CHAPTER III 



SEAWEEDS 



All Seaweeds are Algae, but all Algae are not seaweeds. 

 We have already described several forms of Green Algae 

 that occur in fresh water, and, as a matter of fact, the 

 great majority of Green Algae are fresh- water forms. 



Even a casual observer of Seaweeds will be struck by 

 the variation of their colours, and it happens that 

 colour is quite a good basis for classification. This 

 colour basis establishes three main groups of Algae : 



1. The Green Alg^; =Chlorophycece. 



2. The Brown Alce —Phceophycece. 



3. The Red Algje =Rhodophycece. 



Arrangement according to structure and development 

 practically coincides with this colour classification. 

 The Brown and Red forms are mainly marine, and, 

 doubtless, their peculiarities of structure and colour 

 are due to their environment. It also happens that 

 the colours of Seaweeds have a definite relation to the 

 depth of the water in which they occur. While there 

 are numerous exceptions, the general rule is that dense 

 growths of Green Algae, among which species of Entero- 

 morpha (p. 39) and Vaucheria (p. 65) are conspicuous, 

 occur near the shore, where they are covered by the 

 water only at high- tide. Brown forms occupy the region 

 between tide-marks and, in the same zone, some red 



69 



