GENERA OF ALG^. 27 



* Without dorsal spine; cell walls thick . . . Gongrosira, 21 



* With dorsal spine . . Aphanochmte, 20 



H. Immersed in firm jelly ... . . . Chcetophora, 18 



H. Not in a firm jelly (I). 



I. Filanjents dichotomously or trichotomously branched, 



Microthamnion, 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 . Stigeodonium, 17 



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



Sciadium, 46 

 L. Distinctly many celled (P). 

 L. Articulate (apparently multicellular) (Q). 

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



layer ..... .... Coleochmte, 9 



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



Aphanochxte, 20 

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

 herent . . . . . . Prasiola, 14 



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



Enteromorpha, 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, . . Vat/cheria, 30 

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



earth . . Botiydium, 31 



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



Spharoplea, 1 2 

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

 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, 24a 

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



