Structure and Development of Sieve Elements in the Root of Isoetes muricata Dur.

Root tissues of Isoetes muricata Dur. were fixed in glutaraldehydeand postfixed in osmium tetroxide for electron microscopy. Veryyoung root sieve elements can be distinguished from contiguousparenchyma cells by the presence of crystalline and/or fibrillarproteinaceous material in dilated cisternae of rough endoplasmicreticulum (ER). Similar crystalline-fibrillar material accumulatesin the perinuclear space. During differentiation, the portionsof ER enclosing this proteinaceous substance become smooth surfacedand migrate to the cell wall. Along the way many of them formmultivesicular bodies which fuse with the plasmalemma, dischargingtheir contents toward the wall. Nuclear degeneration is pycnotic.At maturity, the sieve element contains a degenerate, filiformnucleus, plastids, and mitochondria. In addition, the wall ofthe mature sieve element is lined by a plasmalemma and a parietalnetwork of smooth ER. Sieve-area pores are present in both endand lateral walls of mature sieve elements. Whereas a singlecluster of pores occurs in each end wall, the pores of the lateralwalls are solitary and few in number.     

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.     

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.     

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.     

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 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.     

Composition and fine structure of minor veins inTetragonia leaf

Summary Mesophyll containing the minor veins from leaves ofTetragonia expansa Murr. was examined in preparation for a study of effects of beet yellows virus on the leaf tissues of this plant. The sieve elements throughout the minor veins exhibit the characteristics commonly found in this type of cell in dicotyledons. The cells are connected with one another by sieve plates and with contiguous parenchyma cells by branched plasmodesmata. Mature sieve elements are enucleate and lack ribosomes. No tonoplast was discerned in these cells. Mitochondria, plastids, and sparse endoplasmic reticulum are retained in mature cells. The plastids, which contain a massive fibrous ring of proteinaceous material, resemble the sieve element plastids ofBeta. The P-protein in the sieve elements is occasionally composed of tubules; more commonly it is represented by loose helices. The tracheary elements have scalariform secondary thickenings. In regions free of these thickenings, the primary wall largely disintegrates; only some loosely arranged fibrils remain. The mesophyll and vascular parenchyma cells contain the various organelles characteristic of living plant cells but vary in degree of vacuolation and in density of cytoplasm. Some vascular parenchyma cells have particularly dense protoplasts. They contain numerous ribosomes and their background matrix consists of a dense population of fine fibrils. Occasional vascular parenchyma cells contain the tubular spiny cell component first recognized inBeta. This work was supported in part by National Science Foundation grant GB-5506.     

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.     

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.     

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.     

Ultrastructural features of differentiating protophloem sieve elements in adventitious roots of Salix viminalis

The differentiation of the protophloem in 9- to 14-day-old adventitious roots of Salix viminalis was studied. Ultrastructural observations were mainly made on longitudinal serial sections through an uninterrupted file of 32 differentiating sieve elements. The first cell in the file was located about 50 μm from the apical meristem. At an early stage the nucleus was lobed in outline, and in older cells the nucleoplasm became electron lucent. In the first or second cell from the first mature sieve element the nuclear envelope broke open. The nucleoli decreased gradually in size and disappeared finally. From the 9th cell the plastids contained starch and grew somewhat in size. ER increased in amount and began to form stacks in the 20th cell. These stacks moved to a peripheral position. Callose platelets were first observed on the transverse walls in cell 18. Flattened ER-cisternae covered the sieve pore sites. Gradually the middle lamella was dissolved and the callose aggregations formed cylinders around the pores of the sieve plate. Aggregations of tubular P-protein were present from cell 15. P-protein bodies were also present in parenchyma cells adjoining mature sieve elements. The only cell components remaining in mature sieve elements were plastids, mitochondria, stacked ER, the plasmalemma, remnants of other membranes and bodies consisting of P-protein and of an unidentified granular material. The sieve elements had no ontogenetically related companion cells. At a level where both metaphloem and metaxylem had matured the first formed protophloem sieve elements remained intact.     

