THE RELATIONS OF PLANTS TO ANIMALS 187 



available. And the available supply is used over and over again, 

 perhaps in nitrogenous 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 cor- 

 relation. 



Symbiosis. Plants and animals are seen in a general way to be 

 of mutual advantage to each other. Some plants, called lichens, 

 show this mutual 

 partnership in the fol- 

 lowing interesting way. 

 A lichen is composed 

 of two kinds of plants, 

 one at least of which 

 may live alone, but 

 which have formed a 

 partnership for life, 

 and have divided the 

 duties of such life be- 

 tween them. In most 

 lichens the alga, a green 

 plant, forms starch and 

 nourishes the fungus. 

 The fungus, in turn, produces spores, by means of which new lichens 

 are started in life. The body of the lichen is usually protected 



by the fungus, which is stronger in 

 structure than the green part of the 

 combination. This process of living 

 together for mutual advantage is called 

 symbiosis. Some animals thus com- 



lation of the threadlike fungus bine with plants; for example, the 

 to the green cells of the alga. ^ aium al known as the hydra with 



certain of the one-celled afce, 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 



A lichen (Physcia stellaris). Photographed by 

 W. C. Barbour. 



Stages in the formation of the 

 lichen thallus, showing the re- 



