PREPROPHASIC ESTABLISHMENT OF DIVISION POLARITY IN MONOPLASTIDIC MITOSIS OF HORNWORTS

Preoprophase in the monoplastidic mitotic cells of Phaeoceros laevis and Notothylas orbicularis is characterized by the establishment of a division site in the absence of a typical preprophase band. The future cytokinetic plane is predicted by plastid orientation and development of an elaborate preprophasic microtubule system perpendicular to the division plane. Division of the single plastid is initiated early in preprophase and the constricting plastid migrates to a position perpendicular to the future plane of division. Plastid orientation assures that division of the plastid by mid-constriction will result in distribution of a plastid to each daughter cell. Microtubules parallel the long axis of the plasmid and are most numerous adjacent to the nucleus which becomes elongated in the future spindle axis. The division site apparently is a fundamental component of the cytokinetic apparatus involved in the determination of cleavage plane prior to nuclear division.