CHAPTER III. — ASSIMILATION OF FOOD. 237 



CHAPTER III. 



Assimilation of Food. — Distribution and Storage of Food 

 Materials. — Destructive Metabolism. — Influence of 

 Temperature on the Life of the Plant. — Influence of 

 Light on* the Life of the Plant. — Parasites and Sap- 

 rophytes. — Practical Exercises. 



Assimilation of Food. It is, as has already been seen, one 

 of the functions of living organisms to take in materials different 

 from themselves, change them in chemical composition, and 

 appropriate them to their uses. Some require that the materials 

 be in a complex form, others are able to make use of those which 

 are relatively simple. Organic beings cannot create energy ; 

 they can only dispense or apply to serviceable ends that which is 

 supplied to them. Animals and chlorophylless plants are depen- 

 dent for their vitality on the energy supplied by the oxidation of 

 the complex food-materials which they take into their bodies, 

 but chlorophyll plants are able to do an additional work. They 

 can make use of the energy of the sun's rays in constructive 

 work. By their aid they construct, from mineral constituents of 

 the earth and air, complex organic matter, which is afterward 

 used, partly by themselves and partly by other plants and ani- 

 mals, in carrying on their vital processes and building up 

 organic tissues. The utilization of the sun's rays by the plant is 

 accomplished by the agency of the chlorophyll. This green col- 

 oring matter, this verdure which in grass and leaf gives the chief 

 glory to the summer landscape, has other uses than merely to 

 please the eye of man. By means of it, organic beings are able 

 to draw perpetual supplies of power from the sun ; without it, it 

 is difficult to conceive how life, save possibly in some of its 

 lowest forms, could exist upon the earth. 



The precise function of chlorophyll is to apply the energy of 

 the sun's rays to the production of some form of carbo-hydrate, 

 which is not starch, but some related body. This, by immediate 

 combination with the nitrogen and sulphur taken up in the form 

 of sabs by the plant, forms proteid matter. The exact compo- 



