DEVELOPMENT OF PLANTS 181 



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 

 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- 

 tures already noted in the epidermis, cortex and central region 

 of terrestrial plants. The life history of the brown algae indi- 

 cates that they have undergone the same evolution as the Chloro- 

 phyceae. 



(a) The Simpler Brozvn Algae. — This development is very 

 clearly brought out in Ectocarpus, a filamentous branching 

 form that is almost universally distributed along the sea shore 



