286 CONTROL OF REPRODUCTION 



The experiment was remarkably successful, but its success raises 

 other questions for which I do not have answers. First, why does not 

 chrysanthemum, another short-day plant, flower under similar condi- 

 tions? Such a response would be of real use to the growers, but thus 

 far brief irradiations with far red at the beginning of subcritical dark 

 periods have been completely ineffective. Secondly, how does it happen 

 that flowering was also promoted in two long-day plants, henbane and 

 dill {Anethum graveolens L.), tested in the same experiment with the 

 millet? According to their action spectra the flowering of long-day 

 plants on short days is promoted by interrupting the dark periods with 

 red light. Reversal experiments show further that far red given im- 

 mediately after a red interruption incompletely reverses the red action 

 on flowering and stem elongation. But far red given immediately after 

 a long photoperiod promotes these two responses. The reason for these 

 apparently opposite actions of far red is not apparent. One notes, how- 

 ever, that far red had an inhibitory action when it followed a brief 

 period of light and a promotive action when it followed a long one. In 

 the latter case, in which the red light was present continuously for 

 several hours, certain of the flower-promoting reactions presumably 

 escaped from reversible control whereas others, such as stem elonga- 

 tion, were perhaps still subject to control and favored by far-red treat- 

 ment. 



Differences between Long- and Short-Day Plants 



The basis of the differences between long- and short-day plants is 

 still not clear. Action spectra show that the controHing photoreaction 

 in the two is the same even though the responses are opposite. Reversi- 

 bility also occurs in long-day plants just as it does in short-day ones. In 

 long-day plants red promotes flowering and in short-day ones it in- 

 hibits, but in both types far red counteracts the red. Further study of 

 the occurrence of reversibility in long-day plants is needed. The change 

 in the apparent direction of action of far red in millet and henbane sug- 

 gests that flowering depends in some way on two or more light-influ- 

 enced processes that must proceed concurrently or in close succession. 

 If these are not equally influenced by the same light treatment, possi- 

 bly their relative sensitivity to light is in some way responsible for the 



