10a Cell walls thickened, forming H-pieces (cell at end of filament has two 

 projections); cells same diameter throughout Mi crospora 



10b Cell walls thin, no H-pieces; ends of some cells with transverse bands; 

 cells usually wider at one end Oedogonium 



11a Filament (trichome) with a sheath 12 



lib Filament (trichome) without a sheath 13 



12a Sheath firm, extending well beyond end of trichome; cells wider than long 

 Lynqbya 



12b Sheath watery, barely discernable; cells usually longer than broad . . . 

 Phormidium 



13a Cells rounded, trichome appearing as a string of beads; heterocyst (a dif- 

 erentiated, empty-appearing, thick-walled cell) present Anabaena 



13b Cells disc-shaped, wider than long, trichome appearing as a stack of fris- 

 bees; heterocyst absent Oscillatoria 



14a Cells solitary or in small colonies of up to 128 cells; microscopic , . . 



16 



14b Cells in colonies of hundreds of cells; macroscopic 15 



15a Cells touching, mutually compressed; chloroplasts grass-green, contain 



starch (turn blue-black with addition of iodine); colony assumes the shape 

 of a hollow tube Enteromorpha 



15b Cells distant from one another in a gelantinous matrix; chromatophores 



yellow-green, do not contain starch; colony solid, many branched, slimy to 

 the touch and of a tough consistency, sometimes with a disagreeable odor 

 Hydrurus foetidus 



16a Cells in colonies of four or multiples of four 17 



16b Cells usually solitary 18 



17a Cells arranged in a linear series; usually four cells in a colony .... 



Scenedesmus 



17b Ceils arranged radially in a stellate plate one cell in thickness; usually 



16 to 32 cells in a colony Pediastrum 



18a Cell wall thick and rigid; cell divided into two semicells formed by a median 

 constriction; cells at the most only 3 or 4 times longer than broad (Desmid- 

 iaceae) 19 



18b Cell wall thin; cell not divided into two semicells; cells many times longer 

 than broad, pointed at the ends Ankistrodesmus 



19a Cells not conspicuously divided into semicells, lunate in shape 



Closterium 



19b Cells conspicuously divided into semicells, shape variable 20 



20a Semicells rounded Cosmarium 



20b Semicells with three rigid projections on each of the outer ends 



Staurastrum 



-37- 



