rotoplasm from the simpler carbohydrates.

The _synergic foods_, or "reserve foods" as they are sometimes called,
produced by the excess of synthetized material over that needed for the
immediate use of the plant, are accumulated either in the various storage
organs, to be available for future use by the plant itself or by its
vegetative offspring, or in the seed, to be available to the young seedling
of the next generation. Proteins not only serve as reserve food materials
but also make up the body of the living organism itself. Carbohydrates and
fats serve as synergic and reserve foods.

The _secretions_ may be produced either in ordinary cells and found in
their vacuoles, or in special secretory cells and stored in cavities in the
secreting glands (as in the leaves of mints, skin of oranges, etc.), or in
special ducts (as in pines, milkweeds, etc.) or on the epidermis (as the
"bloom" of plums, cabbages, etc., the resinous coating of many leaves,
etc.). As a general rule, the glucosides, pigments, and enzymes are the
products of unspecialized cells and have some definite connection with the
metabolic processes of the plant; while the volatile oils and the alkaloids
are usually secreted by special cells and have no known role in
metabolism.

CHAPTER III

PHOTOSYNTHESIS

Photosynthesis is the process whereby chlorophyll-containing plants, in the
presence of sunlight, synthetize organic compounds from water and carbon
dioxide. The end-product of photosynthesis is always a carbohydrate.
Chemical compounds belonging to other groups, mentioned in the preceding
chapter, are synthetized by plants from the carbohydrates and simple raw
materials; but in such cases the energy used is not solar energy and the
process is not photosynthesis.

Under the ordinary conditions of temperature, moisture supply, etc.,
necessary to plant growth, photosynthesis will take place if the three
essential factors, chlorophyll, light, and carbon dioxide are available.

PHYSIOLOGICAL STEPS IN PHOTOSYNTHESIS

There are five successive and mutually dependent steps in the process of
photosynthesis, as follows:

(1) There must be a gas exchange between the plant tissue and the
surrounding air, by means of which the carbon dioxide of the air may reach
the protoplasm of the chlorophyll-containing cells.

(2) Radiant energy must be absorbed, normally that of