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  • 1995-1999  (96)
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  • 1
    ISSN: 1432-2145
    Keywords: Key words Arabidopsis thaliana ; Alveoli ; Development ; Endosperm ; Microtubules ; Seeds
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract  The process of endosperm development in Arabidopsis was studied using immunohistochemistry of tubulin/microtubules coupled with light and confocal laser scanning microscopy. Arabidopsis undergoes the nuclear type of development in which the primary endosperm nucleus resulting from double fertilization divides repeatedly without cytokinesis resulting in a syncytium lining the central cell. Development occurs as waves originating in the micropylar chamber and moving through the central chamber toward the chalazal tip. Prior to cellularization, the syncytium is organized into nuclear cytoplasmic domains (NCDs) defined by nuclear-based radial systems of microtubules. The NCDs become polarized in axes perpendicular to the central cell wall, and anticlinal walls deposited among adjacent NCDs compartmentalize the syncytium into open-ended alveoli overtopped by a crown of syncytial cytoplasm. Continued centripetal growth of the anticlinal walls is guided by adventitious phragmoplasts that form at interfaces of microtubules emanating from adjacent interphase nuclei. Polarity of the elongating alveoli is reflected in a subsequent wave of periclinal divisions that cuts off a peripheral layer of cells and displaces the alveoli centripetally into the central vacuole. This pattern of development via alveolation appears to be highly conserved; it is characteristic of nuclear endosperm development in angiosperms and is similar to ancient patterns of gametophyte development in gymnosperms.
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  • 2
    ISSN: 1432-2145
    Keywords: Cytokinesis ; Microtubules ; Microsporogenesis ; Orchids ; Phragmoplast ; Pollen
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Microsporocytes of the slipper orchidCypripedium californicum A. Gray divide simultaneously after second meiosis. The organization and apportionment of the cytoplasm throughout meiosis are functions of nuclear-based radial microtubule systems (RMSs) that define domains of cytoplasm - a single sporocyte domain before meiosis, dyad domains within the undivided cytoplasm after first meiosis, and four spore domains after second meiosis. Organelles migrate to the interface of dyad domains in the undivided cytoplasm after first meiotic division, and second meiotic division takes place simultaneously on both sides of the equatorial organelle band. Microtubules emanating from the telophase II nuclei interact to form columnar arrrays that interconnect all four nuclei, non-sister as well as sister. Cell plates are initiated in these columns of microtubules and expand centrifugally along the interface of opposing RMSs, coalescing in the center of the sporocyte and joining with the original sporocyte wall at the periphery to form the tetrad of microspores. Organelles are distributed into the spore domains in conjunction with RMSs. These data, demonstrating that cytokinesis in microsporogenesis can occur in the absence of both components of the typical cytokinetic apparatus (the preprophase band of microtubules which predicts the division site and the phragmoplast which controls cell-plate deposition), suggest that plant nuclei have an inherent ability to establish a domain of cytoplasm via radial microtubule systems and to regulate wall deposition independently of the more complex cytokinetic apparatus of vegetative cells.
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  • 3
    ISSN: 1432-0894
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract An analysis is presented of extratropical cyclone frequency, the 500 hPa height standard deviation field and the monthly 700 hPa height field in the Northern Hemisphere, together with precipitation in the Mackenzie basin and the Mackenzie River runoff. Spatial and temporal variability in the data are examined for the period 1965 to 1989, and a cross-correlation analyses is performed to determine the relationship between the runoff and the precipitation variations, and between the precipitation and the atmospheric circulation anomalies. It is found that precipitation fluctuations in the Mackenzie River drainage basin are strongly linked to variations in the Mackenzie River runoff and in the North Pacific storm tracks, with the time scale of variability ranging from interannual to decadal. The results are discussed in relation to the interdecadal Arctic climate cycle proposed by Mysak, Manak and Marsden, and revised by Mysak and Power. In particular, the latter authors hypothesized that, as part of this cycle, air-sea interactions and synoptic scale processes over the northwestern North Atlantic influenced, via cyclone movements in the Labrador Sea and Baffin Bay, precipitation in northern Canada and hence river runoff into the Arctic. The results of this study indicate that such influences on the precipitation in the Mackenzie basin are small, and hence that the Mysak-Power feedback loop which describes this climate cycle needs further revision.
