Vernalization and Photoperiodism — 64 — A Symposium 



sensitive, while the sensitivity of the fully-expanded foliage leaves seems 

 to depend somewhat on the age of the leaf, the youngest being the most 

 sensitive, and the oldest, mature leaves being relatively insensitive (2, 13, 

 26, 32, 34, 37). Individual plants become more sensitive to photoperiodic 

 treatment as they grow older, but it may be that this increase in sensitivity 

 is related to the fact that older plants have a greater number of foliage 

 leaves to receive the stimulus. 



In short-day plants, of course, the leaves respond to exposure to short- 

 day conditions. Evidence is accumulating that an effective short day must 

 contain a photoperiod of a certain minimum intensity and duration of illu- 

 mination followed by a period of complete darkness of a certain minimum 

 duration. Hamner (15) has concluded that an effective short day for 

 Xanthium pennsylvanicmn must include a photoperiod of approximately 

 30 minutes or more (the length required is dependent on light intensity) 

 followed by a dark period of more than 8>4 hours. The above sequence is 

 not reversible with Xanthium; the photoperiod must precede the dark pe- 

 riod. With Biloxi soybean (1, 14, 15) an effective short day also must 

 include a photoperiod of over a certain minimum intensity and duration 

 of illumination (the minimum length is about one hour and the maximum 

 length is about 20 hours) and a period of complete darkness of more than 

 10^ hours. Two or more (usually three or more) of these short days 

 must occur in direct succession if flowering is to result. Moskov (28) 

 with Perilla ocymoides concludes that this plant in order to be stimulated 

 to flower must have uninterrupted dark periods of more than 8 hours and 

 must have light periods of more than three hours. He (29) noted that 

 short-day plants must be exposed to a cyclic alternation of light and dark- 

 ness although he emphasized the importance of the length of the dark pe- 

 riod. Cailahjan (7) also concludes that short-day plants respond to a 

 definite cyclic alternation of light and darkness. Until evidence to the con- 

 trary is forthcoming, it seems desirable at this time to conclude that the 

 specific length and character of both the photoperiod and the dark period 

 determine the results of photoperiodic induction in all short-day plants, and 

 it appears that determinative reactions take place during both phases of the 

 cycle and also that there is an interaction among them (14). 



Long-day plants seem to have no requirements with respect to dark- 

 ness ; the initiation of floral primordia takes place in continuous light as 

 well as in the long day (14, 33). A certain minimum intensity of illumi- 

 nation is apparently necessary in order to stimulate flowering, but if the 

 plant is illuminated continuously, it will flower provided it is intermittently 

 exposed to fairly intense light. Naylor (33) has shown that continuous 

 illumination does not stimulate flowering in beet unless the illumination in- 

 tensity is above 700 foot candles. With dill continuous light increased in 

 effectiveness with increasing intensities up to 300 foot candles. Either of 

 these plants will flower if exposed to natural light during the day and to 

 low intensities of light (less than 5 foot candles) at night. It may be 

 concluded that the only effect which dark periods have on long-day plants 

 is to inhibit or delay flowering. 



It is of interest to compare the responses of long-day with short-day 

 plants with respect to the influence of darkness. With long-day plants, a 



