228 THE BOTWM OF THE SEA. 



How strange a tree would look which, torn up by the 

 tempest, should rise through the atmosphere and float 

 above the clouds ! Such phenomena occur continually 

 in the ocean. The marine currents detach plants of 

 all kinds from the bottom of the ocean. They collect 

 in parts of the ocean where the currents are weaker. 

 There they form immense floating islands, which 

 sometimes hinder tlie progress of ships. 



Light is as necessary for marine plants as for 

 terrestrial ones, and this prevents them from living 

 at a great depth ; still they grow many hundred feet 

 below the surface, wliere light penetrates but feebly. 

 Shells are distinctly visible in certain parts of the 

 Arctic Ocean at a depth of 460 feet ; at a depth of 

 940 feet the light is still of sufficient intensity to 

 permit the objects to be seen dimly. The bottom 

 of tlie sea is equally visible at the same depth in 

 the Antilles, but the objects are not distinct. Abso- 

 lute darkness prevails at a depth of 1000 feet. The 

 rays of the moon penetrate only to a depth of about 

 40 feet. 



It is, therefore, within a limit of 1000 feet in depth, 

 or about one-tenth of the average depth of the seas, 

 that marine plants vegetate, forming a belt around 

 our continents and islands, or crowning the summits 

 of submerged mountains. 



H^s light any influence on the colour of vegetation 



