A WORLD IN THE WATER 93 



instance, are closely related to the brownish 

 Batrachospermum, the slimy fresh-water weed 

 which has been mentioned. On the other 

 hand, amongst the great class of brown sea- 

 weeds we shall find any number of forms which 

 are exclusively marine in habit, or can only 

 support an existence where the water is brack- 

 ish, as in the estuary of a river. 



Although so varied in habit, the algae of the 

 sea faU roughly into three groups — the green, 

 the brown, and the red. These terms must 

 not be taken in an arbitrary sense, however, so 

 far as the colour is concerned. Many of the red 

 seaweeds may be of a purple or even a brown 

 colour, whilst some of the green seaweeds (such 

 as Cladophora, a bushy plant with rigid branches 

 quite common in pools which have been left by 

 the tide) vary in colour from brown to grey. 



There has for some years been a great deal 

 of discussion as to the significance which should 

 be attached to the colouring of seaweeds. 

 Even if we cannot draw very hard and fast 

 lines from the actual tinting in all cases, the 

 plants, in a general way, fall naturally into 

 these groups. One theory on the subject is 

 interesting, although it is not quite a complete 



