50 STUDIES IN PHYSIOLOGY 



duction of energy and in the process of growth. In many 

 plants, however, considerable quantities are stored away for 

 future use. In the potato plant, for instance (see Fig. 15), 

 a great amount of the starch manufactured in the leaves is 

 changed to sugar, is carried down beneath the ground through 

 the tubes in the stem, and is there stored away as starch in 

 the swollen tubers that we call potatoes. In a similar way 

 sugar is collected below ground in the beet root and above 

 ground in the stem of the sugar cane. 



Proteid Manufacture. Proteids, as we have already learned 

 (p. 18), are among the most complex of chemical compounds. 

 In addition to the carbon, hydrogen, and oxygen found in 

 the carbohydrates, the proteids contain nitrogen and sulphur, 

 and sometimes phosphorus and other elements are present. 

 It is very probable that plants make proteids out of carbo- 

 hydrates, the additional nitrogen, phosphorus, and sulphur 

 being furnished by the materials that are carried up from 

 the soil by the sap. 



4. USES OF THE NUTRIENTS 



The processes by which foods are changed into protoplasm 

 and by which they supply the body with heat and muscular 

 energy are extremely complex. Much study, however, has 

 been given to the subject, and we are now reasonably sure 

 as to some of the uses of the different nutrients. 



Uses of Proteids. We have learned that proteid is the 

 most important substance found in protoplasm. This class 

 of nutrients is therefore essential for the growth and repair 

 of muscle, nerve, and all the other body tissues. It is clear, 

 then, that milk, meats, bread, beans, peas, oatmeal, and 

 other foods that contain considerable amounts of proteid 

 must be of first importance in the nutrition of the body, 

 especially during its period of growth, although, as we shall 

 see later, it is probably true that the average adult eats more 

 than is necessary of this kind of nutrient. Proteids can 



