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  • 1
    ISSN: 1615-6102
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The vacuoles of the sieve elements ofSelaginella kraussiana contain a crystalline protein which appears to degenerate in mature cells. Although it occurs in sieve elements we have elected not to call it “P-protein” because of ontogenetic and possibly functional differences between the two. The nucleus undergoes unique structural changes during development of the sieve element, ultimately being converted to a mass of tubules. The structures referred to by earlier workers as “refractive spherules” inSelaginella are probably plastids. As the size of the sieve pores in lateral and end walls falls into the same size range, the sieve elements ofSelaginella kraussiana can be considered to be sieve cells.
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Protoplasma 83 (1975), S. 217-232 
    ISSN: 1615-6102
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: 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 Ā 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.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Protoplasma 83 (1975), S. 365-388 
    ISSN: 1615-6102
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Metaphloem sieve elements from various parts of the plant body ofZea mays L. were examined with the electron microscope. No qualitative differences were found among sieve elements from sources, sinks, and intermediate regions of the plant. At maturity all sieve elements are lined with a parietal layer of cytoplasm, consisting of a plasmalemma, an anastomosing network of endoplasmic reticulum (ER), occasional stacks of ER, mitochondria, and plastids with protein crystalloids. Many mature sieve elements contained persistent, although apparently degenerated nuclei. In addition, in many mature sieve elements the vacuoles apparently continued to be delimited from the parietal cytoplasm by the tonoplast. In most relatively uninjured sieve elements the plasmalemma-lined sieve-plate pores contained little callose and cytoplasmic components and, hence, were largely unoccluded. P-protein was not encountered in any sieve elements at any stage of development.
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  • 4
    ISSN: 1615-6102
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The sieve areas inWelwitschia are essentially similar to those of coniferous sieve cells, consisting of groups of plasmalemma-lined pores, which are joined in the middle of the wall by a median cavity. The median cavity contains membranes which apparently are connected with aggregates of endoplasmic reticulum bordering the sieve areas. The median cavity is formed through union of smaller median enlargements, the median nodules, each initially associated with a plasmodesma, during perforation of the young sieve area. Callose platelets are not associated with developing pores. All fully-developed pores were lined with callose. The sieve cells are connected with only one other cell type, the albuminous cell. On the sieve-cell side of the wall these connections are similar to sievearea pores, on the albuminous-cell side to plasmodesmata. These connections are also characterized by median enlargements.
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  • 5
    ISSN: 1615-6102
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Penetration of leaves of barley,Hordeum vulgare L., by the corn leaf aphid,Rhopalosiphum maidis (Fitch), was studied with light, phase, and electron microscopes. Penetration of epidermis and mesophyll was largely intercellular, that of vascular bundles or veins largely intracellular. Like other aphids,R. maidis secretes a salivary sheath which surrounds the stylets. When mesophyll cells and parenchymatous elements of the veins were penetrated by stylets, their protoplasts were pushed to one side by intruding sheath material; hence, the protoplasts were not punctured by the stylets, although sometimes the plasmalemma of penetrated cells was ruptured by sheath material. The salivary sheaths ended more or less abruptly outside the walls of sieve elements being fed upon, the maxillary stylets projecting beyond the sheaths and into the sieve elements. Before penetrating a functional sieve element the aphid apparently flushes its stylets in order to clear them for ingestion of food. Salivary and food canals merge near the tips of the maxillary stylets to form a single canal, which ends short of the tips.
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  • 6
    ISSN: 1615-6102
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The sieve areas inPinus resinosa consist of groups of plasmalemma-lined pores joined in the middle of the wall by large compound median cavities. Numerous membranes extend into the pores from aggregates of smooth, tubular endoplasmic reticulum opposite the sieve areas and merge in the median cavities. A compound median cavity arises through the union of smaller simple median cavities, each of which is initially associated with a single plasmodesma. Callose platelets were not associated with developing pores, but the pores of most fully-developed sieve areas were lined with callose. Sieve cells also have connections with albuminous cells. The connections on the sieve-cell side of the wall are similar to sieve-area pores, those on the albuminous-cell side to plasmodesmata. These connections also are associated with compound median cavities containing many membranes. The albuminous cells contain protoplasmic components similar to those of other parenchymatous cells including chloroplasts, cytoplasmic fibers, and microbodies.
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  • 7
    ISSN: 1615-6102
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: 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.
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  • 8
    ISSN: 1615-6102
    Keywords: Beta vulgaris ; Chloroplast ; Ultrastructure ; Ultraviolet radiation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The effect of UV-C (254 nm) and UV-B (290–320 nm) radiation on leaves ofBeta vulgaris L. at the ultrastructural level was investigated. Although the damage caused by UV-C radiation was more striking than that resulting from UV-B radiation, several structural changes were seen in the UV-B treated material. Generally the effects of UV-B and UV-C radiation were different, suggesting different mechanisms of action, discernible even at the ultrastructural level.
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Protoplasma 109 (1981), S. 217-231 
    ISSN: 1615-6102
    Keywords: Anatomy ; Artemisia ; Cell wall composition ; Compositae ; Phloem ; Sieve element
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: 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.
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  • 10
    ISSN: 1615-6102
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary During advanced stages of sieve-element differentiation inUlmus americana L., dispersal of the P-protein (slime) bodies results in formation of a peripheral network of strands consisting of aggregates of P-protein components having a striated, fibrillar appearance. The tonoplast is present throughout the period of P-protein body dispersal. Perforation of the sieve plates is initiated during early stages of P-protein body dispersal. Small P-protein bodies consist of tubular components, most of which measure about 180 Å in diameter. With increase in size of the P-protein bodies narrower components appear. At the time of initiation of P-protein body dispersal, most of the components comprising the bodies are of relatively narrow diameters (most 130–140 Å) and have a striated, fibrillar appearance. Both wide and narrow P-protein components are present throughout the period of sieve-element differentiation and in the mature cell as well, and a complete intergradation in size and appearance exists between the two extremes. Both extremes of P-protein component have a similar substructure: an electron-transparent lumen and an electronopaque wall composed of subunits, apparently in helical arrangement. The distribution of P protein in mature sieve elements was quite variable. The parietal layer of cytoplasm in matureUlmus sieve elements consists of plasmalemma, endoplasmic reticulum cisternae in two forms (as a complex network closely applied to the plasmalemma and in stacks along the wall), mitochondria, and plastids.
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