Notes on Torus Development in the Wood of Osmanthus Americanus (L.) Benth. ' Hook. Ex Gray (Oleaceae)

The development of the torus in the wood of Osmanthus americanus was investigated using transmission and scanning electron microscopy. Torus formation on either side of the pit membrane did not begin until after the development of the associated pit border was well underway. No plasmodesmata were encountered in the torus at any time during its ontogeny. Synthesis of torus material was correlated with a mass of randomly oriented microtubules and dictyosome vesicles. The two halves of the torus did not develop synchronously; deposits of torus material were evident first in the older of two adjacent cells. Selective hydrolysis of the matrix material of the margo also began fIrst on that side of the pit membrane associated with a mature tracheary element. Evidence is presented for a fibrillar as weIl as a matrix component in the torus.

Changes in the membrane components of nondividing cambial cells

Horse-chestnut cambial cells are characterized by the formation of numerous plasmalemma invaginations and the accumulation of membrane whorls in the vacuoles during the transition from activity to rest. This suggests an active membrane trafficking which was investigated with conventional electron microscopy methods combined with selective cytochemical staining. After the cessation of meristematic activity, cell wall thickening is accompanied by increased dictyosome activity. The incorporation of dictyosome vesicles into the plasma membrane produces an increase in plasmalemma surface area in these nongrowing cells. This increase is compensated for by endocytosis accomplished by the formation of saclike plasmalemma invaginations into the peripheral cytoplasm. These invaginations often contain vesicles and tubules. When these invaginations come in contact with a vacuole, they appear to push the tonoplast into the vacuole and form double-membrane protrusions which may eventually separate from the plasmalemma. ER cisternae situated in the intermembrane zone also appear to be transported into the vacuole. Other cisternae may be directly sequestered into the vacuole or take part in the formation of the myelinlike structures which were observed in the cytoplasm. Thus, the vacuoles appear to fill progressively with complex membranous structures of various origins (plasmalemma, tonoplast, ER). It is suggested that their subsequent disappearance during the winter is a consequence of the hydrolytic action of vacuolar contents.

Megagametophyte development in Hordeum vulgare. 1. Early megagametogenesis and the nature of cell wall formation

Megagametophyte development in barley (Hordeum vulgare 'Atsel') was studied using Nomarski-interference optics and transmission electron microscopy. Stages described include the functional megaspore to cell wall formation. Aspects of the transition from the free nuclear stage of the embryo sac to the cellular embryo sac indicate involvement of elongate cell plates associated with clusters of microtubules. Initial cell walls among micropylar and chalazal nuclei are composed of beads derived from dictyosome vesicles. Fusion of growing cell plates occurs, especially within the antipodal apparatus.

The organization of microtubules in guard cell mother cells of Zea mays

The cortical interphase microtubules of the guard cell mother cells (GMCs) of Zea mays form a well-grouped band (interphase microtubule band, IMB) lining the midregion of the lateral and periclinal walls, which are the only expanding walls during interphase. In advanced interphase GMCs, another population of microtubules emerge from the cortical cytoplasm of the midregion of the periclinal walls, entering deep into the cytoplasm. Elongated proplastids converge on the above cortical regions, possibly aligned by the microtubules with which they are associated.The IMB depolymerizes prior to mitosis and a preprophase microtubule band (PMB) is organized adjacent to the proximal, distal, and periclinal walls. In transverse sections the preprophase – early prophase nucleus appears slightly elliptical or spindle-shaped, sometimes exhibiting acute angular profiles at its poles. Extranuclear microtubules closely juxtaposed with the nuclear envelope converge on the "poles" of the nucleus, close to the regions of the PMB adjacent to the periclinal walls. The observations suggest a local interplay between the PMB and (or) the PMB cytoplasmic site on the one hand and the nuclear envelope and (or) the extranuclear microtubules on the other.The microtubules of both bands lining the periclinal walls and the sites of their junctions with the anticlinal ones are more closely grouped than those running along the anticlinal walls, and they exhibit intimate associations with numerous dictyosome vesicles. This preferential gathering of dictyosome vesicles, among others, possibly manifests a mechanism promoting the thickening of the expanding regions of the above walls.The inhibition of the symmetrical divisions of the GMCs by a continuous colchicine treatment leads to the formation of epidermal idioblasts possessing some of the characteristidcs of the guard cells. Furthermore, in the absence of microtubules, local wall thickenings are deposited in the middle of the periclinal walls and at their junctions with the anticlinal ones.From the observations it seems likely that guard cell differentiation commences in GMC, and that the cortical cytoplasm and (or) the plasmalemma of the midregion and the edges of the periclinal walls of the GMC possess some factor(s) favouring their preferential thickening. Cortical microtubule organizing centres (MTOCs) appear to be localized in these regions and activated in a programmed fashion.

