134 The Study of Animal Life parti. 



carried by the vessels to all parts of the plant. So we get 

 a first notion of how a plant is fed. Starch is a com- 

 pound containing carbon and the elements of water. The 

 carbon, we know, comes from the carbonic acid of the air ; 

 the water is absorbed by the roots from the soil. In some 

 way the living matter of the cells, by means partly of the 

 green colouring matter, is able to transform the energy of the 

 sun's rays into potential energy of a combustible substance 

 — starch ; so we get clear evidence of a machinery for the 

 transformation of energy. We have taken a plant as our 

 example throughout, partly because the cells are more 

 evident than in animals, and partly because the chemical 

 processes give evidence of a transformation of kinetic into 

 potential energy more clearly than do those of animals ; for 

 the animals eat the plants, and so by using the potential 

 energy of plant substance are able to live and move. 



We have now some idea of the sources of the energy of 

 life. Plants get their food from the air by their leaves, and 

 from the soil by their roots, which absorb water and salts 

 dissolved in water. By aid of the energy of sunlight they 

 build these up into complex substances, which they use for 

 the growth of their living matter, for the formation of sup- 

 porting structures, and for other purposes. Animals eat 

 these substances. They build up their own bodies of living 

 matter and supporting structures, and they move about. 



In order to get a clearer notion of the nature of living 

 matter we must attempt to trace the manner in which these 

 various substances are built up. We have first to discover 

 what are the substances that are made. In all living 

 creatures there are, in addition to water and salts, such as 

 common salt and soda, three groups of stufifs : — 



Carbohydrates, such as starch and sugar, made of carbon 

 with hydrogen and oxygen in the same proportions as they 

 occur in water ; 



Fats — substances containing the same three elements, 

 but with a smaller proportion of oxygen ; 



Proteids — substances containing always carbon, hydrogen, 

 oxygen, and nitrogen, with a small percentage of sulphur. 



The constitution of proteids is difficult to determine. 