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  • 4
    ISSN: 1432-0894
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract. An analysis is presented of extratropical cyclone frequency, the 500 hPa height standard deviation field and the monthly 700 hPa height field in the Northern Hemisphere, together with precipitation in the Mackenzie basin and the Mackenzie River runoff. Spatial and temporal variability in the data are examined for the period 1965 to 1989, and a cross-correlation analyses is performed to determine the relationship between the runoff and the precipitation variations, and between the precipitation and the atmospheric circulation anomalies. It is found that precipitation fluctuations in the Mackenzie River drainage basin are strongly linked to variations in the Mackenzie River runoff and in the North Pacific storm tracks, with the time scale of variability ranging from interannual to decadal. The results are discussed in relation to the interdecadal Arctic climate cycle proposed by Mysak, Manak and Marsden, and revised by Mysak and Power. In particular, the latter authors hypothesized that, as part of this cycle, air-sea interactions and synoptic scale processes over the northwestern North Atlantic influenced, via cyclone movements in the Labrador Sea and Baffin Bay, precipitation in northern Canada and hence river runoff into the Arctic. The results of this study indicate that such influences on the precipitation in the Mackenzie basin are small, and hence that the Mysak-Power feedback loop which describes this climate cycle needs further revision.
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  • 5
    ISSN: 0016-7835
    Keywords: Key words Fission-track dating ; Apatite ; Thermochronology ; KTB
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  A detailed fission-track analysis was carried out on 41 apatite samples from the 4001-m-deep pilot hole of the German Continental Deep Drilling Program (KTB). The investigations include dating by the population method and length measurements of horizontal confined tracks. Age and length information indicate that all samples above the present-day partial annealing zone cooled from temperatures above 120 °C in the Cretaceous. The top two kilometres of the profile show less variation of age with depth than was expected. Modelling of the profile indicates that the distribution of fission track ages with depth requires that the profile was disturbed during or after cooling through the partial annealing zone. Modelling of individual samples reveals that at least the upper 2 km experienced reverse faulting, effectively thickening the upper part of the rock column by up to 1000 m in the Tertiary. A present-day partial annealing zone for apatite was observed between 2000 and 4000 m (∼60–110 °C). This confirms earlier in situ observations of the position of the partial annealing zone.
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  • 6
    Publication Date: 2018-06-05
    Description: State of the an programmable devices are utilizing advanced processing technologies, non-standard circuit structures, and unique electrical elements in commercial-off-the-shelf (COTS)-based, high-performance devices. This paper will discuss that the above factors, coupled with the systems application environment, have a strong interplay that affect the radiation hardness of programmable devices and have resultant system impacts in (1) reliability of the unprogrammed, biased antifuse for heavy ions (rupture), (2) logic upset manifesting itself as clock upset, and (3) configuration upset. General radiation characteristics of advanced technologies are examined and manufacturers' modifications to their COTS-based and their impact on future programmable devices will be analyzed.
    Keywords: Computer Programming and Software
    Format: application/pdf
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  • 7
    ISSN: 1615-6102
    Keywords: Erythrocladia subintegra ; Red alga ; Freeze-fracture ; Cellulose ; Linear terminal complexes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The formation and development of linear terminal complexes (TCs), the putative cellulose synthesizing units of the red algaErythrocladia subintegra Rosenv., were investigated by a freeze etching technique using both rotary and unidirectional shadowing. The ribbon-like cellulose fibrils ofE. subintegra are 27.6 ± 0.8 nm wide and only 1–1.5 nm thick. They are synthesized by TCs which are composed of repeating transverse rows formed of four particles, the TC subunits. About 50.4 ± 1.7 subunits constitute a TC. They are apparently more strongly interconnected in transverse than in longitudinal directions. Some TC subunits can be resolved as doublets by Fourier analysis. Large globular particles (globules) seem to function as precursor units in the assembly and maturation of the TCs. They are composed of a central hole (the core) with small subunits forming a peripheral ridge and seem to represent zymogenic precursors. TC assembly is initiated after two or three gobules come into close contact with each other, swell and unfold to a nucleation unit resembling the first 2–3 transverse rows of a TC. Longitudinal elongation of the TC occurs by the unfolding of globules attached to both ends of the TC nucleation unit until the TC is completed. The typical intramembranous particles observed inErythrocladia (unidirectional shadowing) are 9.15 ± 0.13 nm in diameter, whereas those of a TC have an average diameter of 8.77 ± 0.11 nm. During cell wall synthesis membranes of vesicles originating from the Golgi apparatus and which seem to fuse with the plasma membrane contain large globules, 15–22 nm in diameter, as well as ‘tetrads” with a particle diameter of about 8 nm. The latter are assumed to be involved in the synthesis of the amorphous extracellular matrix cell wall polysaccharides. The following working model for cellulose fibril assembly inE. subintegra is suggested: (1) the ribbon-like cellulose fibril is synthesized by a single linear TC; (2) the number of glucan chains per microfibril correlates with the number of TC subunits; (3) a single subunit synthesizes 3 glucan chains which appear to stack along the 0.6 nm lattice plane; (4) lateral aggregation of the “3-mer” stacks leads to the crystalline microfibril.
