HOW TO KNOW THE FRESH-WATER ALGAE 



Figure 185 



264a Thallus in the form of a cushion, giving rise to compactly ar- 

 ranged upright branches; cells broadest near the tip of the fila- 

 ments; growing on wood or shells (sometimes on other plants). 



Fig. 185 GONGROSIRA 



Fig. 185. Gongrosira Debaryana 

 Rab., horizontal and erect 

 branches with terminal sporangia. 

 Like Gomontia (Fig. 184) these 

 plants grow on shells and sub- 

 merged wood, or on aquatic 

 plants, but form external cushion- 

 like masses (often encrusted with 

 lime) rather than penetrating the 

 substrate. The erect branched 

 portion of the thallus is more ex- 

 tensively developed than in Go- 

 montia. The chloroplast is parietal 

 and usually more definite in outline than that of Gomontia which 

 may be padded and irregularly netted. 



264b Thallus composed of loosely branched filaments, the branches 

 arising unilaterally. Fig. 186 CHLOROTYLIUM 



Fig. 186. Chlorotylium catarac- 

 tum Kuetz., portion of plant 

 showing characteristic habit of 

 branch development. 

 The attached, lime - encrusted 

 thalli of this branched filamentous 

 plant are usually found in flow- 

 ing water. The filaments present 

 a distinctive appearance when 

 seen microscopically because pairs of short, green cells (often with a 

 reddish tinge) alternate with a more elongate and sometimes nearly 

 colorless cell. 



265a (263) Thallus in the form of a tuft of dichotomously branched, 



radiating, yellowish-green filaments. Fig. 187 LEPTOSIRA 



Fig. 187. Leptosira Mediciana Borzi, 

 portion of plant showing horizontal 

 and erect branching systems. 

 This" species is known only from 

 Massachusetts and Kentucky in this 

 country. Filaments occur in yellow- 

 ish tufts and are usually attached to 

 subtrates in the water. The irregularly 

 branched filaments of bead-like or bar- 

 rel-shaped cells arising from a prostrate 

 portion of the thallus help in making 

 Figure 187 . identification. 



Figure 186 



117 



