In addition, some evidence has been found in mature sieve elements for the presence of a membrane which delimits the parietal layer of cytoplasm, including its network of slime strands, from the vacuolar region of the cell.     

THE FINE STRUCTURE AND DEVELOPMENT OF THE COMPANION CELL OF THE PHLOEM OF ACER PSEUDOPLATANUS

At maturity the companion cell of the phloem of the sycamore Acer pseudoplatanus has a large nucleus, simple plastids closely sheathed with rough endoplasmic reticulum, and numerous mitochondria. The cytoplasm contains numerous ribosomes, resulting in a very electron-opaque cytoplasm after permanganate fixation. Bodies similar to the spherosomes of Frey-Wyssling et al. (4) are collected in clusters and these also contain bodies of an unidentified nature similar to those found by Buttrose (1) in the aleurone cells of the wheat grain. The pores through the wall between the companion cell and sieve tube are complex and develop from a single plasmodesma. Eight to fifteen plasmodesmata on the companion cell side communicate individually with a cavity in the centre of the wall which is linked to the sieve tube by a single pore about twice the diameter of an individual plasmodesma. This pore is lined with material of an electron opacity equivalent to that of material bounding the sieve plate pores. The development of the cell organelles, the possible role played in the phloem tissue by the companion cell, and the function of the complex pores contained in its wall are discussed.     

Plastids in sieve elements and companion cells of Tilia americana

Summary Contrary to an earlier report, the sieve elements and companion cells of Tilia americana contain plastids. In young sieve elements and companion cells the plastids contain a moderately electronopaque matrix and internal membranes; the latter are very numerous in the plastids of the sieve elements. Plastids of mature sieve elements contain an electron-transparent matrix, apparently fewer internal membranes than the plastids of young elements, and a single starch grain each. The plastids of companion cells undergo little or no structural modification during cellular differentiation, and apparently contain no starch.This research has been supported by the National Science Foundation, grants GB-5950 and GB-8330.     

COMPARATIVE LEAF STRUCTURE OF SEVERAL SPECIES OF HOMOSPOROUS LEPTOSPORANGIATE FERNS

The structure of the mature leaves of 13 species from 9 families of homosporous leptosporangiate ferns was examined by light and electron microscopy. In 11 species (Adiantum pedatum L., Athyrium angustum Roth., Cyathea dregei Sm., Lygodium palmatum Sw., Mohria caffrorum (L.) Desv., Oleandra distenta Kuntae, Pellaea calomelanos (Sw.) Link, Pityrogramma calomelanos (L.) Link var. austro-americana (Domn.) Farw., Trichomanes melanotrichum Schlechtend., Vittaria guineensis Desv., and Woodwardia orientalis Sw.) the lamina veins are collateral; in two (Phlebodium aureum and Platycerium bifurcatum), bicollateral as well as collateral veins are present. The vascular bundles in the midribs of C. dregei and those in the petioles and midribs of Phlebodium and Platycerium are concentric. All of the vascular bundles in the homosporous leptosporangiate ferns studied are delimited by a tightly arranged cylinder of endodermal cells with Casparian strips. Within the veins without parenchymatic xylem sheaths, some sieve elements commonly abut tracheary elements with hydrolyzed primary walls. The majority of vascular parenchyma cells contact both sieve elements and tracheary elements, although some parenchyma cells are associated with only one type of conducting cell. Transfer cells (parenchyma cells with wall ingrowths) occur in the veins of 6 species examined. Most of the vascular parenchyma cells, however, have no distinctive structural characteristics. The sieve elements of the homosporous leptosporangiate ferns are very similar structurally and each consists of a plasmalemma, a parietal, anastomosing network of smooth endoplasmic reticulum (ER), and variable numbers of refractive spherules, plastids and mitochondria. The sieve elements of L. palmatum also contain plasmalemma tubules. The parenchymatic cells of the leaf (mesophyll, endodermal and vascular parenchyma cells) are united by desmotubule-containing plasmodesmata. The sieve elements are connected to each other by sieve pores and to parenchymatic cells by pore-plasmodesma connections. The sieve-area pores contain variable amounts of membranous material, apparently ER membranes, but do not occlude them. These membranes commonly are found in continuity with the parietal ER of the lumen. Based upon the relative frequencies of cytoplasmic connections between cell types, the photosynthates may move from the mesophyll to the site of phloem loading via somewhat different pathways in different species of homosporous leptosporangiate ferns.     

Structure and development of the sieve-cell protoplast in leaf veins ofWelwitschia

Summary Tissue of one-year-old leaves ofWelwitschia mirabilis was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. Mature sieve cells contain nuclei composed of peripherally-distributed chromatin material and an intact envelope with pores. During sieve-cell development many mitochondria become closely associated spatially with the nucleus. In addition to a nucleus and mitochondria, the mature, plasmalemma-lined sieve cell contains plastids and abundant smooth endoplasmic reticulum, which generally occurs in massive aggregates at the sieve areas. Dictyosomes and ribosomes are lacking and a tonoplast is not discernible in mature sieve cells. P-protein is not present at any stage of development.This work was supported in part by a grant from the South African Council for Scientific and Industrial Research and in part by the U.S. National Science Foundation (GB 31417).     

