222 LIFE HISTORIES AND EVOLUTION OF THE ALGM 



Vaucheria, Chara, and Fucus, or they may form zoospores, which 

 swim off and grow at once into plants like the parents, as in 

 (Edogonium, Ulothrix, and Coleochcete, the latter type developing 

 the zoospores somewhat indirectly through a group of cells. But 

 in all these forms the essentials of the life history are expressed 

 by the formula 



^>sex. s. P<^>sex. s. P, etc., 



y y 



g and sex. s. standing for gamete and sexually formed spore, 

 respectively. 



It must always be remembered that the algae with sexual 

 methods of reproduction frequently have also asexual zoospores 

 or other forms of asexual reproduction, which may produce a 

 number of successive generations between the development of 

 sexual plants. This happens very frequently among the green 

 algse (as in Ulothrix, (Edogonium, Coleochaete, and Vaucheria) 

 and in the lower brown algae (as Ectocarpus}. The life history 

 formula of the sexual algae may then be variously broken by the 

 introduction of successive generations developed asexually. 



In the red algae the sexually formed cell corresponding to 

 an oospore does not give rise at once to a generation like the 

 parent plant, but an asexual generation is inserted between 

 successive sexual plants, alternating with them. This asexual 

 phase may be represented only by the tissue which produces 

 the carpospores, as in Nemalion, or it may be represented by 

 this structure plus a tetrasporic plant, as in Polysiphonia. There 

 is, then, in the red algse an alternation of generations, an asexual 

 phase, or sporophyte, alternating with the sexual plant, or game- 

 tophyte. This is the most complex type of life history in the 

 algae and may be expressed by the formula 



/cystocarp and \ /carpospore and 



$[ tetrasporic plant, I asex. s.l tetraspore, 

 \when present / \when present 



i> S asex. s. G, etc., 



