Desmidiacew 151 



Cells commonly with a central protuberance. 

 IT Cell-wall smooth, granu- 

 late, verrucose, etc. 

 Central protuberance 



present or absent 18. Cosmarium. 



ITT Cell- wall with regularly 

 arranged spines, com- 

 monly in pairs. Cen- 

 tral protuberance al- 

 ways present 19. Xanthidium. 



Cells without a central pro- 

 tuberance; angles spinate 20. Arthrodesimis. 

 Cells from the vertical view com- 

 monly radiating, triangular, quad- 

 rangular, or up to 11 -radiate, rarely 



fusiform 21. Staurastrum. 



** After division the cells remain attached to form colonies. 



t Colonies spheroidal ; cells not in contact, but joined by gelatinous 



bands. 

 Gelatinous bands narrow; few cells 



forming a microscopic colony 22. Cosmocladium. 



Gelatinous bands very broad; many 



cells forming a macroscopic colony 23. Oocardium. 

 ft Colonies thread-like; cells attached by their apices to form 



long filaments. 

 Cells attached by special apical processes. 



\ Apical processes very short 24. Sphcerozosma. 



1J Apical processes long and over- 

 lapping the apices of the ad- 

 joining cells 25. Onychonema. 



Apices of cells plane and flat. 

 J Cells deeply constricted. 



Cells in vertical view elliptical 26. Spondylosium. 

 Cells in vertical view quad- 

 rangular with produced 



angles 27. Phymatodocis. 



\\ Cells very slightly constricted... 28. Hyalotheca. 

 b. The point of division of the cell, where the new and old parts of the 

 cell-wall are obliquely fitted together, develops a girdle-like thickening or 

 ingrowth, which projects both ways into each of the old semicells during 

 division. Cells attached to form thread-like colonies. 



* Cells joined by special apical processes 29. Streptonema. 



** Cells joined by their flat apices or by flattened apical projections, 

 t Cells short, in vertical view fusiform, trian- 

 gular or quadrangular (rarely circular 



with produced angles) 30. Desmidium. 



tt Cells elongate, cylindrical 31. Gymnozyga. 



