7. Plant mass forming erect tufts from a horizontal expanse, 



especially when growing on moist substrates Symploca 



7. Plant mass not forming erect tufts 8 



8. Trichomes lying parallel, forming planktonic fascicles Trichodesmium 



8. Trichomes entangled, forming mucous, sometimes 



thick, layers on a substrate Phormidium 



9. Sheaths wide, mucous, soft and sticky; freely branching 10 



9. Sheaths firm and narrow; not freely branching; 



trichomes densely compacted and entwined Schizothrix 



10. Trichomes loosely arranged, 2-4 in a sheath - Hijdrocoleum 



10. Trichomes densely entangled, many within a sheath Microcolem 



SPIRULINA Turpin 1827, p. 309 



A spirally twisted, unicellular trichome, cylindrical throughout 

 and not tapering toward the apices (although briefly attenuated in 

 some species); spiral loose and lax, or close and tightly coiled ac- 

 cording to species, the spiral usually very regular; trichomes free- 

 floating and planktonic or intermingled with other algae in tycho- 

 plankton, sometimes forming layers on moist soil, especially where 

 water has subsided. 



This genus should be compared with Arthrospira, in which the 

 trichomes are multicellular. According to the interpretations of some 

 phycologists the two genera should not be separated. This would 

 seem to be justifiable when the dissepiments of Arthrospira cannot 

 be demonstrated and the trichome appears to be unicellular. See 

 Crow ( 1927 ) on the characteristics of these two genera. 



Key to the Species 



1. Trichomes very tightly coiled S. subsalsa 



1. Trichomes loosely coiled - - - " 



2. Spirals 10-12-(16)m or more wide S. princeps 



2. Spirals less than lO/j. wide - ^ 



3. Spirals 4-6/^ wide, very loose * 



3. Spirals averaging less in width, more tightly coiled - 5 



4. Trichome 2-2.5m in diameter — S. laxa 



4. Trichome 0.7-0.8m in diameter - S. laxissima 



5. Spirals 5^ wide; trichomes 2m wide S. Nordstedtii 



5. Spirals 2.5-4^ wide; trichomes 1.2-1.7ai wide, very long S. major 



Spirulina laxa G. M. Smith 1916, p. 481 

 PI. 108, Fig. 10 

 Trichomes loosely spiralled, forming a dark blue-green mass, 

 2-2.5/A in diameter, spiral 4-6/^ wide; distance between spirals 15-20/* 

 wide; cell contents blue-green. 

 Tychoplankter. Wis. 



[479] 



