The concept of the genus Ulothrix (Chlorophyta) strengthened by comparative cytology.

The revelation of the ultrastructure of the pyrenoid, flagellar apparatus and mitosis/cytokinesis in the green algal genus Ulothrix, in its traditional broad sense, resulted in the assignment of species to three newly defined green algal classes. The largest group consisting of 11 species (including the type species Ulothrix tenuissima) is classified in the Ulvophyceae, the species U. verrucosa and U. crenulata are transferred to the genus Klebsormidium in the Charophyceae while Ulothrix species with pointed apical cells (e.g. U. belkae) appear to belong to the Chlorophyceae. It is shown that cytokinesis in Ulothrix sensu stricto may vary from a strict furrowing mechanism in U. mucosa to a pattern in which the centripetal ingrowth is combined with a cell plate-like structure in U. palusalsa and U. zonata. Based on the growth habit, the type of life history and the ultrastructural features pertaining to cytokinesis and the flagellar apparatus, phylogenetic trends are suggested for the Ulotrichales sensu lato.