Ultrastructure of Sporogenesis in a Moss, Ditrichum pallidum II. Metaphase I through the Tetrad

Abstract
Events of meiosis subsequent to prophase are reported for a moss, D. pallidum. Condensed metaphase I bivalents are deployed along the equator of an open spindle consisting of continuous microtubules (MT) and chromosomal MT attached at kinetochores. Structurally simple spindle poles are located in the cytoplasm within opposite cleavage furrows between pairs of cytoplasmic lobes into which telophase II nuclei will be distributed. No discrete structure was observed associated with lobing or with spindle MT orientation. After a short inframeiotic interphase, meiosis II occurs in the undivided cytoplasm with spindles perpendicular to each other. Each of the poles is located near the plastid within each of the 4 lobes. Intersporal septa are formed by coalescence of vesicles simultaneously along cleavage planes predetermined by precocious lobing. Fused vesicle membranes contribute plasma membrane to the proximal surfaces of the spores, and the fibrillar contents of the vesicles, indistinguishable from the polysaccharide fibrils of the sporocyte wall, separate the young spores within the tetrad.