DEVELOPMENT OF PLANTS 201 



are produced that bear gametes, thus completing the life 

 history. 



72. Noteworthy Features of the Chlorophyceae. — Two features 

 in the study of green algae should be kept clearly in mind, 

 because they are closely connected with the tendencies that will 

 appear in the development of the mosses. First: The gameto- 

 spore is essentially a dormant or resting cell that tides the life 

 of the plant over the conditions unfavorable for growth. Second : 

 In passing from lower to higher types, the gametospore tends 

 to vary in its nature and possibilities of growth. At first it 

 forms directly a sexual or gamete-bearing plant as in Sphaerella 

 but in higher types, it develops a generation of zoospores from 

 which the sexual plant is derived as in Ulothrix and Oedogonium. 

 Finally in Coleochaete, we find a further advance in that several 

 non-motile cells are formed by the gametospore. These cells, 

 to be sure, are essentially like the zoospore because they develop 

 directly into zoospores, but this condition emphasizes the fact 

 that the nature of the gametospore is steadily departing from 

 that of the gamete-bearing plant. 



This development could be summarized by saying that there 

 is a tendency to separate the life history of the plant into two 

 stages ; a sexual or gametophyte generation and an asexual sporo- 

 phyte generation. These two generations follow each other in 

 the life history of all the higher plants. This peculiar relation- 

 ship is called the alternation of generations and the reason for 

 it will be more apparent when we come to study the mosses. 



Class B. The Brown Algae or Phaeophyceae 



73. General Features. — With few exceptions the Phaeophyceae 

 are marine plants. They are commonly known as brown algae, 

 owing to the brownish pigments which conceal in a measure 

 the chlorophyll, thus producing their characteristic brown or 

 yellow color. They are of common occurrence along rocky 

 shores and attain enormous dimensions in the northern and 

 southern seas and on the Pacific Coast. As a rule, the plant 

 body is better differentiated than in the green algae and shows 

 a higher type of specialization than yet seen. It would appear 



