Morphological variation of four planktic nostocalean cyanophytes - membersof the genus Aphanizomenon or Anabaena?

Morphological variation of trichomes, heterocytes and akinetes in four spec ies classified into the nostocalean genera Aphanizomenon Morren ex Bornet e t Flahault or Anabaena Bory (Cyanophyta/Cyanobacteria) was studied from nat ural samples by LM and illustrated by drawings and micrographs. All investi gated species, Aphanizomenon gracile (Lemmerm.) Lemmerm., A. aphanizomenoid es (Forti) Komarek et Horecka, Anabaena bergii Ostenfeld, A. minderi Huber- Pestalozzi, occur in the plankton as solitary trichomes, similar to many re presentatives of the genus Anabaena. Main generic diagnostic characters of Aphanizomenon and Anabaena are discussed.