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Several morphological characteristics of cyanobacterial filaments generally observable in the light microscope can be of use in grouping genera. Are the filaments branched? Do they have a gradient of cell diameters (tapering)? Do they have more than one type of cell (cell differentiation)? Is there a visible sheath outside the cells (clear or colored, and thin or thick) that may extend past the last cell in the trichome?


Choices in order of appearance in PhycoKey:

1. unbranched; pseudobranches; or true branches

2. constant width; or tapered toward one or both ends

3. no cell differentiation (all vegetative); or presence of akinetes and/or heterocysts

4. sheath visible (light microscopy); or no sheath visible


Some colonial cyanobacteria comprised of filaments (e.g. Gloeotrichia, Microcoleus, Nostoc, and Rivularia) may be found in both colonial and filamentous keys.