MITOSIS,CYTOKINESIS, AND COLONY FORMATION IN THE GREEN ALGA SORASTRUM1

Synchronous mitotic divisions produce multi-nucleate cells of Sorastrum. Perinuclear envelopes of endoplasmic reticulum and a virtually intact nuclear envelope enclose mitotic nuclei. Cytoplasmic cleavage, which shirts before the last round of Synchronous mitoses, gives rise to uninucleate fragments which differentiate to form zoospores. These zoospores are released into a spherical vesicle, presumably derived from the inner layer of the parental cell wall, in which they swarm actively before aggregating as a spherical colony. The roughly conical shaped zoospores apparently adhere laterally before withdrawing their flagella and extending horns and a stipe, which, following wall deposition, interconnects the cells at the center of the colony. The probable role of the microtubules, which underlie the plasmalemma of aggregating cells, in determining the shape of both the cells and the colony itself is discussed.