Ultrastructural Studies Onsphaeroplea Annulina(chlorophyceae). Vegetative Structure and Mitosis¹

The ultrastructure of the taxonomically disputed filamentous green alga Sphaeroplea annulina (Roth) Agardh has been investigated. The filament is a linear coenobium, the individual coenocytes of which are separated by remnants of cytoplasm termed “plugs.” Vegetative growth is coupled with a form of asexual reproduction in which multinucleate autospores are formed but not liberated. Mitosis is of the semi‐open and centric type. A complete disintegration of the nucleolus in metaphase is not seen. A typical phycoplast to which the centriole pairs migrated at the end of telophase was present. Cytokinesis was not coupled with mitosis. These observations rule out possible affinities with the orders Siphonales (sensu lato) and Cladophorales, and, apart from similarities to the filamentous (uninucleate) Microspora, the Ulotrichales as well. Certain similarities with coenocytic chlorococcalean algae occur, but this investigation supports the concept of a separate order: the Sphaeropleales.