128 THE PLANTS 



which a certain alga belongs is easily recognised ; for 

 differences of structure and fructification correspond 

 with the differences of colour. 



Algae are found on all coasts in both hemispheres if 

 the outward conditions are favourable for their growth. 

 We find them in the Arctic as well as in the tropics, 

 forming a more or less well-defined belt along the coast. 

 Some algae grow so high up that only the spray of the sea 

 as it is dashed upon the beach or against, the rock can 

 reach them ; others live in deeper water, from which 

 only a dredge can bring them up. This belt of algae 

 is divided into two regions : the littoral region, com- 

 prising that tract of the coast which lies dry at every 

 ebb tide ; and the sublittoral region, comprising all 

 vegetation below low-water mark. The algae of the 

 littoral region are adapted to a period of exposure at 

 every ebb, the algae of the sublittoral region to a con- 

 tinually submerged existence. Both the littoral and 

 the sublittoral regions are again divided into an upper 

 and a lower zone, according to the different algae found 

 in each ; for, though many algae have a wide distribu- 

 tion, yet almost every species has its own privileged 

 locality in a vertical, as well as in a horizontal, direction. 



Its internal organization and the external natural 

 agencies determine the place it will occupy. These ex- 

 ternal agencies are light, the nature of the bottom, the 

 salinity of the water, the strength of the currents, etc., 

 and a few remarks may here be given about them. 

 To understand their influence well, one has to study 

 different localities, for agencies exercising a certain 

 influence on the coast of Greenland will have an 

 entirely different effect in the tropics, according to 

 their different combination. 



Light is the chief factor ; every plant wants light to 

 reduce the carbonic acid needed for its existence. 



