GENERAL PHENOMENA OF NUTRITION. 87 



X 



ployed by the animal to repair its losses. Still, no animal 

 can live upon inorganic matter alone, and the food must 

 therefore have been derived from a pre-existent organism. 

 A few plants agree with animals in this respect, and may 

 therefore be looked upon as animals so far as their food is 

 concerned. 



The great majority of plants, on the other hand, are 

 endowed with the power of converting inorganic materials 

 into organic compounds, and they thus differ altogether from 

 animals. The formative power of plants in this respect is, 

 however, limited by very definite bounds. The tissues of 

 plants consist mainly of carbon, hydrogen, oxygen, and 

 nitrogen; but plants cannot avail themselves of these 

 elements as such. Thus, nitrogen is largely present in the 

 atmosphere by which terrestrial plants are surrounded, but 

 plants do not derive their supply of this element from this 

 source. The nitrogen of plants is, on the contrary, obtained 

 by them from ammonia, which they absorb from the soil. 

 Similarly, the carbon of plants is obtained from the carbonic 

 acid which is contained in the atmosphere, or is dissolved 

 in the water taken up by the roots. 



Hitherto we have been dealing with an adult organism, 

 and we have seen how the result of nutrition to maintain 

 the living body in a practically unchanged condition, by the 

 continual formation of fresh matter to take the place of that 

 which has been destroyed by vital work. In this process 

 it is obvious that an average condition of the organism can 

 only be maintained so long as the production of new matter 

 is more or less exactly equivalent to the destruction of old 

 matter. The processes of repair and waste must go hand 

 in hand, and neither must exceed the other for any length 

 of time. The formation of new matter may, however, fall 

 short of the destruction of old matter, or it may exceed it ; 

 and in either case we have a fresh series of phenomena to 

 observe. 



When the waste of the organism caused by the discharge 



