CHAPTEK XX 

 THE ALG;E, THE LOWEST GREEN PLANTS 



206. The algae.* For the present we may think of the thallo- 

 phytes as the immense assemblage of plants below the liver- 

 worts, mosses, ferns, and seed plants. In number of species and 

 divergent evolutionary lines the group is much the largest of 

 the four divisions of the plant kingdom (ThallopJiyta, BryopJiyta, 

 Pteridophyta, and Spermatophyta). 



The algse are thallophytes whose plant bodies are colored 

 because the cells contain chromatophores. Almost all of the 

 fresh-water forms are green, but the majority of the marine 

 algse, or seaweeds, are either brown or of beautiful shades of red. 

 The green color is, of course, due to chlorophyll, while the brown 

 and red tints are caused by other pigments. The algse are 

 divided into four classes as follows : 



Class I. The blue-green algse, or Cyanophycece. 



Class II. The green algse, or Chlorophycece. 



Class III. The brown algse, or Phceophycece. 



Class IV. The red algae, or Rhodophycece. 



It might appear from the above that the algse are classified by 

 their color, but this is not true. These four groups are defined 

 by peculiarities of cell structure, life history, and methods of 

 reproduction which can only be understood through a study of 

 types in the laboratory, and the summaries of these characters 



*To THE INSTRUCTOR : This chapter describes many more types than it 

 would be desirable to present in a general course. The instructor should 

 make selections according to the material available (which varies greatly in 

 different sections of the country), and the time at the disposal of the class. 

 A brief discussion of the best and most available types, and the reasons why 

 they are desirable for laboratory work will be found in the laboratory 

 manual of the authors. 



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