Mar. i7, 1923 
88l 
Further Studies in Photoperiodism 
open type of flowering indefinitely, thus illustrating a narrower type of 
everblooming ( 8 ). In this case the whole effect of the range in length 
of day from February to the summer solstice is merely to change the 
type of blossom (involving change in relative fertility). 
Thus, beginning with the equatorial length of day of 12 hours as the 
standard, it may be said that a group of plants normally will flower 
under any range downward to less than 6 hours, while another group 
will flower under any range upward to at least 18 hours and probably 
up to continuous illumination. Other plants will flower only within a 
comparatively narrow range on either side of the 12-hour standard. 
Still others are capable of flowering throughout these ranges, their 
response being quantitative rather than qualitative in character. 
balance between flowering and vegetative activity 
Within the range of duration of light exposures which will admit of 
alternative forms of expression there are in most cases certain expo¬ 
sures favorable only to sexual reproduction and other exposures favor¬ 
able only to vegetative activity. That is, there are more or less definite 
optima in light exposures for the two alternative forms of expression. 
The question arises as to the response of the plant to light exposures 
intermediate between those specifically favorable to the two types of 
activity. In reality the effects of these intermediate exposures are 
important and are both quantitative and qualitative in character. 
There are two aspects of the problem. Under artificial control, fixed 
day lengths of intermediate duration may be studied, whereas, in nature, 
there is a graduated change from one optimum toward the other and of 
course the change may be in either direction. 
One of the effects of an intermediate light exposure is a tendency 
toward the condition of everflowering in which the plant divides its 
activity between the vegetative and the reproductive phases of develop¬ 
ment. As the day length approaches the optimum for flowering, the 
vegetative phase of activity is reduced more or less and vice versa. 
Thus, the Yellow Dent variety of com grown in summer in the green¬ 
house, with electric light at night to give an illumination period of 18 
hours, produced larger, taller, and longer-lived stalks while the ears 
were longer, with longer stems, but were poorly filled as compared with 
the controls exposed to the normal summer day length. Again, under 
a io-hour day Biloxi soybeans gave per plant a dry weight of 10.9 gm. 
of stalk and 36.6 gm. of seeds, a ratio of 1 to 3.3. The corresponding 
values under a 13-hour day were 65.8 gm. of stalk and 95.3 gm. of seeds, 
a ratio of 1 to 1.4. Under the io-hour day growth ceased as soon as 
flowering began, but under the 13-hour day there was a tendency toward 
everbearing, for growth and fruiting proceeded simultaneously. A con¬ 
siderable volume of data which need not be considered in detail here 
has been obtained with soybeans grown under artificially regulated day 
lengths and planted in the field at stated intervals during the season, all 
of which further exemplify this principle of the influence of intermediate 
day lengths dh the quantitative relation between vegetative and repro¬ 
ductive activities. 
