Subsection IV

Cyanobacteria / Subsection IV / Incertae Sedis / Subsection IV Members of Subsection IV are filamentous cyanobacteria that divide exclusively by binary fission in one plane , perpendicularly to the long axis of the trichomes. Cell division occurs at similar rates throughout the trichome or, in some genera, may be more rapid in “meristematic” zones of the filaments (Castenholz, 1989b; Komárek and Anagnostidis, 1989; Whitton, 1989). With the exception of some representatives of the genus Nostoc , the trichomes are always clearly uniseriate. The ensheathed filaments of some genera (e.g., Calothrix or Scytonema ) may exhibit false branching , a developmental process distinct from true branching that occurs only in members of Subsection V (see Hoffman and Castenholz, this volume) and involves cellular division in more than one plane. Several genera are defined by a basal–apical polarity of the filaments (i.e., tapering trichomes), the apical extremity often terminating in thin and colorless cells, termed “hair” cells (Komárek and Anagnostidis, 1989; Whitton, 1989). The trichome diameter among members of Subsection IV presently in culture varies greatly, ranging from about 2 µm ( Cylindrospermopsis ) to 15 µm (i.e., members of Scytonema, Calothrix , and Rivularia ).