-mentioned groups of
organic components of plants, an attempt will be made to point out what
significance these particular compounds have in the plant's life and
growth. Certain terms will be used to designate different roles, which it
is probably necessary to define.

There may be two possible explanations of, or reasons for, the presence of
any given type of compound in the tissues of any particular species of
plant. First, it may be supposed that this particular type of compounds is
elaborated by the plant to satisfy its own physiological needs, or for the
purpose of storing it up in the seeds as synergic food for the growth of
the embryo, in order to reproduce the species. For this role of the various
organic food materials, etc., we will employ the term "physiological use."
On the other hand, it is often conceivable that certain types of compounds,
which have properties that make them markedly attractive (or repellent) as
a food for animals and men, or which are strongly antiseptic in character,
or which have some other definite relationship to other living organisms,
have had much to do with the survival of the particular species which
elaborates them, in the competitive struggle for existence; or have been
developed in the plant by the evolutionary process of "natural selection."
For this relation of the compound to the plant's vital needs, we will use
the term "biological significance." Such a segregation of the roles which
the different compounds play in the plant's economy may be more or less
arbitrary in many cases; but it will be clear that when _physiological
uses_ are discussed, reference is being made to the plant's own internal
needs; while the phrase _biological significance_ will be understood to
refer to the relation of the plant to other living organisms.

PHYSIOLOGICAL USES OF THE ORGANIC COMPONENT GROUPS

From the standpoint of the role which each plays in the plant economy, the
several groups of organic compounds may be roughly divided into three
classes. These are: (_a_) the framework materials, including gums, pectins,
and celluloses; (_b_) synergic foods, including carbohydrates, fats, and
proteins; and (_c_) the secretions, including the glucosides, volatile
oils, alkaloids, pigments, and enzymes.

The _framework material_, as the name indicates, constitutes the cell-wall
and other skeleton substances of the plant. It is made up of carbohydrate
complexes, produced by the cell p