Mar. 17, 1923 
Further Studies in Photoperioaism 
913 
For this reason decrease of temperature with advance of the season could 
not be expected to account for the initiation of flowering in soybeans in 
August and September, the tendency being in the opposite direction. 
Direct evidence on this point is furnished by plantings of the Peking 
and Biloxi varieties made in the greenhouse on December 14 under con¬ 
trolled conditions of temperature, with and without electric illumination 
from sunset till midnight. With a mean temperature of approximately 
72 0 F. and a daily temperature seldom exceeding in either direction a 
range from 65° to 75 °, the soybeans germinated December 19 and, with 
natural illumination only, the Peking began flowering January 25 and the 
Biloxi February 25. With the added artificial illumination, flowering 
was inhibited. With a mean temperature of approximately 55 0 and a 
daily temperature seldom exceeding in either direction a daily range 
from 50° to 6o°, the soybeans which germinated December 27 had not 
flowered March 1 under either light exposure. In the first case the 
mean temperature (close to 72°) was somewhat below the mean tempera¬ 
ture for the month of July at Washington (76°.8). In the second case 
the mean temperature (approximately 55 °) was considerably below the 
Washington mean of 68° for the month of September. It is clear that 
the lower temperature delayed the time of flowering. 
A second phase of the temperature effect seems to relate more directly 
to internal conditions of nutrition, especially affecting, perhaps, the 
balance between income and outgo of carbohydrate because of the 
change in the rate of respiratory activity. In this way the relation 
between the duration of the light period and the prevailing temperature 
may become of decisive importance. The requirements for successful 
flowering necessarily are more critical than for vegetative growth, since 
the floral structures are dependent on other parts of the plant for their 
supply of suitable forms of carbohydrate. Unfavorable conditions of 
balance, therefore, either because of reduced income or excessive outgo 
of carbohydrate, may easily inhibit flowering and fruiting. These con¬ 
ditions would not necessarily check development of the green parts of 
the plant, since such structures tend to be self-supporting with respect, 
to carbohydrate supply. It need only be added that in these consider¬ 
ations the form of the carbohydrate as well as the quantity must be 
taken into account. 
It has been demonstrated that for many species, probably the majority 
of them, the extremely short days of winter in higher latitudes will 
inhibit increase in stature, and in many cases flowering and fruiting also. 
Largely as a result of the transition from the longer summer days to the 
shorter days of winter many annuals are caused to die and many peren¬ 
nials are forced into a state of more or less complete dormancy. Hence, 
the income from photosynthesis is reduced to a minimum. It is obvious, 
therefore, that exposure to a relatively high temperature during the 
short days of winter may easily prove disastrous to the plant. The 
importance of exposure to cold during the winter rest period has been 
recently emphasized by Coville (6). Exposure to the forcing action of 
high temperatures in winter is likely to be especially unfavorable to 
reproductive structures. It is well known, also, that in some way 
exposure to cold may exercise a beneficial action on nonchlorophyllous 
resting organs, resulting eventually in renewed activity. In general, 
when the dormant structure has entered upon the rest period with the 
necessary “reserve” materials the initial processes of renewed activity 
may be stimulated by favorable temperature conditions, such as a period 
