Subsection IV.I

In cyanobacteria of subsection IV.I the trichomes never exhibit basal–apical polarity , all vegetative cells being of uniform size . With the exception of Scytonema , cell divisions seem to occur at similar rates throughout the trichomes. Members of some genera undergo a distinct developmental cycle , involving hormogonia formation. Although some representatives may produce ensheathed trichomes, false branching is extremely rare , occurring frequently only in Scytonema . Under nitrogen‐limiting conditions, some members differentiate heterocysts exclusively in terminal or intercalary positions ; others exhibit both terminal and intercalary heterocysts. Akinetes (if present) may be formed singly or in short chains (two to three akinetes) exclusively adjacent to, or very close to, heterocysts. In some members, they are initiated equidistant between two heterocysts, successive differentiation on either side of the first akinete often leading to very long chains of akinetes, often comprising the majority of the cell population . Some botanical genera (e.g., Anabaenopsis and Cylindrospermum ) are easily identified by the location and/or spacing of heterocysts or akinetes. Cyanobacteria / Subsection IV / Subsection IV.I / Subsection IV.I