SYMBIOSIS 



105 



plant dies, the nitrogen is returned to the soil, 

 nitrogen is kept in circulation.^ 



Thus the usable 



Carbon dioxide 

 I (CO2) 



Carbon dioxide 

 I (CO2) 



Ammonia! 



Plants 



with chlorophyll 



buildup complex 



organic substances 



They store up 



energy from the sun 



in the process 



and 



Animals 



and plants without 



chlorophyll 



I ^°^^. [which tear down complex! Ammonia 

 orgam I organic substances 1 (NH3) 

 food of \ , , ' 



^ and set free energy 



in the process in 

 form of heat 



Energy from sun. Energy set free 



as heat. 

 The relations between green plants and animals. 



Symbiosis. — We have seen that in the balanced aquarium 

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

 partnership. The living together of different organisms for mutual 

 advantage is called symhio'- 



Animal Life 



qsing Bacteria 



sis. Some animals thus 

 combine in a close part- 

 nership with plants ; for 

 example, the tiny animal 

 known as the hydra with 

 certain of the one-celled 

 algae. Similarly, some 

 animals live together sym- 

 biotically, as witness the 

 sea anem'ones living on 

 the hermit crab, seem- 

 ingly protecting it and 



being carried by the crab to where food is plentiful. If we accept 

 the term symbiosis in a wide sense, all green plants and animals 

 live in this relation of mutual give and take. The interrelation- 



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.) 



Nitrites 

 Nitric Bacteria 



The nitrogen cycle. Trace the nitrogen from 

 its source in the air until it gets back again 

 into the air. 



