4 THE ALGAE 



some groups, e.g. Chrysophyceae, Bacillariophyceae and Xantho- 

 phyceae, and between these as a whole and the Phaeophyceae; also 

 between the Dinophyceae and Cryptophyceae, and the Rhodo- 

 phyceae and Myxophyceae. These relationships are, however, 

 discussed more fully later (see pp. 315, 321). 



It should be noted that chlorophyll a, one of the components of 

 the green pigment, is the most abundant chlorophyll in all types of 

 algae, that jS carotene is equally widely distributed, that starch is 

 not a widely distributed food reserve outside the Chlorophyceae 

 and that cellulose is not the universal wall constituent. There are a 

 large number of xanthophylls and each algal class possesses its own 

 characteristic members. The Siphonales, an order of the Chloro- 

 phyceae, is treated separately because of the carotinoid or yellow 

 pigment differences. The phycobilins of the Rhodophyceae and 

 Myxophyceae are not absolutely identical and hence are differen- 

 tiated by the prefix r- and c- respectively. In several classes of 

 algae, e.g. diatoms, Cryptophyceae, Chlorophyceae, colourless 

 forms are known. Such organisms have to lead a saprophytic 



existence. 



Using a combination of the pigment and biochemical differences, 

 together with morphological differences, the algae as a group can 

 be classified as follows : 



Euphycophyta: 



Myxophycophyta : 

 Chrysophycophyta : 



Charophyceae 

 Chlorophyceae 

 Phaeophyceae 

 Rhodophyceae 



Myxophyceae 



Chrysophyceae 

 Xanthophyceae 

 Bacillariophyceae 



Pyrrophycophyta : 



Cryptophyceae 

 Dinophyceae 



Some workers consider that the Chlorophyceae, Phaeophyceae 

 and Rhodophyceae should have independent rank (e.g. Chloro- 



