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TEXTBOOK OF BOTANY 



these salts; consequently they cannot synthesize amino acids. Some 

 can live only where peptones or other protein compounds are present 

 in the external medium. 



The kinds of algae. For many years algae were grouped on the basis 

 of tlieir color as red algae, brown algae, green algae, and blue-green 

 algae. This classification is still adequate for a general consideration of 

 algae, though anyone unaware of other bases of classification would 

 be confused on being told that some algae are colorless, that the red 

 color of arctic snow is due to the presence of a red-colored green alga, 

 and that the Red Sea was so named because of the abundance of a red- 

 colored blue-green alga in it. 



Most students of the algae now recognize 9 or more classes (Table 

 17). This more critical classification is based in part on color, but also 

 on other distinctive characters of (1) the structure of the vegetative 

 cells and tissues, (2) the reproductive structures, (3) the successive 

 stages in the life cycles, (4) the kinds of accumulated food, and (5) 

 the forms of the chloroplasts or chromatophores. 



Table 17. Classes of Algae and Their Characteristic Pigments 



The various colors of algae are due largely to the relative abundance 

 of certain pigments in the cells. Chlorophyll is present in all algae except 

 the colorless ones. Carotene and xanthophyll occur in most of them. The 

 pigment fucoxanthin ( C4(.H.-,(iO<; ) is characteristic of the brown algae, 



