l energy to the
organism shall be designated as _synergic foods_, while those which contain
no potential energy shall be known as _anergic foods_. On this basis,
practically all of the food of animals, excepting the mineral salts and
water, and all of the organic compounds which are synthetized by plants and
later used by them for further metabolic changes, are synergic foods; while
practically all of the intake of green plants is anergic food.

It is with the latter type of food materials that this chapter is to deal;
while the following and all subsequent chapters deal with the organic
compounds which are synthetized by plants and contain potential energy and
are, therefore, capable of use as synergic food by either the plants
themselves or by animals. It will be understood, therefore, that in this
chapter the word "food" is used to mean the anergic food materials which
are taken into and used by green plants as the raw materials for the
synthesis of organic compounds, with the aid of solar energy, or that of
previously produced synergic foods. In all later chapters, the term "food"
will be used to mean the organic compounds which serve as the synergic food
for the green parts of green plants and as the sole supply of nutrient
material for the colorless parts of green plants and for parasitic or
saprophytic forms (see page 16).

PLANT FOOD ELEMENTS

The raw materials from which the food and tissue-building compounds of
plants are synthetized include carbon dioxide, oxygen, water, nitrogen,
phosphorus, sulfur, potassium, calcium, magnesium, and iron. The two gases
first mentioned are derived directly from the air, through the respiratory
organs of the plant. Water is taken into the plant chiefly from the soil,
through its fibrous roots. All the other elements in the list are taken
from the soil, nitrogen being derived from decaying organic matter (the
original source of the nitrogen is, however, the atmosphere, from which the
initial supply of nitrogen is obtained by direct assimilation by certain
bacteria and perhaps other low forms of plant life), and the remaining ones
from the mineral compounds of the soil.

Carbon dioxide and oxygen, being derived from the air, are always available
to the leaves and stems of growing plants in unlimited supply; but the
supply available to a seed when germinating in the soil, or to the roots of
a growing farm crop, may sometimes become inadequate, es