IV. 



THE ALGJE. 



The Algae are all chlorophyl-bearing plants. A very 

 large number of plants is here included. They are all 

 comparatively simple, but differ among themselves widely 

 in the degree of complexity of structure. They all live 

 in the water, though certain forms can exist in a dried-up, 

 inactive condition for indefinite periods of time ; these 

 latter forms multiply and develop only when moisture is 

 present. 



In the processes of reproduction there is a considerable 

 range of variation. It is believed that both a sexual and 

 an asexual mode of reproduction occur in nearly all cases; 

 this point is not, however, settled in quite a number of 

 families, owing to the lack of sufficient investigation. 

 The mode of asexual multiplication differs in the different 

 families from the simple fission of the cells to the forma- 

 tion of nonmotile tetraspores in one class up to the for- 

 mation of self-motile zoospores, which is the prevailing 

 mode. 



The possession of chlorophyl is not the principal reason 

 for associating together the plants known as Algee ; it is 

 the general similarity of the course of development of the 

 plants which has determined this point in the classification. 

 Neither is it merely the possession of the green, brown, 

 and red colors which determines the separation of the 

 Algae into their three principal classes: it is rather be- 

 cause, in each of these classes, the course of development 

 shows distinctive characteristics. 



70 



