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- 
1 For example, a recent hybrid is "photosynthetic carbon-assimilation"! 
