Mar. I, 1920 



Effect of Length of Day on Plant Growth 



511 



Table IV .—Length of the vegetative period of tobacco and aster as affected by the length 

 of the daily exposure to light 



a. These controls and the test plants having a vegetative period of S2 to 72 days were in 8-inch pots. 



The composite Mikania scandens L. is of interest as presenting a new 

 type of plants so far as concerns behavior under long-day and short-day 

 conditions. Under short-day conditions which were maintained for 

 nearly 12 months this plant lost its power of blossoming. In other 

 words, the plant became sterile. The early varieties of soybeans and 

 the Connecticut Broadleaf tobacco blossom and fruit freely through the 

 range of seasonal changes in the length of the day which obtains for the 

 latitude of Washington, while the late varieties of soybeans, the giant 

 types of tobacco, and the aster are essentially sterile when under the influ- 

 ence of the long summer days; and Mikania, on the other hand, is sterile 

 during all seasons of the year except summer when long days prevail. 

 It is worth noting that the Mikania was unable to develop flowers during 

 the summer months when kept under the influence of a short daily expo- 

 sure to light, notwithstanding that it had been growing in the greenhouse 

 for several months previously. 



The bean from the Tropics, Phaseolus vulgaris, included in the tests, 

 brings us a step nearer to complete sterility in the latitude of Washington 

 (approximately 39°), for whether it is able to blossom here will depend 

 on the early or late occurrence of killing frost. Evidently it could not 

 blossom very far northward of Washington. Under the influence of a 

 7-hour daily illumination this bean blossomed in 28 days, and one month 

 later some of its seed pods were mature; yet under outdoor conditions 



