NUTRITION 363 



plants; but they manage to secure energy in ways unknown to us, and 

 build these substances into their bodies. 



Food a source of energy. After all, foods are of value to plants, as 

 we conceive things, because they supply them with energy as well as 

 with material. The energy income in this way is indeed the important 

 feature. The green plant locks up in the food it constructs a fraction 

 of the solar energy which reached it as light; and thus this energy 

 becomes available to other organisms, since after further transforma- 

 tions of the foods they can release it by decomposition and apply it to 

 other reactions. 



Food and growth. Because with our best appliances we are unable 

 to know yet the real nature of nutrition, the use which a plant makes of 

 food can be determined only by the extent to which it promotes growth 

 and development of the body. The term economic coefficient has been 

 used to express the ratio which the increase in the weight of a crop (say 

 of a fungus) bears to a given quantity of a particular food. Manifestly 

 there are other ways in which the plant uses a food besides incorporating 

 it into the permanent structure of the body, and many complicated rela- 

 tions may be disturbed by too limited nutrition. Yet this economic co- 

 efficient expresses, in a crude way, the differences in the availability of 

 foods for body building, and so impresses the fact that the processes 

 of nutrition differ widely in different plants. 



2. PHOTOSYNTHESIS 



The fundamental fact in the nutrition of all living things is the capacity 

 of green plants to make certain complex organic compounds, carbohy- 

 drates namely, out of carbon dioxid and water, by the aid of light. This 

 unique process is known as photosynthesis. 



The term used. When the food of green plants was described as 

 inorganic, this transformation of inorganic materials into carbohydrates, 

 which was taken to be their incorporation into the body, was called assimi- 

 lation, after the analogy of the transformations undergone by the. food 

 of animals. As the radical differences between the food making of a green 

 plant and true assimilation in both plants and animals began to appear, 

 an attempt was made to obviate the confusion by using the term carbon 

 assimilation. These terms are still in common use in other countries, 

 but will gradually disappear. 1 Clearness demands the use of the dis- 



* For example, a recent hybrid is " photosynthetic carbon-assimilation " ' 



