enomenon will be
distinguished by using appropriate adjectives, as "physiological
adaptations," "chromatic adaptations," "morphological adaptations," etc.

Two markedly different types of responses to altered conditions, or of
adjustment to environment, may be recognized. In the first of these, for
which we will use the term "physiological adaptation," the species of plant
simply acquires the ability to exist and grow normally under conditions
which formerly inhibited its growth. Thus, we may speak of the phenomena
mentioned above as "acclimatization" as the _physiological adaptation_ of
the crop to the new conditions of growth. In general, physiological
adaptations include such variations in the characters or habits of growth
of plants as results in differences in resistance to heat or to cold,
relations to water, aggressiveness in competition with other plants, etc.
In such cases, no modification of the morphological characters of the plant
can be observed, the changes which take place in the structure of the plant
(if, indeed, there be any such changes) must be only minor adjustments of
the protoplasm to meet the new environmental needs.

In the second type of adaptations, for which we will use the term
"morphological adaptations," the structure, or color, or some other
morphological character of the plant is actually changed in some easily
recognizable way, in order that the plant may be better adjusted to its
environment. As examples of _morphological adaptations_, there may be cited
the change in color of sea-weeds with increasing depth in the sea, and
other examples of chromatic adaptation which are discussed below; the
development of fewer, or a larger number, of buds on the above-ground stems
of plants, in response to decreases, or increases, in the available supply
of food; the alteration in the size and shape of the leaves of many plants
when they are grown in shade; the dwarfing of plants at high altitudes, or
under conditions of severe drought; the development of underground storage
organs for certain species of shrubs and trees which grow in regions that
are subject to periodical burning-over, in such a way as to destroy the
above-ground storage stems, etc.

Hence, the two terms, as we will use them here, may be defined as follows:
_morphological adaptation_ is a change in the structural character of the
species in order that it may be better fitted to meet the needs of the new
conditions of growth; while