ORIGIN OF CARBON. 487 



been obtained in available quantities but by 

 the evaporation of the whole seas ! 



We have now to seek the origin of the carbon 

 in sea-plants. Here, it is very unlikely that its 

 source should be in the ocean bed. No beds of 

 softened humus line the ocean floor, or form a 

 resting-place for the few and simple roots of 

 the marine plant. It is compelled to derive all 

 its food from the medium in which it lives, and 

 has its being. Just as in terrestrial plants, the 

 source of the carbon in sea-plants is the car- 

 bonic acid of the element which surrounds them. 

 Sea-water, in common with all water in a state 

 of nature, contains a certain quantity of this 

 gas in a dissolved state. It is derived from 

 the respiration of fishes and other of the marine 

 tribes, and from a number of chemical processes 

 constantly taking place in the contents of the 

 water, or in the materials which form its bed. 



In the decomposition of carbonic acid by 

 marine plants, a simple but highly important 

 part of the chemistry of the ocean is involved. 

 The fact of this decomposition under water 

 may be as strikingly exhibited as that effected 

 by land-plants upon air. If, when winter has 

 sealed with ice the waters of a wayside pond 

 or ditch, we carefully examine the spot on a 

 sunny day, beneath which some aquatic plants 

 are growing, we shall often perceive their leaves 



