PLANTS AND ANIMAI-s 



up their own complex substances and to undergo oxidation, 

 and so to give out energy, and, on the other hand, to build up 

 again more of their own substances from simpler substances, 

 belongs to living matter alone. It is the possession of this 

 double power of breaking down and building up which con- 

 stitutes life. 



Physiology is the study of the means by which these 

 processes arc carried out in the cells and in the body as a 

 whole, and of the modes by which the energy liberated is used 

 by the animal for various purposes. 



Plants and Animals compared. Plants as well as 

 animals consist of living cells, and have in common with animals 

 the properties essential to living matter, namely 



1. They are constantly building up living matter and 



complex substances from simpler substances, and so 

 storing up energy. 



2. They are constantly taking up oxygen and undergoing 



oxidation, and so breaking down living matter. 



3. They form new plants or animals like themselves. 



Plants require oxygen for their life, just as animals do, but 

 their oxidation goes on much more slowly. In plants, the 

 building up of complex substances is far in excess of their 

 breaking down. A tree, as a rule, goes on growing all the 

 time it lives, and when it dies leaved a mass of complex sub- 

 stances, and from them a large amount of energy may be 

 obtained when these substances are, as by the burning of the 

 tree, oxidised into simpler substances again. On the other 

 hand, in an adult animal, an animal no longer growing, the 

 breaking down of the complex substances goes on as fast as 

 the building up of the new. 



Plants are able to do what animals cannot. A plant can 

 build up its living matter and form the complex substances of 

 which it consists by obtaining the elements it needs from very 

 simple chemical compounds. All green plants obtain carbon 

 from the carbonic acid gas of the air, and the other elements 

 they require from simple chemical compounds in the soil. 

 Most of these compounds are mineral or inorganic compounds. 

 Animals, on the contrary, cannot live on inorganic substances ; 

 they can build up their living matter only from the peculiar 



