250 STRUCTURE AND LIFE HISTORIES 



generations is not alike. The sporophyte, for example, 

 begins life as a parasite on the gametophyte, while the 

 gametophyte leads an independent existence from the 

 start. The reader may also recall other differences. The 

 question we have here raised, however, is a very funda- 

 mental one, and will be further discussed in Chapter 

 XXII. 



ULOTHRIX 



237. Habitat. The genus Ulothrix occurs everywhere, 

 from pole to pole, in fresh water. 



238. Description. The plant body (Fig. 262) is usually 

 a simple thread of cells, though in exceptional cases the 

 cell-divisions result in a cell-plate instead of a thread. 1 All 

 the cells are similar in appearance and structure, except 

 the basal one, by which the plant is attached to some solid 

 body. This ' cell is somewhat larger than the others, 

 possesses less pigment, and is suitably modified to serve 

 as an organ of attachment, or hold-fast. The protoplast 

 of each cell possesses one nucleus, surrounded by the green, 

 cylindrical chloroplast. No other pigment occurs, and 

 therefore Ulothrix belongs to the Chlorophyceae, or green 

 algae. Photosynthesis, of course, takes place, and a 

 portion of the photosynthate may become transformed 

 into starch, the presence of which is easily demonstrated 

 by the usual test with iodine. 



239. Asexual Reproduction. Every cell of the plant, 

 except the hold-fast, is capable of functioning as a re- 

 productive cell, and two methods of reproduction are 

 common. In one case the entire protoplast of a cell 



1 These two forms of plant bodies are sometimes designated by the 

 terms "linear aggregate" (a filament) and "superficial aggregate" (a 

 plate). 



