104 



RELATIONS OF PLANTS TO ANIMALS 



Energy 



(Sun) 



This diagram shows that plants and 

 animals on the earth hold the same re- 

 lation to each other as plants and ani- 

 mals in a balanced aquarium. Explain 

 the diagram in your notebook. 



Man does not feed to a great extent upon leaves, but he eats 

 many roots, stems, fruits, and seeds. When he does not feed 

 directly upon plants, he eats the flesh of plant-eating animals, 



which in turn feed directly upon 

 plants. And so it is the world 

 over ; the plants are the food 

 makers and supply the animals. 

 Green plants also give to the 

 atmosphere e^^ery day a very 

 considerable amount of oxygen, 

 which the animals use. 



The Nitrogen Cycle. — The 

 animals in their turn supply much 

 of the carbon dioxide that the 

 plant uses in starch making. 

 They also supply some of the nitrogenous matter used by the 

 plants, another part being given the plants from the dead bodies 

 of other plants, and still another part being prepared from the 

 nitrogen of the air through the agency of bacteria which live 

 upon the roots of certain 

 plants. These bacteria are 

 the only organisms that 

 can take nitrogen from the 

 air. Thus, in spite of all 

 the nitrogen of the at 

 mosphere, plants and ani- 

 mals are limited in the 

 amount available. And 

 the available supply is used 

 over and over again. Eaten 

 in protein food by an ani- 

 mal, it may be given off 

 as nitrogenous waste, get 

 into the soil, and be taken 

 up by a plant through the 

 roots. Eventually the ni- 

 trogen forms part of the food supply in the body of the plant, 

 and then may become part of its living matter. When the 



Nitrogen from air 



Nodule 

 containing 

 nitrogen' 

 ■fxing 

 gcter/9 



Bacteria which take nitrogen from the air 

 and put it into a usable form, are found in 

 nodules on the roots of certain plants. 



