120 
Journal of Agricultural Research 
Vol. XXVII, No.} 
quently with some form of tuberization when exposed to a short light 
period, but are capable of developing flowering stems under the influ¬ 
ence of a longer daily light period or under certain conditions even when 
exposed to continuous illumination. Plants of this type are designated 
as long-day plants. 
In both of the above-named groups the rate of growth of the stem 
increases with increase in the length of the illumination period, maximum 
growth being associated with sterility in the first group and with flower¬ 
ing and fruiting in the second group. There is a third group, however, 
in which maximum stem growth is attained with a light period of inter¬ 
mediate length approximating the equatorial length of day. In this group 
a light period in excess of the optimal for growth favors flowering and 
fruiting and tuberization. In all cases observed the height of the flower 
stem, like that of the vegetative stem, increases with increase in the length 
of the light period, provided the limits favorable to flowering are not ex¬ 
ceeded. The bearing of these group relationships on the natural distribution 
of plants and on the behavior of a given species in different latitudes will be 
apparent at once to the ecologist. The duration of the light period may in¬ 
duce or modify various other forms of response such as branching habits, 
root growth, leaf fall and dormancy, but the present discussion will be 
limited primarily to the fundamental relationships of increase in stature, 
sexual reproduction and tuberization. 
One of the most striking features of the response to duration of the 
light period so far as concerns initiation of flowering is that the important 
factor is not merely the total number of hours of illumination during the 
24-hour period but the number of hours of uninterrupted light in each 
period of exposure. Thus, darkening soy beans and other species during 
the middle of the day for periods of four, five, or six hours, begining at 
10 a. m., has but little effect in either hastening or delaying the advent 
of flowering, though the growth rate may be materially checked. 
Another significant fact is that in many cases electric light of intensity 
as low as 5 foot-candles when used for prolonging the daylight period is 
capable of exercising a definite formative action in either initiating or 
inhibiting flowering. It is difficult to explain the action of light of such 
low intensity on the basis of photosynthesis alone. While the effect on 
sexual reproduction is clearly expressed, the general nutrition of the plant 
is not permanently maintained and sooner or later decline sets in, usually 
resulting in death. 
Since sexual reproduction and other forms of expression can be readily 
controlled by proper regulation of the daily light period this factor 
furnishes a means for making detailed study of the internal conditions of 
the plant which are associated with alternative forms of development, at 
least in so far as analytical methods are available for such work. Under 
proper conditions and with suitable material the time required for definite 
expression is easily determined and varies only within narrow limits. 
Moreover, it is not necessary to control accurately other environmental 
factors in order to obtain the expected response. It is believed that a 
large field for profitable study of internal environment in relation to 
response is thus opened up and the present paper is intended as a begin¬ 
ning in this field. But little is available in the way of a scientific back¬ 
ground for such studies, for thus far the duration of the light period as 
a factor in plant development has received almost no attention from 
physiologists and biochemists. Even in the case of photosynthesis, 
while there is a voluminous literature on the effect of intensity and 
