processes of ripening and preparation for winter
defoliation. The rapid and brilliant changes in foliage coloring after a
sharp frost which kills the tissues and makes rapid translocation of the
food material of the leaves to the storage organs immediately necessary,
have been explained as the response of the pigmentation of the leaves to
the need for increased heat-absorption. On the other hand, the red pigments
of the beet-root, etc., which seem to be identical in composition with the
other anthocyanin pigments, can have no such function as those which have
just been described. Furthermore, the fact that the pigment often varies in
color from red to yellow or brown, depending upon the temperature under
which the tissue is ripening, makes it an open question whether the pigment
is the regulating agency or whether its nature is the result of the
environmental conditions. Or, in other words, it is a question whether
these changes in color are a mechanism by which the plant cell adjusts its
absorptive powers, or whether they are only the inevitable result of the
changes in temperature upon a pigment material which is present in the cell
for an entirely different use.

A very interesting side-light upon the color changes which many species of
plants undergo when the external temperature falls has been shown by the
investigations of the relation of the sugar content of the plant tissues to
their pigmentation. It is a well-known fact that not only do many species
of deciduous plants show the characteristic reddening of their leaves after
frost in the autumn but also many evergreens (_Ligustrum_, _Hedera_,
_Mahonia_, etc.) exhibit a marked reddening, or purpling, of their foliage
during the winter months, with a return to the normal green color in the
spring. Earlier investigations, which have been confirmed by several
repetitions, showed that the red or purple leaves always contain higher
percentages of sugar than do green ones of similar types. More recent
studies have shown that artificial feeding of some species of plants with
abnormally large portions of soluble sugars produces a reddening of the
foliage tissues which is apparently identical with that which these tissues
undergo as the result of low temperatures. Thus, the connection between the
natural winter reddening of foliage and the development of sugar in the
tissues during periods of low temperatures (see page 64) seems to be
clearly demonstrated. It appears that a