Identify that alga:
Web-based key to genera - 4

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  Unbranched, uniseriate filaments: Click on the thumbnail for a larger image
 

chloroplast a band or bands:

    
 

Chloroplast a simple axile band (lying up the centre of the cell): may be twisted.

(a) has pyrenoids:

(b) no pyrenoids : Pyrenoids are round organelles, usually visible, but more easily seen using Lugol's Iodine: they stain black because they store starch
 

Chloroplast(s) parietal, twisted into a spiral encircling the cell contents
Spirogyra: one of the most readily recognisable filamentous algae, also called "Pond-silk" from its very smooth feel
 

2 or more parietal band chloroplasts lying parallel or slightly spiral, but not by more than half a turn
Sirogoniumsuperficially very similar to Spirogyra
       
 

  Chloroplast with pyrenoids: cell walls parallel without constrictions at end

 

 
  Mougeotia
     

Mougeotia is the commonest genus answering to this description, but other similar ones exist, which can only be distinguished if zygospores (the product of conjugation) are found. These are:


Debarya - like one of the narrower Mougeotia (9-16µm): no illustration available.

Mougeotiopsis : lacks pyrenoids - see below

Temnogametum: similar to Mougeotia, but margin of chloroplast has fine cytoplasmic extensions, cells about 18µm wide. (no illustration available)

Cylindrocapsa : actually a member of the Ulotrichales, but can resemble a rather distorted Mougeotia, Zygnema, Microspora, or Ulothrix, with thickened cell walls and a chloroplast which may be mistaken for a band or net (actually stellate). The presence of swollen oospores gives it away.

 
 

Cells constricted at ends (rounded)

  
Zygnemopsis
       
 

If the chloroplast is an even texture, lack of pyrenoids indicates Mougeotiopsis (fairly rare)
  Mougeotiopsis
       
 

Chloroplast stellate

 
 

 

(a) 2 chloroplasts. Filament has a smooth outline, similar to Mougeotia or Spirogyra; cells long-cylindric.


  Zygnema
    (b) 1 chloroplast (in most cells - 2 before cell division). Cells are square-cylindric, wall may be thickened and look like Microspora
  Zygogonium
    (c) 2 chloroplasts, but each cell consists of 2 semicells (is a Desmid). Filament is enclosed in a wide mucilage sheath (shown up with Indian Ink in the accompanying illustration).
  Hyalotheca
       
 

Chloroplast a parietal band:

 
 

 

 

 partially or fully encircling the cell; cell wall clearly evident, no sheath.
  Ulothrix
    partially or fully encircling the cell, with 1 or more pyrenoids; cell wall indistinct, filament in an even-textured mucilage sheath.
  Geminella
    usually in middle of cell, but may be indistinct: cells in pairs or spaced out in a lamellate mucilage sheath
  Binuclearia 
    band or girdle-shaped, cells in a mucilage sheath similar to Geminella, but no pyrenoid   Gloeotila (no illustration available)
    several per cell, band or ring-shaped, cell elongated, contents divided up by clear areas, multinucleate   Sphaeroplea (no illustration available)
       
 

Choloroplast one or more discs

 
 

Chloroplast a parietal disc or plate, not completely filling the cell's circumference
  Hormidium (Klebsormidium)
      Similar genera:
Stichococcus has very short filaments which readily break up, cell diameter from about 1-4µm.
Uronema has pointed terminal cells
       
    Numerous disc-shaped chloroplasts, wall made of H-pieces
  Tribonema
       
 

Chloroplast a network

  
    Cells barrel-shaped, walls made up of H-pieces, visible at ends of filaments or where they overlap at the middle of the cell 
  Microspora:Compare with Tribonema, which has elongated, barrel-shaped cells and wall made of H-pieces, but numerous disc-shaped chloroplasts
    Filament irregular in thickness, cells tapering or bulbous, with rings towards one end of a cell
  Oedogonium
    Filament composed of longer cells than Microspora, similar to an unbranched Cladophora (see amongst branched filaments), but cells not as long; may show "knee-bends" like Klebsormidium, and occasionally rhizoid-like short branches (like Mougeotia)   Rhizoclonium
       
 

Chloroplast shape indistinguishable

  
    This is a frequent finding and causes identification problems Sometimes it is had to be sure even if there is a chloroplast or not, particularly with preserved or old material: the section below attempts to deal with some of these difficult cases
 

 

 

 Chloroplast present: filament composed of dumbell-shaped cells (or otherwise composed of semicells) linked together, possibly in a mucilage sheath: one of several filamentous Desmid genera

Filamentous desmids: many look like Cosmarium cells linked end-to-end


Bambusina: cells barrel-shaped, inflated in the centre, with a slight median incision, longitudinal striations on the end sections but not the inflated median area. Cells linked at the apices
     
 Desmidium: cell width exceeds height, with open median incision. End view is triangular so when seen side-on in a filament some cells show one corner projecting outwards: the cells are linked at a slight angle to each other so these projections form a band which twists round the filament.
       
  Hyalotheca: fairly quadrate cells, some with a slight median incision. Stellate chloroplast, mucilage sheath
      Onychonema: semicells fairly rounded, with 2 long processes at each end which link the cells together, mucilage sheath
     
 Sphaerozosma: cells with an open or closed median incision, linked by a single process at each end
     
  Spondylosium : semicells with open or closed median incision, linked at apices without granules; with sheath. Semicells often different sizes.
      Teilingia: median incision is open. Cells linked by small granules at apices
       
    cells in a thick walled filament (a) wall an even texture
(b) wall appears composed of plates or scales
(c) wall lamellate
       
 

(a)

 

Wall an even texture

 
  Often it is hard to tell whether the wall shows traces of structure or not, as in this illustration. The chloroplast here may be stellate, in which case this is Zygogonium
 
     
 
(b)
 Wall composed of plates or scales
  probably Microspora: the H-pieces which compose the wall can become thickened and partially detached - more than shown here. Note the chloroplast ought to be a net but is contracted to one end of the cell. Another possibility is:
      Klebsormidium (crenulatum).
 
(c)
 Wall lamellate:    
 

 

 

 

striations between cells, which may be arranged in pairs
  Binuclearia: related to Ulothrix (see the chloroplast)
   

Wall lamellate,or may be plain, cells long-cylindric, chloroplast may look similar to Mougeotia but indistinct; may show distinctive swollen oospores


  Cylindrocapsa: (may sometimes be multiseriate)

 
       

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John Kinross