886 
Journal of Agricultural Research 
Vol. XXIII, No. II 
tively, as it were, through the change in day length in the fall when the 
change is away from the optimum for vegetative activity and toward the 
optimum for sexual reproduction. One importamt qualification must be 
made to this principle, namely, that it holds good only so long as the day 
length does not become too short for the flowering and fruiting processes. 
The long-day plants, on the other hand, through late spring and early 
summer are subjected to a changing day length which is approaching 
the optimal for sexual reproduction, hence it is to be expected that these 
plants would flower and fruit abundantly during this period. A large 
proportion of spring flowering plants consists of. woody and herbaceous 
perennials in which the flower primordia are organized during the summer 
preceding the spring in which the blossoms unfold. In this case it 
seems probable that the optimal day length for flower development 
following the summer solstice passes by before the embryonic flower 
buds have made much progress in their development and the direction 
of the change in day length through late summer and the fall is away 
from this optimum for flowering and fruiting and toward or through the 
optimum for vegetative activity. Under these conditions further 
development of the flower buds would be attended with difficulty so that 
their growth would be slow. If the onset of cold weather is delayed, 
some species may flower in the fall instead of in the normal spring season. 
In some, and probably in most instances, however, flowering would be 
inhibited by the short days of late fall and winter. On the other hand, 
the cold weather of winter may exercise a distinctly helpful influence 
on the completion of the flowering process by establishing a favorable 
balance of income over outgo. Consequently, the earliness of flowering 
in the spring will depend largely on how soon the temperature rises to 
the point where development can actively proceed. The necessary 
internal conditions for flowering having been established, it remains for 
rise in temperature in spring to speed up the unfolding of the blossoms. 
Nevertheless, the increasing length of the day in the spring undoubtedly 
remains a factor, particularly in those species which do not unfold their 
blossoms till spring is well advanced. This is seen, for example, in the 
case of the beet (p. 888). 
APOGEOTROPISM, OR INCREASE IN STATURE 
Apogeotropism or increase in stature is a response of great importance 
in photoperiodism; and, in lact, the experimental data thus far secured 
as a whole seem to indicate that in a sense this is the fundamental 
phenomenon underlying the various other responses, including flowering 
and fruiting, which are discussed in this paper. It appears that, in 
general, there is an optimal length of day for apogeotropic growth and 
any change to suboptimal conditions will result in checking the apogeo¬ 
tropic type of vegetative development, to be followed by various other 
forms of expression. With sufficient departure from the optimal light 
period the primary axis may be completely suppressed and aerial growth 
practically confined to leaf development, as typically shown in the leaf 
rosette of stemless plants. Another typical form of expression result¬ 
ing from a rather extreme suboptimal light period is the prostrate or 
creeping habit of growth, stem elongation taking place without increase in 
stature. As the day length is advanced toward the optimum the rate 
of vertical elongation, as well as the final stature attained, are increased. 
