558 Journal of Agricultural Research voi. xviii. no. ir 



soybeans still showed the abnormal behavior in the winter even when the 

 temperature in the greenhouse was kept quite as high as prevails out of 

 doors during the summer months. This observation seemed to dispose of 

 temperature as a possible factor of importance in the "winter effect." 

 It is clear that the quantity of solar radiation received by plants is less in 

 winter than in summer, for both the number of hours of sunshine per day 

 and the intensity of the light are reduced during the winter months. 

 The quality of the light also is affected, since the angle of elevation of the 

 sun's path during the winter is less than during the summer and the 

 selective absorptive action of the atmosphere comes into play. It hap- 

 pened that in the investigation on oil formation in seeds a number of 

 experiments had been made with soybeans to determine the effect of 

 light intensity on this process and, incidentally, it was observed that in 

 no case was the date of blossoming materially affected by the intensity of 

 the light. It had been found, also, that partial shading was without 

 decided effect on the blossoming of the Mammoth tobacco. In view of 

 these experiences it hardly seemed likely that the other primary factor 

 controlling the maximum amount of radiation received by the plant — 

 namely, the length of the daily exposure — could be responsible for the 

 effects in question. Nevertheless, the simple expedient of shortening 

 artificially by a few hours the length of the daily exposure to the sun by 

 use of a dark chamber was tried, and some very striking results were 

 obtained, as detailed in the following paragraphs. 



PLAN OF THE EXPERIMENTS 



The first experiments with the use of the dark chamber were begun in 

 July, 1918. A small, ventilated, dark chamber with a door which could 

 be tightly closed was placed in the field. The soybeans used in the tests 

 were grown in wooden boxes 10 inches wide, 10 inches deep, and 3 feet 

 long. These containers have been extensively used in growing soybeans 

 and other small plants under controlled conditions, and it has been found 

 that normal plants are easily obtained in this way. The dark chamber 

 and the type of box used for growing soybeans and similar plants are 

 shown in Plate 64, A. Larger plants like tobacco have been grown in 

 large galvanized iron buckets or, in some cases, in ordinary flower pots. 

 When the test plants have attained the desired stage of development the 

 procedure has been to place them in the dark chamber at the selected 

 hour in the afternoon each day. The plants were left in the dark cham- 

 ber till the hour decided upon in the following morning, when they were 

 again placed in the sunlight. This procedure was followed each day till 

 the test was completed. Appropriate control plants were left in the open 

 throughout the test in each case. By this method the number of hours 

 of exposure to sunlight during the 24-hour period could be reduced as 

 far as desired. 



