248 CONTROL OF REPRODUCTION 



quantitative one, as first shown by experiments of Hamner with soybean 

 (1940). In this experiment soybean plants were removed from long- 

 day conditions and placed in short-day conditions for varying numbers 

 of days, after which they were returned to long-day conditions. After a 

 further time — 2 weeks in this case — the plants were classified as to 

 number of flowers formed per plant. The results of this experiment are 

 clear-cut. Number of flowers per plant is a linear function of the 

 number (greater than one) of short-day, long-night cycles to which 

 the plants have been subjected. Similar experiments may be done with 

 the short-day plant Xanthium, as has been shown by Lockhart and 

 Hamner (1954) and by Salisbury and Bonner (1955). In the case of 

 Xanthium we use as a measure of the extent and effectiveness of photo- 

 periodic treatment the subsequent rate of development of reproductive 

 buds. Xanthium plants are grown on long day, treated with one, two, 

 or more short-day, long-night cycles, and returned to long day. After a 

 further period of time the plants are dissected and the stage of develop- 

 ment of each bud noted. Rate of development of the reproductive buds 

 of Xanthium is a linear function of the number of short days to which 

 the plant has been subjected. Similarly, rate of development of repro- 

 ductive buds is a linear function of the extent by which the length of 

 the dark period exceeds the critical value, 8.5 hr. 



It would appear then that the substances that promote flowering 

 which are produced in one favorable dark period can be accumulated 

 and added to those produced during the next dark period. The effects 

 of successive cycles of photoperiodic treatment are additive. Thus, 

 although induction of flowering of itself is an all or none phenomenon, 

 the effectiveness of photoperiodic treatment as measured by number 

 of flowers or rate of flower bud development is a modulated response, 

 and this modulation persists over weeks or months. Plants treated with 

 a minimum of photoperiodic induction flower slowly over long periods 

 of time (Nay lor, 1941 ). Plants treated with a greater number of cycles 

 or with dark periods of greater length flower rapidly and vigorously 

 over long periods. 



The facts described above help us to interpret many details con- 

 cerning the photoperiodic behavior of plants. Thus, if we give many 

 successive long dark periods to a short-day plant, these dark periods 

 need be only infinitesimally longer than the critical value in order to 



