Mar. 17, X923 
Further Studies in Photoperiodism 
919 
One of the characteristic effects of change in the light period from 
optimum to suboptimum for stem elongation is to promote branching. 
Thus, the apical bud loses its dominance and lateral buds become ac¬ 
tive. As stages in the response to progressive departure of the light 
period away from the optimum for elongation of the axis may be noted 
increased branching at the top, middle, or base of the aerial portion of 
the axis; various degrees in erectness of the resulting branches; change 
from aerial to underground type of stem and change downward in direc¬ 
tion of growth of these underground stems. 
The decreasing length of day of fall is an important factor in causing 
perennials to enter upon the winter period of dormancy. Abscission 
and leaf fall, also, appear to be induced by this shortening of the light 
period. Abrupt and sufficiently pronounced change from long-day to 
short-day conditions, however, tends to cause deciduous woody peren¬ 
nials to behave like evergreens in retaining the foliage leaves, and this 
tendency also is discernible even in certain typical annuals. Further¬ 
more, the light period, through quantitative features of its action on 
the formation of vegetative resting and reproductive structures, may 
affect to some extent the conditions of emergence from dormancy. 
During the winter months the relation between the length of the 
daily light period and the temperature becomes especially important. 
Because of the short days prevailing in this period the income of the 
plant through photosynthesis is reduced to the minimum and growth 
conditions are otherwise unfavorable, so that high temperatures are 
likely to establish an unfavorable ratio of income to outgo and thus 
may easily prove disastrous to the plant. 
It seems probable that the annual cycle of length of day, affording 
as it does the only consistently rhythmic feature of the external en¬ 
vironment, is a dominant causal factor in phenomena of plant periodicity 
which have been regarded by many as explainable only on the basis of 
an “internal rhythm.” This view is strengthened by the fact that 
definite periodicity in plant development is observable under various 
climatic conditions, from the equatorial regions to the poles, while the 
only consistently rhythmic factor of the environment applicable through¬ 
out this range is the length of day. In arriving at a correct estimate of 
the significance of the annual cycle in length of day in regulating periodic 
plant responses it must be understood that full expression of periodicity 
is conditioned on the specific requirements of the particular plants in 
question as to temperature and other environmental factors. The 
internal mechanism concerned in the marked regulatory action of the 
length of day on plant growth has not been fully determined, but there 
is strong evidence that in some way the degree of hydration of the 
living cell content is brought under very delicate control by the ratio 
of the number of hours of sunlight to the number of hours of darkness 
in the 24-hour period. 
LITERATURE CITED 
(1) Bernard, Noel. 
1902. Etudes sur la tub&risation. In Rev. G6n. Bot., t. 14, p. 5-25, 58-71, 
101-119, 170-183, 219-234, 269-279; fig. 7-12, 26, 36-38. 
(2) - 
1902. CONDITIONS PHYSIQUES DE LA TUB^RISATION CHEZ LES v£g*$TAUX. In 
Compt. Rend. Acad. Sci. [Paris], t. 135, p. 706-708. 
(3) Bonnier, Gaston. 
1895. INFLUENCE DE LA LUMI^RE ttLECTRIQUE CONTINUE SUR LA FORME ET LA 
structure des plantes. In Rev. Gen. Bot., t. 7, p. 241-257, 289-306, 
332-342, 409-419* P 1 * 6 ~ 1 5 ‘ 
