THE RELATIONS OF PLANTS TO ANIMALS 163 



available supply is used over and over again, perhaps in nitrog- 

 enous food by an animal, then it may be given off as organic 

 waste, get into the soil, and be taken up by a plant through the 

 roots. Eventually the nitrogen forms part of the food supply in 

 the body of the plant, and then may become part of its living 

 matter. When the plant dies, the nitrogen is returned to the soil. 

 Thus the usable nitrogen is kept in circulation. 1 



Symbiosis. - We have seen that in the balanced aquarium 

 the animals and plants, in a wide sense, form a sort of unconscious 

 partnership. This process of living together for mutual advantage 

 is called symbiosis. Some animals thus combine with plants ; 

 for example, the tiny animal known as the hydra with certain of 

 the one-celled alga3, and, if we accept the term in a wide sense, all 

 green plants and animals live in this relation of mutual give and 

 take. Animals also frequently live in this relation to each other, 

 as the crab, which lives within the shell of the oyster ; the sea 

 anemones, which are carried around on the backs of some hermit 

 crabs, aiding the crab in protecting it from its enemies, and being 

 carried about by the crab to places where food is plentiful. 



A Hay Infusion. - - Still another example of the close relation 

 between plants and animals may be seen in the study of a hay 

 infusion. If we place a wisp of hay or straw in a small glass jar 

 nearly full of water, and leave it for a few days in a warm room, 

 certain changes are seen to take place in the contents of the jar; 

 after a little while the water gets cloudy and darker in color, and a 

 scum appears on the surface. If some of this scum is examined 

 under the compound microscope, it will be found to consist almost 

 entirely of bacteria. These bacteria evidently aid in the decay 

 which (as the unpleasant odor from the jar testifies) is beginning 

 to take place. As we have learned, bacteria flourish wherever the 

 food supply is abundant. The water within the jar has come to 

 contain much of the food material which was once within the 

 leaves of the grass, - - organic nutrients, starch, sugar, and pro- 

 teins, formed in the leaf by the action of the sun on the chlorophyll 



1 A small amount of nitrogen gas is returned to the atmosphere by the action of 



the decomposing bacteria on the ammonia compounds in. the soil (See figure of 

 nitrogen cycle,) 



