ost cases.

Hence, it will be understood that in all the following discussions of plant
activities, except where specific exceptions are definitely mentioned, it
is the green, or autotrophic, plants to which reference is made in each
case.

From the standpoint of the sum total of its activities, a green plant is
essentially an absorber of solar energy and a synthetizer of organic
substances. Each individual autotrophic plant takes up certain amounts of
the anergic foods which are discussed in the preceding chapter and
manufactures from them a great variety of complex organic compounds, using
the energy of the sun's rays, absorbed by chlorophyll, as the source for
the energy necessary to accomplish these synthetic reactions. The ultimate
object of these processes is to produce seeds, each containing an embryo
and a sufficient supply of food for the young plant of the next generation
to use until it has developed its own synthetic organs; or (in the case of
perennials) to store up reserve food materials with which to start off new
growth after a period of rest and often of defoliation. To be sure, animals
and men often interfere with the completion of the life cycle of the plant,
and utilize the seeds or stored food material for their own nutrition, but
this is a biological relation which has no influence upon the nature of the
plant's own activities.

Since all of these synthetic reactions must go on at ordinary temperatures,
active catalyzers are necessary. These the plant provides in the form of
enzymes (see Chapter XIV) which are always present in active plant
protoplasm. Proper conditions for rapid chemical action are further assured
by the colloidal nature (see Chapter XV) of the protoplasm itself.

TYPES OF CHEMICAL CHANGES INVOLVED IN PLANT GROWTH

The whole cycle of chemical changes which is involved in plant growth
represents the net result of two opposite processes; the first of these is
a constructive one which has at least three different phases: namely, a
synthesis of complex organic compounds, the translocation of this
synthetized material to the centers of growth, and the building up of this
food material into tissues or reserve supplies; and the second is a
destructive process of respiration whereby carbohydrate material is broken
down, potential energy is released, and carbon dioxide is excreted.

The synthetic processes which take place in plants are of two types;
namely, photosynthes