Fine Structure ofPsilotum NudumCells During Division

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Abstract
An ultrastructural study was made of mitotic cell division in sporogenous cells of Psilotum nudum. During prophase a « clear zone », devoid of large organelles but containing many ribosomes, vesicular cisternae and two sizes of microtubules, 18 mμ and 28 mμ in diameter, surrounds the nucleus. The microtubules of the metaphase spindle and of the phragmoplast are of the same size and appearance as the larger microtubules in the clear zone. Since many more microtubules were present in the phragmoplast than in the interchromosomal region in early anaphase, a new population of microtubules must form in this region in telophase. The organization of the cell plate around these tubules may result in the formation of plasmodesmata. Interphase chromatin appears in two forms, as condensed masses (heterochromatin) and as a diffuse matrix of chromatin (euchromatin). These two forms appear to be connected. The chromatin as well as the condensed chromosomes at all stages of division are composed of 10 mμ fibrils which in turn appear to be a helical arrangement of one or two 35 A strands. Masses of nucleolar material consisting largely of an 18 mμ granular component, often persist throughout cell division at the periphery of the spindle; however, these eventually disintegrate and are not included in the daughter nuclei. Clumps of loosely organized fibrous material in newly formed nuclei may represent the first stage of nucleolar formation. Three types of interchromatin substances were observed. Dense 40 mμ granules resembling Balbiani ring granules were seen, profiles of 25–30 mμ fibrous elements were abundant, and a 30 mμ diameter granule with a dense central partide and 6 or 7 similarly dense peripheral particles appeared in association with chromatin.