Polysomes and intracisternal accumulations in enucleate sieve elements of rice (Oryza sativa L.)

Polysomes in sieve elements of rice (Oryza sativa L.) were studied with the electron microscope. The polysomes were found on the rough endoplasmic reticulum (ER) present in immature sieve elements and also on the cisternae of aggregated ER in the parietal layer of mature, enucleate sieve elements. In the immature sieve elements the ER cisternae existed as narrow profiles while in the mature sieve elements the ER cisternae were considerably dilated and contained a fibrillar material and, occasionally, electron-opaque inclusions. In addition to the aggregated ER, single profiles of ER were found applied to the lateral walls and also the sieve plates. These cisternae also bore ribosomes and were separated from the plasmalemma by a narrow, dense space. In the mature sieve elements much of the surface of the ER membranes was covered with polysomes. The dimensions of the polysomes are described and the possibility that they contribute to the formation of the fibrillar material in the intracisternal space is discussed.Abbreviations ER endoplasmic reticulum     

漆树乳汁道分泌细胞的超微结构及其与生漆产生和分泌关系的研究

漆树(Rhus verniciflua)乳汁道分泌细胞含有丰富的质体、内质网和嗜锇物质。电子显微镜的现察结果表明,嗜锇的生漆成分合成的可能场所是质体和内质网,并且通过内质网分子和小泡群与质膜相互接触并融合以及质膜内褶包被等三种形式释放到质膜和细胞壁之间的间隙中;再经过细胞壁中乳汁道腔形成时断裂了的胞间连丝通道和扩散渗透两条途径,越过细胞壁分泌到乳汁道腔中。细胞核、线粒体、高尔基体以及细胞质基质或多或少也参与了上述过程。     

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.     

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.     

Ultrastructure of minor veins inCucurbita pepo leaves

Summary The minor veins ofCucurbita pepo leaves were examined as part of a continuing study of leaf development and phloem transport in this species. The minor veins are bicollateral along their entire length. Mature sieve elements are enucleate and lack ribosomes. There is no tonoplast. The sieve elements, which are joined to each other by sieve plates, contain mitochondria, plastids and endoplasmic reticulum as well as fibrillar and tubular (190–195 diameter) P-protein. Fibrillar P-protein is dispersed in mature abaxial sieve elements but remains aggregated as discrete bodies in mature adaxial sieve elements. In both abaxial and adaxial mature sieve elements tubular P-protein remains undispersed. Sieve pores in abaxial sieve elements are narrow, lined with callose and are filled with P-protein. In adaxial sieve elements they are wide, contain little callose and are unobstructed. The intermediary cells (companion cells) of the abaxial phloem are large and dwarf the diminutive sieve elements. Intermediary cells are densely filled with ribosomes and contain numerous small vacuoles and many mitochondria which lie close to the plasmalemma. An unusually large number of plasmodesmata traverse the common wall between intermediary cells and bundle sheath cells suggesting that the pathway for the transport of photosynthate from the mesophyll to the sieve elements is at least partially symplastic. Adaxial companion cells are of approximately the same diameter as the adaxial sieve elements. They are densely packed with ribosomes and have a large central vacuole. They are not conspicuously connected by plasmodesmata to the bundle sheath.     

Transport of assimilates in the developing caryopsis of rice (Oryza sativa L.)