On the differential divisions and preprophase microtubule bands involved in the development of stomata of Vigna sinensis L

The manifestation of premitotic cell polarity and the resultant structural asymmetry of the differential divisions participating in the development of stomata of Vigna sinensis vary considerably. However, two morphologically distinct types of differential division were distinguished: (a) ‘asymmetrical differential divisions’, in which the premitotic polarization of the cell, the eccentric position of the nucleus during division and the differences in size and organization of the daughter cells are obvious; and (b) differential divisions in which the above features are inconspicuous or almost absent. The former occur in the ordinary protodermal cells, the latter in some meristemoids. The organization of a sharply demarcated preprophase microtubule band (PMB) precedes, all differential and non-differential divisions. In the first type of differential division the PMB is formed eccentrically, while in the second it may display either an approximately symmetrical or a clearly asymmetrical disposition, always indicating with surprising accuracy the sites where the succeeding cell plate will join the parent walls. The PMB foreshadowing the highly curved cell plates in meristemoids I of the mesoperigenous process, as well as in meristemoids I and II of the mesogenous one, are apposed only on one anticlinal wall and therefore do not encircle the nucleus or traverse the cell. In the symmetrical divisions of guard cell mother cells (GMC), as well as in those of protodermal cells, the PMB runs right round the internal plasmalemma surface in an equatorial position, coinciding with that of the future cell plate. In the former cells the wall abutting the cortical cytoplasm traversed by the band becomes locally thickened. The variability in the pattern of the microtubules of the band along the walls of the GMC is directly mirrored in the pattern of the thickening. It seems that in GMC the PMB mediates a directed exocytosis of dictyosome vesicles. In contrast to what is now generally accepted in dicotyledonous plants, each meristemoid I of both the mesogenous and mesoperigenous stomata in Vigna sinensis leaves does not inhibit but induces the formation of other meristemoids close to it.

Ultrastructural studies on the oil bodies of Marchantia paleacea Bert. I. Early stages of oil-body cell differentiation: origination of the oil body

The differentiation of the idioblastic oil-body cells (OBC's) of Marchantia paleacea begins with the formation of protoplasmic and organelle complement in some thallus cells, which are meristematic in appearance. An increase of cytoplasm quantity and density, a proliferation of rough endoplasmic reticulum (ER) membranes, free ribosomes, dictyosomes, plastids, and mitochondria, as well as the appearance of cytoplasmic microtubules and the establishment of ER–plastid relationships were observed especially. These associations, together with the increased cytoplasm quantity, constitute accurate criteria for the identification of very young OBC's.The above protoplasmic changes are accompanied by the cell polarization; the nucleus is displaced at the one end of the cell and the vacuoles at the other end or peripherally. At these stages, the dictyosomes appear active and produce numerous smooth and coated vesicles.Gradually, at a more or less central area free of vacuoles, a number of ER membranes, dictyosomes, and dictyosome vesicles are preferentially localized. Microtubules fan out from this area toward the cell walls. The oil body (OB) appears in the form of a rudimentary 'vacuole,' at the centre of this area. Its bounding membrane is identical with the one of dictyosome vesicles and quite different from the ones of ER and typical vacuoles. Microtubules are associated with its contour, while rough ER membranes may surround it partly. The nascent OB grows further by the fusion of dictyosome vesicles, a phenomenon demonstrated for the coated vesicles and suggested for the smooth ones. The microtubules form a dense framework around the growing OB, while some of them are detected bridged with its limiting membrane. Actually, these microtubules appear to radiate out from the OB and persist until the first stages of lipophilic material elaboration.From the presented observations it is suggested that the OB's originate by the fusion of dictyosome vesicles through a mechanism in which microtubules play a key role. The ER membranes also seem to participate in the development of the newborn OB.

