Identify that alga:
Web-based key to genera - 6

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filaments

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branched
+chloroplast
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unbranched
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Colonies and coenobia
+chloroplast
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planktonic,
with sheath
 
(search11)
planktonic,
without sheath  
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 attached
cushion

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 attached,
flat plate
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attached,
sheath

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Colonies and coenobia : Cyanobacteria
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planktonic,
with sheath
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Unicells

(under development)
 + chloroplast(s)
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(under development)
Desmids

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non-flagellate cells
without case
(under development)
 in silica case
= diatoms
(under development)
flagellate cells

(under development)
 
 
 
 

no chloroplast:
Cyanobacteria
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Branched filaments: (without chloroplasts = branched filamentous cyanobacteria)
      Click on the thumbnail for a larger image
  Cells rounded, arranged in series within a thick sheath:
 

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.

Stigonema
 

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.
 
  Asterocystis (Chroodactylon): a Rhodophyte
       
  cells short-cylindric, forming smooth filaments (trichomes), with a 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

Scytonema: there is no heterocyst at the point of branching; branching is usually mid-way between heterocysts. This double form of branching is typical, but single branching also occurs:

         
 

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

 

 
Gloeotrichia: may form spherical planktonic colonies
 

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  

Rivularia: (no illustration available)

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John Kinross