are _external_
to the plant and are either physical, such as changes of temperature,
amount of light received, etc.; or chemical, such as variations in the
salts received from the soil, or common anaesthetics applied to the plants
by man. A plant grows normally under certain conditions to which it has
become adjusted by hereditary acquirements. When these conditions are
altered, the effect upon the functioning of the plant protoplasm may be
either stimulating or depressing. Extreme changes in environmental
conditions generally result in the death of the plant; but changes which do
not result in the lethal condition affect the plant by either stimulating
it to more rapid physiological activity or by depressing its normal growth
or functions. As has been pointed out, the same external influence, either
chemical or physical, which acts as a stimulant if it differs only slightly
from normal conditions, may become depressing, or positively toxic, if
present to a larger extent.

There is also the possibility of the elaboration by the plant itself of
_internal_ agents, or substances, which may have a definite stimulating or
inhibitory effect upon its metabolism and growth. The study which has been
given to these matters has practically all been carried on within very
recent years and is still in progress. Most of it is still in the
experimental stage, in which no definite conclusions are as yet possible.
Hence, the most that can be done at present is to give a brief review of
the suggestions which have been made thus far, as indicative of the
uncertainty of our present knowledge of these matters and of the general
trend of the investigations which are now in progress.

Substances which are elaborated by plants and which are supposed to have a
definite stimulating or beneficial effect upon the activities of the plant
which produces them, or to influence the physiological activities of other
plants with which these substances come in contact through either the
parasitic or the symbiotic relation, have been variously discussed under
the names "hormones," "auximones," and "vitamines"; while injurious
substances are generally known as "toxins." Whether these different terms
actually represent different definite types of substances, or whether there
are actually different groups of stimulating or inhibitory agents produced
in plants, is uncertain; but the following brief statements will serve to
indicate the general nature of the s