d for this reason, most such examples of
adaptation have been studied in connection with possible temperature
reactions upon the growing organism.

However, recent investigations seem to point strongly to the conclusion
that the amount of _light_ rather than the _temperature_ of the new
surroundings is the most important influence in determining the
physiological processes known as the "acclimatization" of plants. For
example, a very elaborate series of investigations has shown that the
flowering stage in the development of plants is determined by the length of
the daylight period per day, irrespective of the actual amount of
vegetative growth which the plant has made. Thus, tobacco plants, which
during a period of long days grow to the height of 8 or 10 feet before
blossoming, if grown at the same temperature in periods of short days (or
if kept in the dark during a portion of the longer days) will blossom when
less than 3 feet in height and when the total mass of vegetative material
which has been produced is less than one-third of that of the "gigantic"
plants of the same variety grown with longer periods of illumination per
day. This same principle has been found to hold good for many widely
different types of plants. In some species, however, flowering is favored
by long days, and vegetative growth by short daylight illumination. But in
all species which have been studied, there seems to be a direct relation
between the length of day, or the total illumination per day, and the
normal or abnormal functioning of the plant. It is apparent that at least
the physiological function of sexual reproduction (flowering and
seed-production) is determined by the length of daylight illumination. The
duration of daylight per day which is necessary to induce the blossoming of
the plants varies for different species, but it is constant for individuals
of the same species. This adaptation of stage of growth to duration of
daily illumination must, therefore, be an evolutionary character of the
species.

Hence, it appears that in many cases physiological adaptation may be a
direct response of the life-processes of the plant to the daily length of
photochemical stimulation which it receives from solar light. But there is,
as yet, no explanation of how this (or any other) influence actually
changes the vital processes of the plant protoplasm so as to bring about
either a morphological adaptation of structure or a physiological
adaptat