42 THE PLANT: A GENERAL EXTERNAL VIEW 



need to understand the difference between food and food 

 materials. Food is a general term. You may think you 

 know what you mean by it, but you would find it hard 

 to define. Just now we need a definition. We may define 

 as food all substances which contribute directly to the energy, 

 growth, or repair of living bodies. You may have been 

 thinking that plants get their food from the soil and from 

 the air. It is true that they do take in substances from 

 the soil and from the air, but it is also true that these sub- 

 stances are not food. They are substances out of which 

 food is made, but they are no more food itself, strictly 

 speaking, than bits of gold and iron ore are a beautiful and 

 complicated watch. Gold and iron ore may furnish the 

 materials from which the watch is made, but surely they 

 are not the watch itself. Here, then, at last, we have 

 found a fundamental difference between the nutrition of 

 plants and the nutrition of animals. Plants manufacture 

 food, animals do not. Plants get from outside themselves 

 the crude materials from which food is made, but these 

 materials are not the food itself. The food itself is made 

 inside the body of the plant. All this is true only of 

 green plants. Some plants have no green parts. A toad- 

 stool is an example. Such plants get their food from out- 

 side their own bodies, just as you and I do. 



B. Digestion and Assimilation. — You have probably 

 learned already that the food which we eat, in order to be- 

 come of use to us, must be digested and assimilated. Diges- 

 tion is the transformation of food into such liquid form as may 

 go to all parts of the body. Assimilation is the transformation 

 of food into living parts of the body. Plants, like ourselves, 

 must digest and must assimilate their food. These are 



