Cell Division in the Filamentous Pleurastrum and Its Comparison With the Unicellularplatymonas(chlorophyceae)1

SUMMARY At prophase inPleurastrum,extranuclear spindle microtubules develop from the region of centrioles, which lie lateral to the nucleus midway between the future sites of the metaphase spindle poles. The microtubules then move laterally to overarch the nucleus and finally become incorporated into the spindle. The centrioles do not migrate and therefore lie in the same plane as the chromosomes at metaphase. At telophase, 2, more different systems of microtubules develop from the vicinity of the centrioles—a phycoplast and extensive arrays of microtubules that ensheath the daughter nuclei. Cell division in the filamentousPleurastrumis compared to that in the green flagellate,Platymonas.The similarities between cell division in the 2 algae are interpreted as evidence: (i) that rhizoplasts (which inPlatymonasresemble myofibrils) are somehow homologous to microtubules; and, (ii) that cell division inPleurastrumdiffers from cell division in other examined filamentous chlorophycean genera because Pleurastrum has an independent evolutionary origin from a monad withPlatymonas‐like characteristics.