576 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1920. 



In additional tests with the above-mentioned varieties of soy bean it 

 was found that further decrease in the length of the day, even to as 1 

 low as 5 hours, had practically no greater effect than the 12-hour day 

 in forcing the plants into flowering and fruiting. On the other 

 hand, the rate of growth was found to be directly proportional to 

 the length of the day and, as a consequence, the size attained by the 

 plants at the time of flowering was materially reduced by shortening 

 the length of the daily light period below 12 hours. The action of 

 the artificially shortened day in forcing early flowering and fruiting 

 as well as in reducing the size attained by the plants is well shown 

 in plates 6, 7, 8, and 9. 



Another experiment with soy bean gave interesting and rather un- 

 expected results. The length of the daily exposure to light was 

 reduced by transferring the plants to the dark house at 10 a. m. and 

 returning them to the sunlight at 2 p. m. The plants thus really 

 received two periods of illumination daily, the total daily exposure 

 to sunlight through the summer and autumn being 11 to 8£ hours. 

 Under these conditions the plants behaved about the same as those 

 remaining in sunlight throughout the day, or, in other words, the 

 noon "siesta" was without effect in hastening the formation of 

 flowers and seed. This is in striking contrast with the effect pro- 

 duced when the length of day is shortened by cutting off the sunlight 

 in the morning and afternoon, as is clearly shown in plate 9, figure 2. 

 The most likely explanation would seem to be that internal processes 

 set up through the action of the morning sunlight continue in opera- 

 tion through the greater portion of the artificially darkened noon 

 period, the effect thus being about the same as if the plants had 

 remained in the light throughout the day. 



RELATION BETWEEN EARLY-MATURING AND LATE-MATURING VA- 

 RIETIES OF SOT BEAN. 



To secure varieties of various useful plants adapted to the various 

 regions of the country and to secure different varieties of many of 

 these plants adapted to the different seasons of the year in any given 

 region are among the chief problems of the plant breeder and the 

 plant introducer. Hence it is important to arrive at a correct under- 

 standing of the relation between early and late maturing varieties 

 in so far as difference in behavior is directly attributable to response 

 to external conditions. The soy bean is admirably adapted to throw 

 light on this subject. It has already been pointed out that of differ- 

 ent varieties of this plant which, when planted in the spring, require 

 all the way from a month up to more than 100 days to reach the 

 flowering stage, all can be made to blossom within three or four weeks 

 by properly shortening the length of the day. It is at once seen 

 that if this behavior holds true under natural conditions in the field 



