156. Chloroplast a broad axial plate or band; wall thin .... Mougeotia (in part) 



156. Chloroplast a parietal net; wall thick and usually 



lamellose - Rhizoclonium ( in part) 



157. Thallus a pad or parenchymatous layer, composed of compactly 

 arranged, erect filaments, growing on submerged wood and 



shells - Gongrosira 



157. Thallus not a cushion of erect filaments - 158 



158. Plant brick- or rusty-red (in mass), the haematochrome 

 masking the chlorophyll; usually aerial, rarely in aquatic 

 habitats.- Trentepohlia (in part) 



158. Plants not colored by haematochrome 159 



159. Plants growing on the shells of turtles; branches arising only 



from the basal region of the filament Basicladia 



159. Plants not growing on turtles —160 



160. Plants attached (sometimes floating in age), with basal-distal 

 differentiation; thalli arbuscular 161 



160. Filaments not forming arbuscular thalli 163 



161. Thallus very small (50-100^ high); branches cut off from the 

 cell of the axis by a wall distinctly above the point of origin ( PI. 



11, Fig. 5); branches scarcely tapering toward the apices .. Microthamnion 



161. Thallus larger; branches cut off by cross walls at the point of 



origin; branches tapering toward the apices —.162 



162. Thallus arbuscular, attached or free-floating; branches upwardly 

 directed; highly variable and sometimes but very little branched, 

 especially in old, free-floating forms Cladophora 



162. Thallus arbuscular., attached, with irregularly directed branches, 

 the lower ones growing downward as long rhizoids; branches 

 becoming entangled to form dense tufts Cladophora profunda 



( Aegagropila ) 



163. Filaments frequently branched; cells cylindrical but interspersed 



with swollen and dark, ovate or cask-shaped akinetes Pithophora 



163. Branches few, sometimes multicellular but frequently unicellular; 

 filaments coarse and wiry, irregular in habit, composed of long 



or short cylindrical cells; akinetes wanting— ..Rhizoclonium (in part) 



164. Setae cut off laterally from the anterior end of the cells, bul- 

 bous at the base Bulbochaete 



164. Setae and hairs without bulbous bases -165 



165. Thallus inclosed in soft, watery mucilage, or mucilage wanting 

 altogether 166 



165. Thallus inclosed in firm mucilage; definite in form, tubercular, 

 globular, or arborescent gelatinous masses; macroscopic Chaetophora 



166. Thallus consisting of a cushion-like mass of short filaments which 

 have a prostrate and an erect pulvinate portion; mucilage want- 

 ing; free cell walls bearing setae with 



sheathed bases.- — - -- - -- Coleochaete (in part^ 



166. Thallus not forming pulvinate masses 167 



167. Thallus consisting of an axis of large, barrel-shaped or cylindrical 



cells bearing lateral fascicles of small-celled branches Draparnaldia 



167. Thallus composed of an axis giving rise to long or short, taper- 

 ing branches with cells similar in size to those of the main fila- 

 ment, gradually decreasing in size to the apices Stigeoclonium 



168. Parasitic in the tissues of higher plants; plants coenocytic Phyllosiphon 



168. Not parasitic in the tissues of higher plants -169 



[584] 



