72 PLANT-LIFE 



other environmental conditions. Over 250 species occur 

 on the coasts of the Arctic Sea, nearly 900 are found in 

 the West Indies, while Australia has over 1,100 species. 



In regard to the sites on which Algae grow, it is notice- 

 able that they favour a rocky shore with an abundance 

 of rock-pools, the water of which is renewed at every 

 tide. Such a shore is literally covered with Seaweeds, 

 in marked contrast to stretches of sand, on which few 

 species occur, and only such as are furnished with long 

 threadlike rootlets, by means of which they can get 

 an anchorage in the sand. It should be remarked that 

 Seaweeds have no true roots. The rootlike processes 

 by which they attach themselves to rocks or sand have 

 no relation to nutrition; they are merely holdfasts 

 enabling the thalli to anchor themselves, and thus 

 retain a position in the water most suitable to their 

 needs. All necessary food elements are found in solu- 

 tion in the water, and are absorbed through the per- 

 meable cell-walls of the thallus, or " frond," as it is often 

 termed. Seaweeds are subjected to the movements of 

 water, and are well adapted thereto. Species which 

 thrive where movement is gentle — for instance, in a rock- 

 pool — would have a poor chance in surf. But there 

 are species which love the surf, and are well accommo- 

 dated to its relentless movement by the possession of 

 strong holdfasts, flexible stalks, and blades that swing 

 with the water. 



The careful observer will notice that not only are 

 Seaweeds zoned according to colour, but also that 

 different species occur within the limits of a colour zone, 

 forming zones within zones. Thus, on a typical West of 

 Scotland rocky shore we find in the green zone, near high- 



