osynthesis, has not yet been determined.

But there must be some influence other than response to environmental
conditions which controls the vegetative color in plants, since shrubs, or
trees, which have green, yellow, red, and purple leaves, respectively, will
grow normally, side by side, under identical external conditions of
sunlight, moisture supply, etc. The hereditary influence must completely
overshadow the apparent normal self-adjustment of pigment to
energy-absorbing needs, in all such cases.

Again, it appears that there is some definite connection between pigment
content and respiration. It is known, of course, that the gaseous exchanges
involved in animal respiration are accomplished through the reversible
change of haemoglobin to oxyhaemoglobin, these being the characteristic blood
pigments. The easy change of carotin, C_{40}H_{56}, to xanthophyll,
C_{40}H_{56}O_{2}, and _vice versa_, and the reversible changes of the
yellow anthoxanthins to the red anthocyanins, under the influence of the
oxidizing and reducing enzymes which are universally present in plants,
would indicate the possibility of the service of these pigments as carriers
of oxygen for respiratory activities in plants in a way similar to that in
which the blood pigments serve this purpose in the animal body. The fact,
which has been observed in connection with the experimental studies of the
development of the lycopersicin, that tomatoes which normally would become
red remain yellow in the absence of oxygen, indicates that this
pigmentation, at least, is definitely connected with oxygen supply; and the
further fact that the development of lycopersicin in red tomatoes, red
peppers, etc., is dependent upon the temperature at which the fruit ripens,
may indicate a definite connection of this pigment with the need for more
oxygen (or for more heat, as suggested in the following paragraph) at these
lower temperatures.

Again, many investigators have concluded that at least one function of the
anthocyanin pigments is to absorb heat rays and so to increase
transpiration and other chemical changes. In support of this view, there
may be cited the general presence of such pigments in arctic plants, their
appearance in the leaves of many deciduous trees after a frost in the fall,
etc. Indeed, there is much to support the view that the autumnal changes in
foliage pigments have the physiological function of absorbing heat in order
to hasten the metabolic