HOW TO KNOW THE FRESH-WATER ALGAE 



212a Thallus an mtestiniform, hollow tube, with the wall one cell in 

 thickness. Fig. 152 ENTEROMORPHA 



Fig. 152. Enteromorpha intestinalis (L.) 

 Grev. a, habit of branched thallus; b, 

 cells showing parietal position of chloro- 

 plasts. 



Like Monostroma (Fig. 151) Enteromorpha 

 is primarily a marine alga but becomes 

 adapted rather easily to freshwater habi- 

 tats. The long, hollow tubes are frequently 

 branched, forming slender threads or crin- 

 kled tubes. The plants are always at- 

 tached to submerged plant stems, or to 

 stones, especially in flowing water. There 

 are eight species known from freshwater 

 or brackish situations. 



212b Thallus not an intestiniform tube 213 



213a Plant a lobed or ruffled disc of cells, 10 cm. or less across; at- 

 tached by a central short stipe (usually on rocks in alpine and 

 arctic situations). Fig. 153 PRASIOLA 



Fig. 153. Prasiola crispa (Lightf.) Menegh. a, 

 several forms of thallus; b, diagram to show 



-jxfob 00 op 00 /~) cells in 4 ' s - 



00 00 ®*-L®° L( Four species of Prasiola have been re- 



^00 00 QO j) j^ ported from the United States, mostly from 



^g-j-o- ^ ^^^/ P )/ alpine and subalpine situations. In the Arctic 



^^^y ^V // me plants are common on soil rich in nitro- 



a genous wastes. The thalli are foliose or 



Fiqure 153 frond-like sheets, attached at a central point 



by a short stalk or disc. The genus and 



Schizognium (Fig. 140), comprising the Schiz- 



ogoniaceae, have star-shaped, axial chloro- 



plasts. 



213b Plant otherwise; a filament or a gelatinous strand 214 



214a A filament of cells 221 



214b A gelatinous strand, or a tube, or a plant including a gelatinous 

 tube which may or may not have cross partitions 215 



98 



