as a
voluntary control over the amount of "use" which it makes of its different
organs in response to changes of environment. The common form of statement
that a plant develops an organ, or a process to meet a certain need, or
modifies its habits of growth to meet a change of environment are, of
course, purely metaphorical, and can only be taken to mean that such
processes are mechanical responses to changes in external conditions.

The nature of the mechanism by which these responses are accomplished is,
as yet, wholly unknown. There is accumulating a large mass of experimental
evidence which goes to show that, while both temperature and light are very
important factors in determining the type of changes which will take place
in a living organism, the so-called "photochemical action of light" is by
far the most potent of all the climatic factors which influence the course
of development of a plant. But we have, as yet, no inkling of how the
protoplasm of the plant adjusts or controls its responses to variations in
any of these external factors.

With these general considerations in mind, we may now proceed to the
consideration of certain particular types of adaptations.

CHROMATIC ADAPTATIONS

Adaptations have been observed in both the energy-absorbing pigments of the
general tissues and in the ornamental epidermis pigments of plants. The
former are by far the most important from the physiological point of view;
while the latter may have interesting biological significance.

Under nearly all conditions of growth of land plants, the supply of the
chlorophylls and their associated pigments provides for the absorption of
solar energy far in excess of the amount necessary for the photosynthetic
assimilation of all the carbon dioxide which is available to the plant. It
has been shown that an active green leaf, on an August day, can absorb
eight times as much radiant energy as would be required to assimilate all
the carbon dioxide present in the air over its surface. No land plant,
under normal conditions, develops supplementary pigments in order to
utilize other than the parts of the spectrum which are absorbed by
chlorophyll and its associated pigments.

But deep-sea plants show quite a different phenomenon of pigment
development. Water is a blue liquid. At depths of 40 feet or more, the
light which penetrates is devoid of red rays, feeble in yellow, and is
characteristically green or blue