Mar. *7» *9*3 
Further Studies in Photoperiodism 
905 
transition from the long days of summer to the short days of midwinter 
is required to induce abscission in these plants. 
DORMANCY AND DEATH 
In general, exposure of annuals to the optimal illumination period for 
flowering tends to induce rapid senescence and death. In the same way 
exposure to certain definite day lengths causes perennials to enter into a 
state of dormancy. Deciduous trees and shrubs, in which the laying down 
of resting buds on the stem precedes leaf fall, enter into a form of dor¬ 
mancy involving temporary weakening, but not complete loss, of 
capacity for apogeotropic functioning. Herbaceous perennials enter into 
a form of dormancy in which there is more complete loss of the apogeo¬ 
tropic function. In both cases there is loss of leaves and photosynthetic 
activity is mostly suspended. That the first-named type of dormancy 
may be prevented by maintenance of a relatively long illumination period 
is shown by the experiments with the tulip poplar described in the 
preceding discussion of abscission and leaf fall. That the second type of 
dormancy also may be prevented by maintaining a long illumination 
period has been shown in experiments with Aster linariifolius. 
Too great a departure from the optimal illumination period for apogeo¬ 
tropic growth, however, may cause the plant to assume a sort of pseudo¬ 
dormancy in which the leaves acquire an abnormally dark green color and 
the general appearance above the ground is one of comparative inactivity. 
Annuals, as well as perennials, show this response. These considerations 
lead to the conclusion that there may be an optimal light duration for 
death as well as for dormancy, while on either side of this optimum there 
may be very considerable and long-continued activity by the plant. On 
the one side the response is primarily and characteristically apogeotropic. 
On the other side the general tendency is toward geotropic response, 
visible expression being confined mostly to the surface or underground 
portions of the plant. 
The results of tests with five varieties of soybeans will illustrate the 
fact that exposure to a definite light period results in rapid senescence 
and death in annuals. The varieties known as Mandarin, Peking, 
Tokyo, Biloxi, and Otootan were planted May 31 to June 1 in boxes 
and germinated June 4 to 6. They were exposed to day lengths of 
10, 11, 12, 13 hours, and to the natural length of day of summer. For 
comparison, a lot of each variety also was given 10 hours of light daily 
of low intensity by means of a shade of cheesecloth. The principal 
results of the tests are presented in Table VI. On September 8 the 
seed pods of all varieties were fully matured and dry under all reduced 
day lengths, but only those of the Mandarin variety had matured under 
the natural length of day. Under the shorter light period all varieties 
flower at practically the same time; but as the light duration is increased 
the dates of flowering draw apart, and under the full summer day length 
the differences in time of flowering become very great. As to the 
advance of senescence following the flowering and fruiting stages two 
facts are brought out in the table. It is evident that the rate at which 
senescence advances in a given variety depends on the duration of the 
light period and, also, it is evident that the optimal light period for 
advance of senescence differs with the variety. For the earlier varieties, 
Mandarin and Peking, it appears that the optimal period is approxi¬ 
mately 13 hours, while for the very late varieties, Biloxi and Otootan, 
