A Study of Stelar Ultrastructure in the Heterosporous Water Fern Marsilea quadrifolia L

Sieve tube elements occur in the rhizomes and petioles of Marsileaquadrifolia. These are either thick walled with compound sieveplates in oblique end walls or thin walled with simple sieveplates in transverse end walls. Vessels are restricted to themetaxylem in the roots where the phloem contains sieve cellsonly. The sieve pores are invariably callose lined and as inother pteridophytes, excepting the Lycopsida, refractive spherulesare ubiquitous in the sieve elements of Marsilea. The luminaof the protoxylem tracheary elements in the rhizomes and petiolesare occluded by tyloses but probably remain functional in theroots. Pericycle cells backing on to the root protoxylem armspossess wall ingrowths. Transfer cells are however absent fromthe vascular tissue of the rhizomes and leaves. It is suggestedthat their presence in the root pericycle is related to theretrieval of ions from the xylem sap which may be particularlycritical in water plants. The incidence of transfer cells incryptogams appears to be far more sporadic than in angiosperms.The root endodermis of Marsilea possesses a casparian stripand abundant vacuolar tannin deposits. Plasmalemmasomes arenumerous adjacent to the pericycle transfer cells. vascular ultrastructure, Marsilea quadrifolia L, transfer cells, sieve tube elements, tyloses     

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.     

The Fine Structure of the Sieve Tubes of the Petiole of Nytnphoides peltatum (Gmel.) O. Kunze

A study has been made by electron microscopy of the fine structureof the peti-olar sieve tubes of the water plant Nymphoides peltatum.These are found to have very well-developed nacreous walls.The pores of the sieve plates appear to be filled in functioningsieve tubes with densely staining cytoplasm. The peripheralcytoplasm of the sieve tubes seems to contain an extensive developmentof the endoplasmic reticulum, whose elements become finer nearthe plates and crowd together through the pores. These findingsappear to be compatible with more than one theory of translocation,including the electro-osmotic theory of mass flow.     

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.     

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 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.     

Studies onArtemisia afra Jacq.: The phloem in stem and leaf

Summary The structure of the phloem was studied in stem and leaf ofArtemisia afra Jacq., with particular attention being given to the sieve element walls. Both primary and secondary sieve elements of stem and midvein have nacreous walls, which persist in mature cells. Histochemical tests indicated that the sieve element wall layers contained some pectin. Sieve element wall layers lack lignin. Sieve elements of the minor veins (secondary and tertiary veins) lack nacreous thickening, although their walls may be relatively thick. These walls and those of contiguous transfer cells are rich in pectic substances. Transfer cell wall ingrowths are more highly developed in tertiary than in secondary veins.     

ULTRASTRUCTURE OF THE NACREOUS LEPTOIDS (SIEVE ELEMENTS) IN THE POLYTRICHACEOUS MOSS ATRICHUM UNDULATUM

The physiological phloem equivalents, leptoids, of the polytrichaceous moss Atrichum undulatum appear to be similar to the nacreous sieve elements that occur in many higher plants. These leptoids are elongated cells with nacreous thickenings on their radial and tangential walls. Their oblique end walls, which lack such thickenings, are traversed by numerous pores through which the plasmalemma, endoplasmic reticulum, and cytoplasm are continuous between adjacent leptoids of a longitudinal file. These end walls closely resemble the simple sieve areas of the sieve elements found in Polypodium vulgare. The leptoid sieve pores have a median expanded area and frequently are occluded by small amorphous protein plugs at each end. Also, callose was observed as electron-luscent areas both on the faces of the end walls and as a thin cylinder surrounding the lateral area of each pore. Amorphous and granular cytoplasmic contents of the leptoids appear to be morphologically similar to the slime (P-protein) found in the sieve-tube elements of many angiosperms. Differentiating leptoids are characterized by the formation of numerous membrane-bound protein bodies in close association with polysomes and endoplasmic reticulum. As the leptoid matures, the contents of the protein bodies become dispersed in the cytoplasm. Ultrastructurally and ontogenetically the leptoids in the gametophores of A. undulatum appear almost identical to the sieve elements of P. vulgare and therefore should be considered sieve elements rather than phloem-like equivalents.     

The Phloem of Nelumbo nucifera Gaertn

In common with other aquatic angiosperms, Nelumbo nucifera Gaertn.has a relatively strongly developed phloem tissue. The vascularsystem consists of discrete collateral bundles in which no cambiumdevelops and the phloem and xylem are separated by a narrowlayer of parenchyma cells. The phloem consists of sieve elements,companion cells, and phloem parenchyma cells. The sieve elementshave transverse end walls with simple sieve plates. The cellsattain considerable width in the late phloem (metaphloem). Thecompanion cells are in vertical strands. In the early phloem(protophloem) of large bundles the sieve tubes and companioncells are eventually obliterated. The parenchyma cells alsoform vertical strands which may contain tannin cells. Some parenchymacells and companion cells are binucleate. The sieve elementsshow ultrastructural features common for these cells in dicotyledons.At maturity, they lack nuclei, ribosomes, and tonoplasts, butretain a plasmalemma, mitochondria, and plastids. The latterare poorly differentiated and form starch. The endoplasmic reticulumis in part stacked, in part it forms a network next to the plasmalemma.The P-protein occurs in two forms. One consists of tubules notassembled in any specific type of array. The other, possiblycomposed of much extended tubules, is assembled in crystallineaggregates which are retained as such in mature cells. The sieveplate pores are lined with callose and plasmalemma. The lateralwalls are relatively thin and the nacreous layer varies in degreeof distinctness.     

