118 



BOTANY: PRINCIPLES AND PROBLEMS 



energy and building-materials. We may therefore define food 

 as whatever furnishes a supply of available energy to an organism or 

 contributes materially to the upbuilding of its tissues. It is the car- 

 bohydrates, fats, and proteins which provide the materials for 

 growth, and which, because of their somewhat unstable chemical 

 composition, contain a supply of potential energy readily utilized 

 by the organism. These are the true foods. The essential 

 mineral salts, which constitute a very small portion indeed of the 



Formation 



of 

 Carbohydrates 

 Fats 

 Proteins -i^ 



and 

 Protoplasm 



Fig. 



65. — The organic food cycle. History of the construction and disintegra- 

 tion of the important organic substances found in plants. 



plant body, are neither tissue-builders (except in a very minor 

 degree) nor energy-producers, and hence cannot strictly be 

 regarded as plant foods at all. Their importance lies rather in the 

 fact that they are necessary, in minute quantities, to the 

 construction and successful functioning of protoplasm itself. 

 Together with water and carbon dioxide they may appropriately 

 be called nutrient materials. 



The food of plants and animals is essentially the same and the 

 difference between the two groups, therefore, lies not in the 

 character of the food which they use but in the fact that green 

 plants are capable of synthesizing food from inorganic nutrient 



