THE ALGAE 



183 



the monoploid gametophyte and the diploid carposporophyte, which are 

 never separated from each other. In such a Hfe-cycle meiosis occurs in 

 the formation of the carpospores. In types Hke Polysiphotiia, Chondrus, 



SPIROGYRA 

 TYPE 



DICTYOTA 

 TYPE 



B 



Fig. 175. — Comparison of types of Life-cycle. 

 A, Haplobiontic. B, Diplobiontic. 



Ceratnium and Corallina the carpospores give rise to independent diploid 

 tetrasporic plants, so that here we have in effect an alternation of three 

 generations, two of which, namely, the gametophyte and tetrasporophyte, 

 are quite independent. In this case the gametophytic plants are monoploid, 

 while both the carposporophytic and tetrasporic plants are diploid. Meiosis, 

 however, does not occur in the carpospores, but is postponed until the 

 formation of the tetraspores. 



The Distribution of Algae 



From what has been said already it will be realized that, while the Chloro- 

 phyceae occur chiefly in fresh water, the Phaeophyceae and Rhodophyceae are 

 predominantly marine Algae. It has been shown that there appears to be a 

 direct correlation between the conditions of life and the colour of the pig- 

 ment. Green Algae can only assimilate when growing in bright light, while 

 the Brown Algae can do with rather less, and the Red Algae with least of all. 



From the types we have described it is clear that members of the Chloro- 

 phyceae occupy a great variety of situations. Quite apart from fresh water, 



