Ultrastructure and Differentiation in Cladophora Glomerata. I. Cell Division

Cladophora glomerata is a coenocytic, fresh‐water green alga in which mitosis and cytokinesis occur independently. The mitotic spindle is centric, closed, and develops from two half‐spindles which form from amorphous but well‐defined MTOCs at each pole. The nucleolus is only partially dispersed during mitosis and structured kinetochores are evident on the chromosomes. Anaphase separation of chromosomes is asynchronous and results from spindle elongation plus shortening of the chromosome‐to‐pole distance. Neither a phycoplast nor a phragmoplast is present during cytokinesis. Microtubules are associated with the septum but whether they participate actively in its ingrowth is not clear. Two types of vesicles are associated with the growing septum. The membrane at its leading edge is thicker and more densely stained than elsewhere. The ultrastructure of nuclear and cell division in C. glomerata is sufficiently different from the data on other green algae that conclusions about phylogeny must await further study, especially of other coenocytic green algae.