Primitive-like metaphloem sieve elements in the aerial stem ofEquisetum hyemale

Summary Internodal metaphloem sieve elements located near the nodes of aerial stems ofEquisetum hyemale contain very oblique end walls. During maturation, the connections, or plasmodesmata, in these walls undergo little or no structural modification. By contrast, the endwall connections uniting the protoplasts of mature sieve elements elsewhere in the aerial stem ofE. hyemale are pores.This work as supported by U.S. National Science Foundation grant GB 31417 to R. F.Evert.     

Developmental Features of the Primary Phloem in Phaseolus vulgaris L.

Certain developmental features of the primary phloem were examinedin Phaseolus vulgaris L., chiefly by the use of the pulvinusat the base of the petiole. The cells included in the studywere the sieve element, the companion cell, and the tannin cell.In the sieve element, the sieve plate shows the usual sequenceof conversion of plasmodesmatal canals into pores. The endoplasmicreticulum, which appears as flat cisternae associated with ribosomesin younger cells, later becomes in part stacked and in partaligned parallel with the walls as a network. The stacked ERprecedes the anastornosing parietal ER in time of development,but the parietal ER persists longer. Of the two forms of P-proteincharacteristic of a number of Fabaceae, the crystalline bodyappears considerably in advance of the body composed of tubules.Neither form of P-protein disperses completely in the maturecell, although the crystalline protein may spread out into aggregatesof fine fibrils. The companion cells show the typical denseprotoplasts and branched plasrnodesmatal connections with thesieve elements. The vacuome of these cells is dispersed intonumerous small vacuoles, many of which appear to be concernedwith autophagic digestion of protoplasmic material. The tannincells have large vacuoles in which the tannin material is located.The cells form vertical series in which the end walls becomeperforated.     

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.     

The Ultrastructure of Protophloem Sieve Elements in Leaves of Aegilops comosa var. thessalica

The differentiation and obliteration of protophloem sieve elementsin leaves of the grass Aegilops comosa var. thessalica havebeen studied by electron microscopy. These elements differentiatesimilarly to metaphloem sieve elements of the same plant andother monocotyledons. Plasmalemma, smooth endoplasmic reticulum(ER), mitochondria, P-type plastids and sometimes nuclear remnantsconstitute the protoplasmic components at maturity, all areperipherally distributed. The differentiation of end walls intosieve plates and the presence of sieve areas on the lateralwalls indicate that protophloem sieve elements are componentsof sieve-tube. They may be functional for a brief period butsoon after their maturation they are compressed and finallyobliterated by the stretching of actively-growing surroundingcells. The protoplasmic components of mature elements degenerateand are destroyed during obliteration of the sieve elements. Aegilops comosa var. thessalica, protophloem, sieve elements, differentiation, ultrastructure     

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.     

SOME FINE STRUCTURAL OBSERVATIONS ON DEVELOPING AND MATURE SIEVE ELEMENTS IN THE BROWN ALGA LAMINARIA SACCHARINA

The differentiation of sieve elements from inner cortical cells of the stipe of Laminaria saccharina (L.) Lamour. involves the development of a well-structured protoplast and an end wall possessing evenly spaced pores which are visualized by electron microscopy. The protoplast consists of organelles which are commonly found in brown algal cells, including nuclei, cup- or horseshoe-shaped chloroplasts, dictyosomes, mitochondria, and ER. Mitochondria and clusters of small vacuoles, presumably redistributed by the surging effect which occurs in sieve elements, were routinely observed in the vicinity of the end wall. Chloroplasts were seen in progressively degenerated states in older sieve elements, yet nuclei were determined to be non-necrotic. Numerous pores along the end walls interconnect adjacent sieve elements. Each pore is traversed by a strand of cytoplasm and surrounded by plasmalemma. The pores are open and possess no callose. In this paper the sieve element ultrastructures of L. saccharina are compared to those in L. groenlandica, Alaria marginata, Nereocystis lutkeana and Macrocystis pyrifera, and a possible phylogenetic specialization of sieve elements is presented in table form and discussed.     

