1,721,304 research outputs found
Asplenium trichomanes ssp. trichomanes gametophyte. A light and electron microscope study
No full textIn controlled culture conditions Asplenium trichomanes L. ssp. trichomanes spores germinate 8–10 days after sowing. The first protonemal cell and the primary rhizoid arise from two unequal divisions of the original spore cell. First an unbranched 5–6-celled germ forms which later becomes a spatula-, racket- and finally heart-shaped, bisexual prothallus with archegonia consisting of four rows of four cells and antheridia consisting of opercular, ring and basal cells. An additional notch with associated archegonia and antheridia develops in several-month-old gametophytes. The ultrastructure of the mature gametophyte is characterized by pleomorphic mitochondria, plastids which may be chloroamyloplasts, vacuoles containing osmiophilic materials and poorly developed endoplasmic reticulum of smooth type. The walls are crossed by many plasmodesmata. (Membrane) structures are visible inside the wall materials nearest the plasmalemma. © 1995 Taylor & Francis Group, LLC
Beyond graphene: Clean, hydrogenated and halogenated silicene, germanene, stanene, and plumbene
No full textThe fascinating electronic and optoelectronic properties of freestanding graphene and the possible inclusion of novel two-dimensional (2D) systems in silicon-based electronics have driven the search for atomic layers consisting of other group-IV elements Si, Ge, Sn, and Pb, which form similar hexagonal lattices and are isoelectronic to graphene. The resulting 2D crystals silicene, germanene, stanene and plumbene, referred as Xenes, but also their functionalized counterparts, e.g. the hydrogenated sheet crystals, named as Xanes, silicane, germanane, and stanane, are in the focus of this review article. In addition, halogenated Xenes are investigated. The consequences of the larger atomic radii on the atomic geometry, the energetic stability, and possible epitaxial preparations are discussed. In the case of honeycomb atomic arrangements, the low-energy electronic excitations are ruled by almost linear bands. Spin–orbit coupling opens small gaps leading to Dirac fermions with finite effective masses. The linear bands give rise to an absorbance of the Xenes determined by the finestructure constant in the long-wavelength regime. While for vanishing photon energies the excitonic influence is still an open question, saddle-point excitons and excitons at M0 van Hove singularities appear at higher frequencies. After opening substantial fundamental gaps by hydrogenation, the absorption edges of the Xanes, silicane, germanane, and stanane, are dominated by bound excitons with extremely large binding energies. Other chemical functionalizations, but also vertical electric fields, yield electronic structures ranging from topological to trivial insulators. Even a quantum spin Hall phase is predicted at room temperature. The topological character and the possible quantization of the spin Hall conductivity are studied versus gap inversion, chemical functionalization, and Rashba spin–orbit interaction. The drastic changes of the electronic properties of Xenes with chemical functionalization, interaction with the substrate, and external perturbations, open future opportunities for tailoring fundamental properties and, therefore, interesting applications in novel electronic and optoelectronic nanodevices
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A scanning and transmission electron microscope study of gametangia in the green seaweed 'Halimeda tuna
No full textThe gametangia of the green seaweed Halimeda tuna are spherical bodies of diameter up to 250–300 μm. They are clustered in groups of 8–10 on hundreds of threads sprouting from all pale white segments of the fertile individuals. In addition to gametes, starch-containing chloroplasts, naked starch grains and two types of spherical bodies different in size and ultrastructure are the main corpuscular components. A layer of amorphous material of irregular thickness underlies the walls which are finely and evenly rough, structureless, electron translucent and 1–1.5 μm thick. Gametangia with superficial wall warts were found also. © 1994 Taylor & Francis Group, LLC
Ultrastructure, viability, and in vitro germination of the tricellular Sambucus nigra L. pollen
No full textMature pollen grains of Sambucus nigra L. are tricolporate, isopolar, 12.5 mm wide, 25 mm long, and tricellular. They have a tectate exine, a bilayered nexine, and a thin intine. The vegetative cell has abundant rough endoplasmic reticulum, many well-structured mitochondria, numerous apparently inactive dictyosomes, immature dividing, and starch-filled plastids and lipid bodies. The sperm cells have few and poorly structured organelles. They are linked by cytoplasmic bridges, indicating a persistent physiological unit, and are surrounded
by fibrillar polysaccharide material. Freshly released pollen is 95% viable. Three- and 4-year-old pollen grains stored at 20C are 78.2% and 43% viable, respectively. In vitro germination requirements and pollen tube growth are more similar to those of bicellular pollen than to those of tricellular pollen
An unusual cytoplasmic component Euphorbia peplus L. endosperm
No full textThe coenocytic endosperm of Euphorbia peplus contains a complex system of flat, tightly-stacked membrane saccules which have a sinuous profile and a smooth or smooth-rough surface. They are joined by membrane-to-membrane links. The usual ergastoplasmic cisternae are an integral component of this system which can essentially be regarded as a sui generis from of endoplasmic reticulum. © 1995 Annals of Botany Company
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