^jA Journal of Agricultural Research voi. xviii, no. n 



that this minimum formative period is approximately 21 days for the 

 Mandarin and Peking varieties, 24 days for the Tokyo, and 26 days for 

 the Biloxi, although under suitable conditions these periods might pos- 

 sibly be somewhat further shortened. 



Subjecting the plants to two periods of illumination daily, whereby the 

 total daily exposure averaged 9 or 10 hours, was vastly less effective in 

 inducing early blossoming than a single daily exposure of 12 hours; and, 

 in fact, in the later varieties the effect was of little significance. This is 

 true in spite of the fact that the plants were in darkness during the hours 

 of most intense sunlight — namely, from 10 a. m. to 2 p. m. Obviously 

 it is not merely the total number of hours of sunshine received daily by 

 the plant that may induce such marked shortening of the vegetative period, 

 but the continuity of the exposure also plays an important part. The 

 two plantings of soybeans serving as controls, which first appeared above 

 ground on May 17 and June 16, respectively, did not respond in the same 

 manner to the prevailing seasonal conditions. The vegetative period of 

 the Mandarin was lengthened by two days as a result of the later planting, 

 while the later maturing varieties were affected in the reverse manner. 

 These results are in accord with the fact that the average length of day 

 during the vegetative period was longer for the later planting than for the 

 earlier in the case of the Mandarin, while the reverse is true of the other 

 varieties. The marked action of a decrease in the length of the day, 

 within certain limits, in hastening the arrival of the blossoming stage is 

 equally in evidence throughout the stages of seed formation and matura- 

 tion. This fact is shown by numerous tests; but experiments (a) (i) and 

 (b) (3) with the Mandarin and Peking varieties, respectively, may be 

 cited specifically. 



These tests under controlled conditions clearly show that so far as 

 concerns sexual reproduction the Mandarin soybean is adapted to a 

 relatively long day, since the time required by it to reach the blossoming 

 stage during the long summer days can not be greatly reduced by short- 

 ening the length of the daily exposure to light. On the other hand, the 

 Biloxi is distinctively a "short day" variety; and with a daily light 

 exposure of 12 hours or less it blossoms almost as early as the Mandarin, 

 whereas the control plantings show that it refuses to blossom during the 

 long summer days when normally exposed to the light. It is interesting 

 to note that, on the basis of these results, all of the four varieties tested 

 should behave similarly when grown under a 12-hour day such as pre- 

 vails at the equator. The action of the shortened period of daily light 

 exposure in promoting sexual reproduction offers a satisfactory explana- 

 tion of the fact that there is a marked progressive shortening of the 

 vegetative period in successive plantings of medium and late maturing 

 varieties of soybeans made during the summer months. In this con- 

 nection an examination of figure i , showing graphically this progressive 

 shortening in the vegetative period, is of interest. It should be pointed 



