CULTURING,ULTRASTRUCTURE AND COLONY FORMATION IN PECTODICTYON CUBICUM (CHLOROPHYCEAE,CHLOROCOCCALES) 12

Pectodictyon cubicum Taft collected in California develops typical eight-celled, cuboidal colonies only in alkaline media. Vegetative cell ultrastructure is similar to that in other genera of the Chlorococcales, except for an extensive endoplasmic reticulum system paralleling the plasmalemma (termed a paramural ER). Autosporogenesis proceeds inside the parent cell wall by three mitotic divisions producing eight nuclei in two tiers of four. Cleavage furrows following paths delineated by long ER segments and indistinct microtubules bisect and then radically separate the protoplast into eight pyramidal units. Walls comprised of a thick, presumably cellulose layer and a thinner trilaminar layer (not acetolysis resistant) form as cells become rounded, except where touching. A gelatinous matrix is produced around and between cells with concentration at the three sites of prior cell contact 90 degrees apart. Pulsed production of mucilage in three solid strands forces cells to separate, and the expanding colony becomes a hollow cube with a cell at each corner.