The growth of the pollen tube wall in Oenothera organensis

The growth of the pollen tube wall of Oenothera is effected by the expulsion of fibrillar material from the cytoplasm into the developing wall. This material may also be seen in the cytoplasm, contained in membrane-bound vesicles. It is not clear how the content of the vesicles is discharged, but it appears not to involve the participation of microtubules. The source of the cytoplasmic fibrillar bodies depends upon the stage of development of the pollen tube. The earilest growth is derived from the inclusion into the wall of vesicles containing pre-formed materials present in the grain on pollination. During the next stage of growth the wall is derived from the content of double-membraned inclusions also present in the pollen. The content of the former vesicles is not so similar to the wall as the latter, but intermediates between the 2 types of vesicle may be seen in the cytoplasm, indicating that the former are formed from the latter. Most of the tube wall is derived from the products of dictyosomes in the pollen grain or tube. These dicytosomes are few in number and they must be exceedingly active. This, and the observation that dictyosome vesicles are frequently associated with banked complexes of mitochondria, indicates that some steps in the metabolism of the vesicular content, perhaps phosphorylation, take place distant from the dicytosomes. These different sources of fibrillar material presumably permit the rapid starting of tube growth, without any attendant metabolism. However, it would be impossible to include enough pre-formed wall material in the grain to enable the full growth of the tube, so once started, it seems that the tube then relies on the elaboration of simple reserves for the contruction of its wall. These reserves are likely to be held in the pollen, and may be the large numbers of starch grains characteristic of the pollen cytoplasm.

Pollen tube growth in the stigma of Oenothera organensis following compatible and incompatible intraspecific pollinations

The growth of the pollen tube wall of Oenothera organensis results from the insertion of bodies, composed principally of fibrils, synthesized in cytoplasm. This material for the early growth of the tube wall is derived from double membraned inclusions, present in the pollen on release from the anther. The wall of the later tube is derived from the products of dictyosome vesicles, believed to be non-cellulosic glucans. Some of these vesicles, which are formed in very large numbers on germination of the pollen grain, form an association with banked complexes of mitochondria, and it is proposed that this association indicates that phosphorylation of glucan precursors occurs within the vesicles, rather than in the dictyosome. The mechanism of tube wall growth following incompatible cross-pollinations is identical with that following compatible crosses. The early incompatible tube, however, contains far lower levels of free carbohydrates. Among these carbohydrates must be the precursors required for glucan synthesis, for the tube wall is laid down only thinly at this stage. Once the incompatible tube has passed through the outer layer of the stigmatic cells and their secretion, growth similar to that of a compatible tube starts. Subsequent growth of the tube appears to depend upon the amount of reserve material that passes through the region of low carbohydrate content. While the early interactions between the pollen and stigma are far from understood, it is clear that the self-incompatibility system acts principally during growth of the tube in the viscous fluid coating the outer stigmatic cells.

Symmetric and Asymmetric Mitosis and Cytokinesis in the Root Tip of Hydrocharis Morsus-Ranae L

In the roots of Hydrocharis morsus-ranae, certain cells of the protoderm divide asymmetrically to form a small, highly cytoplasmic trichoblast proximally, and a larger, more vacuolate epidermal cell distally. The former develops as a root hair without further division; the latter divides several times to form ordinary epidermal cells. During mitosis, presumed dictyosome vesicles and fragments or sections of reticulated or serrate sheets of ER, aligned with the spindle microtubules, were observed among the chromosomes as early as metaphase, suggesting that the portions of ER were involved in formation of the cell plate or in some other function in the equatorial region. A pre-prophase band of microtubules was not observed. Asymmetric divisions differ from symmetric ones in the skewed orientation of the metaphase plate, the formation of a curved, rather wavy cell wall and the slightly greater vacuolation of one daughter cell. Less difference in the ultrastructure of the daughter cells resulting from an asymmetric division was observed in this rather slowly growing material than in other examples previously described in the literature.