10 GENERAL ZOOLOGY 



starches under the influence of light is called photosynthesis. 

 Only plants possessing chlorophyll or other similar substances are 

 capable of carrying out this function. In photosynthesis, carbon 

 dioxide and water are absorbed by the plant and oxygen is given 

 off. In animal respiration, oxygen is absorbed and carbon 

 dioxide and water given off. There is thus a reciprocal relation 

 between animals and plants in the use of these substances. 

 Similarly, other waste products of animal metabolism can be 

 utilized by the plants in synthesizing proteins, after such nitrog- 

 enous waste products have been acted on by bacteria present in 

 the soil. 



Photosynthesis does not occur at night or in the dark. Under 

 these conditions oxygen is taken in and carbon dioxide is given 

 off, just as in animals, regardless of light conditions. Respira- 

 tion of oxygen, then, is a characteristic of metabolism in all 

 organisms. Photosynthesis is a process which accompanies 

 respiration in plants, taking place when light is available. Dur- 

 ing photosynthesis, respiration is overshadowed by the construc- 

 tive activities of the plant tissues. Properly speaking, oxygen 

 is not a food, if one defines food as a fuel or a means of repair. 

 Oxygen is a necessary element in processes of oxidation which 

 accompany all energy-releasing reactions of the body. 



Movement. — Nonliving things as a rule move under the influ- 

 ence of gravity. Occasionally, as in a volcanic eruption or an 

 explosion of dynamite or similar substances, nonliving objects 

 may be moved by the action of forces generated by processes of 

 combustion. The power of controlled or independent movement 

 exhibited by animals results from internal processes of combus- 

 tion or oxidation. The energy liberated is utilized in producing 

 movement. Movement is one of the outstanding character- 

 istics distinguishing animals from plants. In plants movement 

 is largely limited to changing positions of the foliage in response 

 to sunlight. It is true of course that in some plants, as in 

 Mimosa, the sensitive plant, whose leaves fold up in response to 

 touch, movements of parts resemble similar movements in 

 animals; but on the whole movement has been much more highly 

 perfected in animals than in plants. 



Irritability. — Animals respond when subjected to disturbing 

 influences. The disturbing influence or stimulus may be pro- 

 duced by impact, heat, light, sound, electricity, chemical action, 