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  • 8
    ISSN: 1615-6102
    Keywords: Confocal microscopy ; Microtubules ; Mitosis ; Orchid ; Phragmoplast ; Pollen
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Cytokinesis following asymmetrical pollen mitosis was studied in the slipper orchidCypripedium fasciculatum using techniques of immunofluorescence, confocal laser scanning, and transmission electron microscopy. Data from stereo reconstructions of double labelled preparations (microtubules/nuclei) show that the contribution of residual spindle fibers to development of the interzonal array is minor; rather, new populations of microtubules are nucleated in association with the two groups of anaphase chromosomes. As kinetochores reach the poles, trailing arms of the chromosomes and nonkinetochore microtubules are displaced outward in the equatorial zone and by early telophase the interzone is left virtually free of microtubules. The interzonal apparatus has its origin in a massive proliferation of microtubules from the polar regions and surfaces of contracting chromosomes. Each polar region appears as a hub from which microtubules radiate in a spoke-like configuration and numerous tufts of microtubules appear to emanate from margins of the chromosomes themselves. These newly organized arrays of microtubules extend to the equatorial region where they interact to form the interzonal apparatus. Increasing organization of microtubules in the interzone results in development of a typical phragmoplast configuration consisting of opposing cone-like bundles of microtubules bisected by an unstained equatorial line.
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  • 9
    ISSN: 1615-6102
    Keywords: Cereals ; Endosperm ; Development ; Polarity ; Microtubules
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The endosperm of cereal grains develops as a multinucleate mass of wall-less cytoplasm (syncytium) that lines the periphery of the central cell before becoming cellular. The pattern of initial wall formation is precisely oriented and is followed by a round of precisely oriented formative cell division that gives rise to initials for the two tissues of endosperm. The initial anticlinal walls form at boundaries of nuclear-cytoplasmic domains (NCDs) defined by radial microtubules emanating from nuclei in the syncytium. Polarized growth of the NCDs in axes perpendicular to the embryo sac wall and centripetal elongation of the anticlinal walls results in a single layer of open ended alveoli overtopped by the remaining syncytial cytoplasm. This arboreal stage, so named because the elongate nucleate columns of cytoplasm resemble an orchard of trees, predicts the division polarity of the imminent formative division. Mitosis occurs as a wave which, like polarization, moves in both directions from ventral to dorsal. Spindles are oriented parallel to the long axis of the alveoli and cell plates give rise to periclinal walls. The outer daughter nuclei (aleurone initials) are thus completely enclosed by walls and the inner nuclei (starchy endosperm initials) are in alveoli adjacent to the central vacuole.
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  • 10
    ISSN: 1615-6102
    Keywords: Confocal laser scanning microscopy ; Cytoplasmic domains ; Meiosis ; Microtubules ; Organelle band ; Polarity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Establishment of division polarity and meiotic spindle organization in the lady's slipper orchidCypripedium californicum A. Gray was studied by immunocytochemistry, confocal and transmission electron microscopy. Prior to organization of the spindle for meiosis I, the cytoplasmic domains of the future dyad and spindle polarity are marked by: (1) constriction of the prophase nucleus into an hourglass shape; (2) reorganization of nuclear-based radial microtubules into two arrays that intersect at the constriction; and (3) redistribution of organelles into a ring at the boundary of the newly defined dyad domains. It is not certain whether the opposing microtubule arrays contribute directly to the anastral spindle which is organized in the perinuclear areas of the two hemispheres. By late prophase each half-spindle consists of a spline-like structure from which depart the kinetochore fibers. This peculiar spindle closely resembles the spline-like spindle of generative-cell mitosis in certain plants where the spindle is distorted by physical constraints of the slender pollen tube. In the microsporocyte, the elongate spindle of late prophase/metaphase is curved within the cell so that the poles are not actually opposite each other and chromosomes do not form a plate at the equator. By late telophase the poles of the shortened halfspindles lie opposite each other. Plasticity of the physically constrained plant spindle appears to be due to its construction from multiple units terminating in minipoles. Cytokinesis does not follow the first meiosis. However, the dyad domains are clearly defined by radial microtubules emanating from the two daughter nuclei and the domains themselves are separated by a disc-like band of organelles.
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