STRUCTURE OF THE LEAF OF PYROSSIA LONGIFOLIA—A FERN EXHIBITING CRASSULACEAN ACID METABOLISM

The leaf of Pyrossia longifolia (Burm.) Morton, an epiphytic fern known to exhibit CAM, was examined by light and electron microscopy. The relatively thick leaf contains a single-layered epidermis, “water-storage” tissue, and a reticulate vascular system embedded in mesophyll tissue not differentiated into palisade and spongy layers. Mesophyll is composed of large, slightly elongate cells each with a thin, parietal layer of cytoplasm and a large central vacuole. The chloroplast-microbody ratio in mesophyll cells indicates that Pyrossia may be a high photorespirer and thus similar in that sense to C₃ plants. Mesophyll is separated from the vascular tissue by a tightly-arranged layer of endodermal cells with Casparian strips. The inner layer of mesophyll cells and the endodermal cells lack suberin lamellae. The collateral veins contain sieve elements, tracheary elements, pericycle and vascular parenchyma cells, the latter conspicuously larger than the sieve elements. The vascular parenchyma is the only cell type in the leaf which contains plastids with a peripheral reticulum. The parenchymatic elements of the leaf are connected by plasmodesmata, all of which lack neck constrictions and sphincters, or sphincter-like structures. The connections between sieve elements and adjacent parenchymatic elements are pore-plasmodesmata characterized by prominent wall thickenings on the parenchymatic-element side of the wall. The distribution and relative frequencies of plasmodesmata between the various cell types of the leaf indicate photoassimilates may move either symplastically or by a combination of symplast and apoplast from the mesophyll to the site of phloem loading in the veins.     

Another View of the Ultrastructure of Cucurbita Phloem

The primary phloem of young internodes of Cucurbita maxima wasstudied with the electron microscope. Phloem parenchyma cellsare highly vacuolated and contain nuclei, endoplasmic reticulum,ribosomes, mitochondria, chloro-plasts, and occasional dictyosomes.As compared with parenchyma cells, the most distinctive featuresof companion cells are their extremely dense cytoplasm, lowdegree of vacuolation, lack of chloroplasts, and numerous sieve-elementconnexions. Companion cells contain plastids with few internalmembranes. At maturity the enucleate sieve element is linedby a plasmalemma, one or more cistema-like layers of endoplasmicreticulum, and a membrane which apparently delimits the parietallayer of cytoplasm from a large central cavity. In OsO_4–-andglutaraldehyde-fixed elements, the central cavity is traversedby numerous strands, which run from cell to cell through thepores of sieve plates and lateral sieve areas, and which arederived ontogenetically from the slime bodies of immature cells.Numerous normal-appearing mitochondria are present in the parietallayer of cytoplasm. The pores of sieve plates and lateral sieveareas are lined with cytoplasm. The ultrastructural detailsof young sieve elements differ little from those of other youngnucleate cells. During sieve-element development, the sieveelement increases in vacuolation. At the same time, slime bodiesdevelop in the cytoplasm. With glutaraldehyde fixation, thesebodies often exhibit a double-layered limiting membrane. Asthe sieve element continues to differentiate, the slime bodiesincrease in size and the parietal layer of cytoplasm becomesvery narrow. Presently, the slime bodies begin to disperse andtheir contents fuse. This phenomenon occurs in the parietallayer of cytoplasm, while the latter is still delimited fromthe large central vacuole by a distinct tonoplast. The initiationof slime-body dispersal more or less coincides with perforationof the pore sites, and many pores are traversed by slime earlyin their development. Before slime-body dispersal, all dictyosomesand associated vesicles disappear from the cytoplasm. Eventually,the tonoplast diappears and the slime becomes distributed throughoutthe central cavity in the form of strands. Nuclei and ribosomesdisappear before breakdown of the tonoplast. Sieve elementsare connected with companion cells and parenchyma cells by plasmodesmata.     

Characteristics of Structure and Differentiation in the Sieve Element of Lower Vascular Plants

The structure and differentiation of the sieve element of lower vascular plants is reviewed using data obtained primarily from ultrastructural investigations conducted during the last ten years. During the last decade the phloem of representatives from every major group of the ferns and fern allies has been examined with the electron microscope and from these studies a rather clear picture has emerged of the structure of the sieve element protoplast in this diverse group of plants. Present data indicate that although the details of sieve-element differentiation may differ, the protoplasts of the mature sieve elements in the various groups of lower vascular plants are remarkably similar in structure. Each consists of a plasmalemma, a parietal, anastomosing network of smooth ER, plastids, mitochondria and, with the exception of the lycopods, variable numbers of refractive spherules. The protoplasts of mature sieve elements are joined by plasmalemma-lined connections, each arising from a single plasmodesma during the course of sieve element differentiation. The size of the connections in the mature elements range from plasmodesmata-like structures to relatively wide sieve-area pores, depending on the species. Moreover, the contents of the cytoplasmic connections vary somewhat according to the species. Whereas in the lycopods, the sieve-area pores are virtually unoccluded by any cytoplasmic material, the cytoplasmic connections of all other lower vascular plants examined with the electron microscope contain variable amounts of membranous material, apparently tubular elements of ER. In Equi-setum hyemale, Psilotum nudum and the eusporangiate and protoleptosporangiate ferns, the ER membranes are very numerous and virtually occlude the pores. Furthermore, the membranes apparently are not connected with the parietal ER in the lumen of the cell. The sieve-area pores of the leptosporangiate ferns also contain ER membranes, however, they are not as abundant as the membranes of the eusporangiate and protoleptosporangiate ferns. In addition, in the leptosporangiate ferns the pore membranes apparently are united with the parietal ER in the lumen of the cell.