22 NUTRITIONAL PHYSIOLOGY 



nitrogen. These, as well as the carbon dioxid, may be of 

 service to the plant, although to be strictly accurate it must 

 be added that they require some modification, usually 

 effected in nature through the action of bacteria. In 

 view of these exchanges one is tempted to the hasty con- 

 clusion that a single-celled green plant and a single- 

 celled animal form a balanced system capable of con- 

 tinued maintenance in short, a microcosm. There is, 

 however, a fatal obstacle to the continuance of the part- 

 nership the animal's need of organic food can only be 

 satisfied by the sacrifice of the plant. To have a truly 

 balanced system of an enduring character we must assume 

 a multiplication of cells descended from the original uni- 

 cellular plant, providing a surplus of vegetable tissue for 

 the animal's consumption. 



The give and take which has been illustrated for single 

 cells is proceeding on a vast scale everywhere. It is 

 hard to realize that the great harvests which support 

 the races of mankind were formed for the most part from a 

 gas present only very sparingly in the atmosphere, from 

 water, and from the mineral salts of the soil. While we 

 rely partly upon animal food (meat, milk, and eggs), 

 this does not alter the fact of our absolute dependence 

 upon the green plants, which in turn owe their growth to 

 the translated energy of the sun. It is amusing to note 

 the apparent travesty upon our human life which can be 

 read into the contrasted conduct of green plants and of 

 animals. The plants are conserving, while the animals 

 are spendthrift. Plants create and distribute wealth. 

 They seem like the thrifty and industrious members of 

 society upon whose charity others less competent depend. 

 One is reminded irresistibly of the parent at home and 

 the son at college. If the father does not literally subsist 

 upon a diet of carbon dioxid and water that the son may 

 have protein and alcohol, the approach to a parallel is too 

 close for complacent attention. 



The Body and the Diet. Turning now to the definite 

 problems of human nutrition, let us consider the respective 



