Filament becoming multiseriate as it increases in size. True branches arise by lateral cell division: branches of various lengths stick out at right angles to the main axis. Plant brown; cells olive-green or brown. Heterocysts may be present.

True branches stick out at right angles to the main filament: filament usually uniseriate. Cells blue-green.

   
Hapalosiphon : Fischerella is rather similar

Looks similar to last, but branching is false and cells have chloroplasts with clearly visible pyrenoids; arranged in series (usually single) in a broad sheath.

Filaments more or less even thickness throughout:

(a) false branches arise at a heterocyst:

(i) each trichome has its own sheath

 
Tolypothrix: a heterocyst lies at the base of each branch

(ii) several trichomes share a common sheath

 
Coleodesmium: looks like a rather bushy Tolypothrix (not evident from these images):
(cf. Dichothrix and Rivularia, which have tapering filaments)

(b) false branches arise between heterocysts

Filaments taper towards tip

(a) no basal heterocyst

Homoeothrix: compare very carefully with (a) Tolypothrix and Scytonema, to ensure that the branches do taper as in the preceeding illustration, and (b) with Calothrix to check there are no basal heterocysts. The sheath is firm over the basal part of the trichome but becomes thinner and may splay out towards the tip.

(b) basal heterocyst

(i) filaments single or in a small group: false branching is infrequent and may not be seen.

 
Calothrix: sheath is firm, covering the thicker, basal part of the trichome, but does not usually cover the heterocyst

(ii) filaments branching frequently, false branches with a basal heterocyst, arising in a clump or separately

   
Dichothrix: sheath is firm over the basal part of the trichome but becomes thinner and may splay out towards the tip

(iii) Filaments forming a clump:

radiating out from the centre, not branched

forming a clump attached to a surface, radiating or running parallel; false branching frequent with a heterocyst at the base of the branch, occasional intercalary heterocysts