8 THE OYSTER. 



stantly pasturing upon them, the multiplication of 

 these plants is kept in check ; but in calm weather it is 

 no rare thing to find great tracts of water many miles 

 in extent packed so full of them that the whole surface 

 is converted into a slimy mass, which breaks the waves 

 and smooths the surface like oil. The so-called " black 

 water " of the Arctic and Antarctic oceans, the home 

 and feeding ground of the whale, has been shown by 

 microscopic examination to consist of a mass of these 

 plants crowded together until the whole ocean is dis- 

 colored by them. Through these seas of " black 

 water " roam the right whales, the largest animals on 

 earth, gulping at each mouthful hundreds of gallons 

 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 vegeta- 

 tion, and the Red Sea owes its name to the coloration 

 of its water by great swarms of microscopic organisms 

 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 

 terrestrial 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 