Assimilates entering the developing rice caryopsis traverse a short-distance pathway between the terminal sieve elements of the pericarp vascular bundle and the aleurone layer. The ultrastructure of this pathway has been studied. Sieve elements in the pericarp vascular bundle are smaller than their companion cells.The sieve elements show few connections with surrounding vascular parenchyma elements but are connected to companion cells by compound plasmodesmata. Companion cells, in turn, are connected to vascular parenchyma elements by numerous compound plasmodesmata present in wall thickenings. Assimilates leaving the sieve element — companion cell complex must laterally traverse cells of the pigment strand before they come into contact with the aleurone layer. The pigment strand cells have modified inner walls made up of a suberin-like material. This material may act as a permeability barrier isolating the apoplast from the symplast of the pigment strand. The walls of the pigment strand cells are traversed by numerous plasmodesmata. Water may be conducted to the endosperm through the isolated cell-wall system of the pigment strand while assimilates possibly move via plasmodesmata. High frequencies of plasmodesmata occur at the junction between the pigment strand and the nucellus and also between adjacent cells of the nucellus. By contrast, plasmodesmata are absent between the nucellus and the aleurone layer and also between the nucellus and the seed coat. A predominantly circumferential and symplastic transport pathway is likely between the pigment strand and nucellus. In view of the total absence of plasmodesmata between the nucellus and the aleurone layer assimilates entering the endosperm may have to cross the plasmalemma of the nucellus. It is possible that constraints to the flow of assimilates may occur in the short-distance pathway between the terminal sieve element — companion cell complexes and the endosperm, and this is discussed.     

Studies on the Relation Between the Ultrastructure of Secretory Cells and Their Synthesis and Secretion of Lacquer in Laticiferous Canals of Rhus verniciflus

Secretory cells of laticiferous canals contain many plastids and endoplasmic reticulum (ER) in Rhus verniciflua. The electron microscopy suggests that osmiophiiic Lacquer component is mainly synthesized in the plastids and ER. They may be eliminated from the protoplasts to the space between the plasmalemma and the cell wall in three ways: (1) by ER elements, (2) by vesicles approaching the plasmalemma and fusing their membrances with the latter, and (3) by their becoming surrounded by plasmalemma invaginations, and then they traverse the wall through the channels of plasmodesmata which became disconnected during the schizogenous development of the canals and percolate through the wall that faded into an even looser mesh of fibrillar material toward the canal lumen. More or less, nucleus, mitochondria, Golgi bodies and ground cytoplasm also take part in the above-mentioned process.     

STRUCTURE AND DEVELOPMENT OF THE SIEVE-ELEMENT PROTOPLAST IN THE HYPOCOTYL OF PINUS RESINOSA

Hypocotyl tissue of Pinus resinosa Ait. was fixed in glutaraldehyde-paraformaldehyde and postfixed in osmium tetroxide for electron microscopy. Although young sieve cells contain all the components characteristic of young, nucleate cells, they can be identified early in their development. Increase in wall thickness occurs early and rapidly. Concurrently, the plastids, which already contain starch granules, form both crystalline and fibrillar inclusions. As the sieve cell approaches maturity, an extensive network of smooth, tubular endoplasmic reticulum (ER), which becomes mostly parietal in distribution, is formed. At maturity, massive aggregates of this ER occur on both sides of sieve areas. These ER aggregates are interconnected with one another longitudinally by the parietal ER. In addition, the mature, plasmalemma-lined sieve cell contains a degenerate nucleus, mitochondria, and intact plastids. Dictyosomes, ribosomes, and vacuolar membranes are lacking. P-protein is not present at any stage of development.     

ULTRASTRUCTURE OF DEVELOPING SIEVE ELEMENTS IN THLASPI ARVENSE L. II. MATURATION

Developing sieve elements of pennycress (Thlaspi arvense L.) were studied with the electron microscope. The maturation of sieve elements involved loss of ribosomes from cytoplasm; degeneration of nulcei; modification of endoplasmic reticulum (ER); loss of tonoplast; and disappearance of dictyosomes and dictyosomes vesicles, coated vesicles, microtubules, and microbodies. Such changes produce a mature, presumably conducting cell that contains no nucleus or central vacuole but which retains a thin layer of peripheral cytoplasm with plastids, mitochondria, and smooth ER. Some similar changes have been described in a variety of developing sieve elements of angiosperms, but coated vesicles and microbodies previously have not been followed through sieve-element maturation. Likewise, few developmental studies have been made of plant sieve elements that exhibit two types of P-protein, the tubular type and the granular P-protein body.