WebKey: DESMIDS

Phylum Chlorophyta; sub-phylum Chlorophyceae; Order Zygnematales

There are two groups of Desmids: the "Placoderm" and the "Saccoderm" Desmids.

Placoderm desmids have cells composed of two halves (semicells), often almost divided from each other by deep median incisions. The cells are symmetrical about 3 planes, and often bear elaborate ornamentation. Genera are Cosmarium, Cosmocladium, Euastrum, Micrasterias, Staurastrum, Staurodesmus, Xanthidium.

Saccoderm desmids are simple shapes, cylindrical or spindle-shaped, rarely with any median constriction. This group forms the family Mesotaeniaceae and includes the genera Cylindrocystis, Mesotaenium, Netrium, Roya, Spirotaenia, Tortitaenia, and, in the suborder Closteriinae, Closterium, Gonatozygon and Penium, and in the suborder Desmidiinae, Actinotaenium.

Saccoderm:      

KEY to Genera:

   
Go to:
 1a
Cells solitary, symmetrical
2
1b
Cells in colonies or linked together in filaments
15
 
 
   
2a
Cells more or less cylindrical
3
2b
Cells a more complex shape
  9
 
 
   
3a
Cells with a median constriction
4
3b
Cells without a median constriction 
6
 
 
   
4a
Median constriction slight

Penium

Actinotaenium
Mesotaenium

 
4b
Median constriction conspicuous
5
  
 
   
5a
Apex without a median notch
  Pleurotaenium  
5b
Apex with a median notch
Tetmemorus (no illustration available)  
 
 
   
6a
Chloroplasts 2 simple ribbons
Gonatozygon  
6b
Chloroplasts 2, stellate or ridged
  7
  
 
   
7a
Walls smooth, without pores
  8
7b
Walls with pores / granulation / striation
 
 
 
   
8a
Choloroplasts stellate
Cylindrocystis  
8b
Chloroplast ridged with notched edges
Netrium  
 
 
   
9a
Cells flattened from front to rear, with distinct median isthmus
(side view) (frontal view) 10
9b
Cells radially symmetrical, with distinct median isthmus
(end-on view)  (Frontal view) 14
 
 
   
10a
Lateral margins not incised
11
10b
Lateral margins slightly incised; apex with a deep notch or slightly depressed
          Euastrum  
10c Lateral margins deeply incised     Micrasterias  
  
 
   
11a
With marginal spines
12
11b
Without marginal spines
  13
 
 
   
12a
Cell face has a central protuberance
  Xanthidium  
12b
No central protuberance
Staurodesmus  
 
 
   
13a
Angles of semicells extended into processes
      Staurastrum  
13b
Angles of semicells not extended into processes
Cosmarium  
       
14a
With spines at angles
    Staurodesmus  
14b
Angles of semicells extended into processes

Staurastrum
 
 
 
   
15a
 Colonies of Cosmarium-like cells connected by mucilage strands
      Cosmocladium  
15b 
 Cells linked together as filaments
  16
 
 
   
16a
 Cells linked by apical processes
  17
16b 
 Cells not linked by apical processes
  18
     
17a linked by a single apical process   Sphaerozosma  
17b linked by paired apical processes Onychonema  
     
18a Cells flattened, not radially symmetrical in apical view   19
18b Cells radially symmetrical in apical view: filament appearing to twist as cells are linked slightly offset from each other   20
 
 
   
19a
  Cells divided by a deep median sinus, somewhat Cosmarium-like: linked together at the poles
  21 
19b   Cells rather barrel-shaped, without a median sinus. Filament in a mucilage sheath sometimes with a fibrillar structure clearly visible: single stellate chloroplast. Hyalotheca  
       
20a Cell with a median notch Desmidium  
20b
  Cells markedly barrel-shaped, with a shallow median sinus
   Bambusina  
 
 
   
 21a
   Cells linked by small apical granules
       Teilingia  
 21b
Cells linked at flattened apices, no granules
Spondylosium planum  
       
22a Cells short, cylindrical Penium  
22b
Cells tapering, straight to strongly curved
    Closterium  
       
 
   

 

This key owes much to the diagrammetic key in Desmids of the English Lake District,
by EM Lind and AJ Brook, Freshwater Biological Association, Ambleside

[return to start  of key]

John Kinross

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