8 THE OYSTER. 



of the little mollusca and Crustacea which feed on the 

 plants. 



In tropical seas, ships sometimes sail for days through 

 great floating islands of this surface vegetation, and 

 the Red Sea owes its name to the coloration of its 

 water by great swarms of microscopic plants which are 

 of a reddish tinge. The plant life of the ocean is 

 ample for the support of all its animal life, just as the 

 vegetation of the land gives a maintenance to all terres- 

 trial animals. 



The source of the food of animals is the vegetable 

 world. What is the source of the food of plants? 



Most of it consists of mineral matter, derived from 

 the crust of the earth ; but before this can be used by 

 plants it must be dissolved in water. The solid rocks 

 cannot maintain life until they have been ground down 

 and dissolved, and in the form of frost and rain, water 

 is continually breaking down and wearing away the 

 hard rocks, and carrying the fragments down to lower 

 levels to form the fertile land of the hillsides and 

 valleys and meadows. As the roots of the plants 

 penetrate this loose material they gather up the mineral 

 food which is dissolved by the rain and convert it into 

 their own substance, and as their leaves fall and their 

 trunks decay, the decaying vegetable matter gradually 

 builds up the leaf-mould and meadow-loam which are 

 so well adapted for supporting vegetable life. Each 

 year, however, the heavy rains wash great quantities 

 of this light, rich soil into the rivers, which in times of 

 flood cut into their banks and carry the arable land, 

 which has been built up so slowly, down to lower 

 levels, until at last it finds its way to the ocean and is 

 lost, so far as its use to man is concerned. 