THE FINE STRUCTURE OF SIEVE ELEMENTS OF NEREOCYSTIS LÜTKEANA

The sieve elements of Nereocystis from the base of phylloids contain numerous small vesicles, cytoplasm, ribosomes, and the usual organelles and membrane systems, including nuclei, plastids, mitochondria, dictyosomes, and endoplasmic reticulum. They have a thick secondary wall layer which is deposited along the longitudinal walls and at the sieve plate excluding the sieve pores. The sieve pores range in diameter from 100 to 400 nm and are lined by plasmalemma. The sieve elements from the hollow basal parts of the pneumatocyst show essentially the same features but have larger and fewer vesicles, relatively little cytoplasm, larger sieve pores, 400–900 nm in diameter, and may lack a nucleus. In old sieve elements there are large deposits of callose on the sieve plate and along the longitudinal wall; the vesicles seem to break down, and the protoplast appears necrotic. It is concluded that the trumpet hyphae and sieve tubes are basically the same type of cell, and that the trumpet-shape of the sieve elements is due to their passive stretching during extension growth of the organ in which they occur. There are minor but significant differences among the sieve elements from different regions of the thallus which may reflect possible levels of structural specialization of the sieve elements within the same plant.     

Le phloème deSelaginella willdenowii: histophysiologie comparée

Metaphloem sieve elements ofSelaginella willdenowii are elongated cells with slightly oblique or transverse end walls. Pores are seen on both lateral and end walls, although they are more numerous on the latter. Parenchyma cells exhibiting strong enzyme activities (acid phosphatase, non specific esterase, succinate dehydrogenase, cytochrome oxidase, peroxidase) are present between sieve elements and tracheids in each vascular bundle. A functional association thus appears to exist between these parenchyma cells and the conducting elements.—The occurrence of transverse to slightly oblique end walls in sieve elements seems to characterize the ligulate Lycopsids (as opposed to the aligulateLycopodium where sieve elements possess slanting, very oblique, end walls).

     

Hyperplastic Phloem and Its Plastids in Spinach Infected with the Curly Top Virus

The hyperplastic growth induced in the phloem tissue by infectionwith the curly top virus was studied in minor veins of leavesof spinach, Spinacia oleracea L., by the use of the electronmicroscope. Proliferation of cells occurs in the phloem andin the parenchyma bordering the phloem. The arrangement of cellsis less orderly when hyperplasia occurs in older than in youngertissue but in both instances the majority of cells differentiateinto sieve elements. As in normal phloem, sieve element plastidshaving a ring of proteinaceous fibrils are a consistent featurein the hyperplastic phloem. Depending on the kind of cell inwhich hyperplasia is initiated, the plastids may originate fromyoung plastids similar to those in normal sieve elements orfrom more or less completely differentiated chloroplasts. Theprotoplasts of the hyperplastic sieve elements, including theplastids, degenerate during differentiation or after maturation.     

Junction Complexes between Sieve Tubes in the Secondary Phloem of Myrtaceae

Junction complexes of unusual structure form between neighbouringsieve tubes in the secondary phloem of Eucalyptus species. Thick-walledribs support thin-walled ‘sieve areas’. In longitudinalsections the structures have a ‘concertina’- likeappearance. They are relatively large, up to 0.2 mm in length.Electron micrographs confirmed that the structures consistedof thin-walled areas perforated with pores, supported by muchthicker ribs. The structures provide a vast surface area fortransfer of metabolites between sieve tubes compared with thatof lateral wall sieve areas of other plants. Hydrolysis of parenchymacell walls occurs during the development of the junction complexes.The structures are only found when sieve tubes are in closeproximity and it is the redifferentiation and partitioning ofintervening parenchyma cells which result in junction complexformation. A survey for the presence of the structures in thephloem of other genera in the family Myrtaceae was made andthey were found in Tristania and Angophora but were not observedin Acmena and Metrosideros. Eucalyptus, sieve tubes, lateral walls, ultrastructure     

Relation of beet yellows virus to the phloem and to movement in the sieve tube

In minor veins of leaves of Beta vulgaris L. (sugar beet) yellows virus particles were found both in parenchyma cells and in mature sieve elements. In parenchyma cells the particles were usually confined to the cytoplasm, that is, they were absent from the vacuoles. In the sieve elements, which at maturity have no vacuoles, the particles were scattered throughout the cell. In dense aggregations the particles tended to assume an orderly arrangement in both parenchyma cells and sieve elements. Most of the sieve elements containing virus particles had mitochondria, plastids, endoplasmic reticulum, and plasma membrane normal for mature sieve elements. Some sieve elements, however, showed evidence of degeneration. Virus particles were present also in the pores of the sieve plates, the plasmodesmata connecting the sieve elements with parenchyma cells, and the plasmodesmata between parenchyma cells. The distribution of the virus particles in the phloem of Beta is compatible with the concept that plant viruses move through the phloem in the sieve tubes and that this movement is a passive transport by mass flow. The observations also indicate that the beet yellows virus moves from cell to cell and in the sieve tube in the form of complete particles, and that this movement may occur through sieve-plate pores in the sieve tube and through plasmodesmata elsewhere.