190 THE BROWN ALGAE 



zoospores from which the sexual plant is derived as in Ulothrix 

 and Oedogonium. Finally in Coleochaete, we find a further ad- 

 vance in that several non-motile cells are formed by the gameto- 

 spore. 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 difi"erentiated than in the green algae and shows 

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

 probable that the brown algae have been derived from green 

 algae at a point considerably in advance of the unicellular forms. 

 At least, there is no satisfactory evidence of a series of unicellu- 

 lar forms leading up to the rather complex types that are repre- 

 sentative of the simplest of the Phaeophyceae. The majority 

 of these lower forms have already reached the filamentous stage 

 and many of the higher genera exhibit a differentiation of the 

 plant body suggestive of the higher plants, as for example, a 

 definite axis with branches and leaf-like outgrowths and root- 

 like organs that anchor the plants to the substratum. Likewise, 

 the tissues of these higher types often reveal many of the fea- 



