Mar. I, 1920 



Efject of Length of Day on Plant Growth 



573 



Table III. — Number of days required by soybeans to reach the flowering stage under daily 

 light exposures of different lengths 



Length of .daily exposure. 



Mandarin. 



Date of 

 germina- 

 tion. 



Date of 



transfer 

 to dark 

 house. 



Time 

 from ger- 

 mination 

 to blos- 

 soming. 



Peking. 



Date of 

 germina- 

 tion. 



Date of 

 transfer 

 to dark 

 house. 



Time 

 from ger- 

 mination 

 to blos- 

 soming. 



10 a. m. to 3 p. m., 5 hours 



9 a. m. to 4 p. m., 7 hours 



Do 



Daylight to 10 a. m. and 2 p. m. till dark, 

 Si4 to IT hours. 



6 a. m. to 6 p. m., 12 hours 



Full daylight, 12^4 to 15 hours 



Do 



May 17 

 ...do 



May 20 

 ..do 



Days, 

 a 23 



May 17 



...do 



...do 



June 19 



Jime 16 

 May 17 

 June 16 



June II 

 Control 

 ..do 



16 

 May 17 

 June 36 



May 20 



...do 



June 7 



Control 

 ..do.... 



Days. 



I,ength of daily exposure. 



Tokyo. 



Date of 

 germina- 

 tion. 



Date of 

 transfer 

 to dark 

 house. 



Time 

 from ger- 

 mination 

 to blos- 

 soming. 



Biloxi. 



Date of 

 germina- 

 tion. 



Date of 

 transfer 

 to dark 

 house. 



Time 

 from ger- 

 mination 

 to blos- 

 soming. 



10 a. m. to 3 p. m., 5 hours 



9 a. m. to 4 p. m., 7 hours , 



Do 



Daylight to 10 a. m. and 2 p. ta. till dark, 

 834 to II hours. 



6 a. m. to 6 p. m., 12 hours 



Full daylight, laj^ to 15 hours 



Do 



May 17 



..do 



...do 



June 19 



16 

 May 17 

 June 16 



May 20 



...do 



June 7 

 16 



Control 

 ...do.... 



Days. 



May 17 



..do 



June IS 



t6 



May 17 

 June 16 



May 20 



..do 



June 24 

 16 



Control 

 ..do... 



Days. 



a 27 



26 



aS 



79 



aS 

 no 

 90 



oin those cases in which the plants were placed in the dark house after they had germinated, only the 

 period elapsing after they had been transferred is taken into accoimt, rather than that beginning with 

 the date of germination. 



It is seen that when the daily illumination consists of a single exposure 

 of 12 hours or less, the usual length of the growing period from germina- 

 tion to blossoming is only slightly shortened in the early variety, Man- 

 darin; but the shortening effect is increasingly accentuated as the usual 

 growing period increases, till, in the very late variety, Biloxi, this period 

 is reduced to less than one-fourth that of the control plants grown under 

 full daylight exposure during the summer months. In reality, all varieties 

 become early maturing ones under these conditions, and there is but little 

 difference in the time required by the four varieties to reach the blossom- 

 ing stage. These tests also show that reducing the length of the illumi- 

 nation period below 12 hours has no further effect in shortening the 

 vegetative period, so that apparently there is a certain minimum 

 period of light exposure, reduction of which is without action in hasten- 

 ing the appearance of the flowering stage. These results seem to indi- 

 cate further that for each variety a certain minimum period of time 

 (ordinarily one of vegetative activity) must elapse from the inception 

 of the stimulating action resulting from the reduced light exposure be- 

 fore the flowering stage can be attained. The data in Table III suggest 



