GENERA OF ALG^E. 27 



* Without dorsal spine; cell walls thick Gongrosira, 21 



* With dorsal spine Aphanochczte, 20 



H. Immersed in firm jelly Chatophora, 18 



H. Not in a firm jelly (I). 



I. Filaments dichotomously or trichotomously branched, 



Microtham nion, 1 9 



I. Filaments not dichotomously branched (J). 

 J. Branches in lateral clusters; cells of main stem with a transverse, 



central chlorophyll band Draparnaldia, 16 



J. Branches not in lateral clusters (K). 



K. Cell membrane usually thick, cells longer than broad; ultimate 



branches, much thinner than the primary, 



Cladophora, 23, or Pithophora, 24 

 K. Cell membrane very thin and hyaline; stem and branches barely 



separably in size and appearance .... Stigeoclonium, 17 

 K. Cell membrane thin; branches radiating, umbel-like, one-celled, 



Sciadtum, 46 



L. Distinctly many celled (P). 

 L. Articulate (apparently multicellular) (Q). 

 M. Articulate, branched, forming a little mass or a subdisciform 



layer Coleochczte, 9 



M. Articulate, prostrate or creeping, a spine on the back, 



Aphanochate, 20 



M. Expanded, leaf-like, smooth, formed of angular cells; always ad- 

 herent Prasiola, 14 



M. Membranaceous, tubular or utricular, fixed only when young, 



Enter omorpha, 15 



M. Forming a green net, visible to the naked eye, Hydrodictyon, 41 

 M. Forming a colony, plane; discoid or stellate, often perforated; 



cells polygonal Pediastrum, 40 



N. Filaments tufted, long, sometimes branched, . . Vaucheria, 30 

 O. Rootlets much divided, descending from the cell base into the 



earth Botrydium, 31 



P. Cells long; chlorophyll enclosing large, regularly placed vacuoles, 



Sphczroplea, 1 2 



P. Cells long, filaments somewhat geniculate, without vacuoles (/). 

 P. Cells short, cylindrical or oblong; without vacuoles; filaments not 



contracted; attached when young .... Cylindrocapsa, 13 

 P. Cells short, broader than long, filaments contracted irregularly, 



Schizomeris, 24.3, 

 Q. Filaments simple, not laterally united, (R). 



