310 



THALLOPHYTES 



cell, they so arrange themselves and grow together at points of 

 contact as to form a miniature net. Through the softening and 

 decay of the wall of the mother cell, the small net is set free 

 and by the mere enlargement of its cells becomes a colony of 

 adult size. The gametes are isogamous and are formed in great 

 numbers by certain cells. As many as 100,000 of them may be 

 produced within a cell. Almost as soon as formed they escape 



from the mother cell and begin to pair 

 and fuse. The zygospore produces 

 zoospores which at first pass into a rest- 

 ing stage and later from new nets. 



Thus in the Protococcales the individ- 

 uals may remain separate or form colo- 

 nies which -are exceedingly complex in 

 the higher forms. In the simplest forms, 

 as Pleurococcus illustrates, reproduction 

 is by cell division in which the parent 

 divides to form two new plants, but in 

 the higher forms there is reproduction 

 by zoospores and isogametes. Since 

 their sexuality does not reach the heter- 

 ogamous condition, they are not so ad- 

 vanced in this respect as the Volvocales 

 are, but they lack motility and this 

 feature is characteristic of the higher 

 plants, which are adapted to live on land 

 rather than in the water. 



Confervoid Algae (Confervales). The Confervales or Con- 

 fervoid Algae are among the most familiar of the Green Algae. 

 Their plant bodies are usually filaments, commonly consisting 

 of much elongated cylindrical cells closely joined end to end in 

 a single row. The filaments may be several inches in length 

 and in some forms much branched. In a few forms the plant 

 body is plate-like instead of filamentous, as the Sea Lettuce illus- 

 trates (Fig. 266). The Confervales are common in lakes, ponds, 

 streams, and water troughs, where many of them grow attached 

 and form green hair-like fringes about rocks and other ob- 

 jects. More than 700 species of them are known, and there 

 is considerable variation in plant body and methods of repro- 

 duction. 



FIG. 266. Sea Lettuce 

 (Ulva), a Confervoid Alga 

 having a plate-like plant 

 body. This plate-like plant 

 body is two layers of cells 

 in thickness, bright green, 

 and resembles a leaf in 

 form. Natural size. Re- 

 drawn from Thuret. 



