No abstract

The spindle of the colonial diatom Fragilaria contains two distinct sets of spindle microtubules (MTs): (a) MTs comprising the central spindle, which is composed of two half-spindles interdigitated to form a region of "overlap;" (b) MTs which radiate laterally from the poles. The central spindles from 28 cells are reconstructed by tracking each MT of the central spindle through consecutive serial sections. Because the colonies of Fragilaria are fiat ribbons of contiguous cells (clones), it is possible, by using single ribbons of cells, to compare reconstructed spindles at different mitotic stages with minimal intercellular variability. From these reconstructions we have determined: (a) the changes in distribution of MTs along the spindle during mitosis; (b) the change in the total number of MTs during mitosis; (c) the length of each MT (measured by the number of sections each traverses) at different mitotic stages; (d) the frequency of different classes of MTs (i.e., free, continuous, etc.); (e) the spatial arrangement of MTs from opposite poles in the overlap; 09 the approximate number of MTs, separate from the central spindle, which radiate from each spindle pole. From longitudinal sections of the central spindle, the lengths of the whole spindle, half-spindle, and overlap were measured from 80 cells at different mitotic stages. Numerous sources of error may create inaccuracies in these measurements; these problems are discussed.The central spindle at prophase consists predominantly of continuotls MTs (pole to pole). Between late prophase and prometaphase, spindle length increases, and the spindle is transformed into two half-spindles (mainly polar MTs) interdigitated to form the overlap. At late anaphase-telophase, the overlap decreases concurrent with spindle elongation. Our interpretation is that the MTs of the central spindle slide past one another at both late prophase and late anaphase. These changes in MT distribution have the effect of elongating the spindle and are not involved in the poleward movement of the chromosomes. Some aspects of tracking spindle MTs, the interaction of MTs in the overlap, formation of the prophase spindle, and our interpretation of rearrangements of MTs, are discussed.J. CELL BIOLOGY 9 The Rockefeller University Press

The x-ray structure (λ<10 Å) of the DPF-78 discharge was investigated with different heavy-gas admixtures to a basic D2 filling. A possible classification of plasma features is proposed. Micropinches with an unusual high aspect ratio of about 10:1 were found. Soft x-ray spectra of single micropinches were registered on film. The use of x-ray spectroscopic methods by means of spectra simulation codes allowed us to estimate the parameters of particular types of pinches with axial size of 0.5 mm and full radial size of 0.05 mm. In the He-like ionization state we obtained ne≊3×1021 cm−3 and Te≊0.8 keV. The electron temperature determined in the H-like ionization stage was considerably higher, Te≊2 keV, indicating a strong ionizing plasma. Possible deviations from the Bennett condition are discussed in connection with the spectroscopic results.

The life history, reproduction and development of Desmarestia menziesii J. Agardh from Antarctica is described. Unilocular sporangia occur singly or in small groups in the outermost cortical layer of the sporophyte. They are formed by periclinal division of cortex cells into a stalk cell and the sporangium initial. Meiospores germinate into dioecious microscopic filamentous gametophytes. As in other perennial Antarctic species of the Desmarestiales, gametangia are formed in culture under short‐day conditions or in darkness. In nature, juvenile sporophytes should therefore be formed in winter. They develop only attached to the oogonium. At first they are uniseriate and elongate by means of an intercalary meristem located in their middle part. Laterals are formed predominantly in this region, and they subsequently give rise to secondary laterals. The branching pattern is opposite to alternate in both young and adult plants. Cortication of the main axis is initiated by filaments growing out from the lowermost cells of the primary laterals. In sporophytes of this developmental stage the meristem of the main axis is confined to a small region where cortication starts and above. Lateral branches elongate and become corticated in the same way as the main axis. In mature plants, cells of the inner cortex can become meristematic again and form a meristoderm which contributes to axis thickness by periclinal and anticlinal divisions. The observations are discussed in relation to possible evolutionary relationships in the genus Desmarestia and in the order Desmarestiales.