Murneek — SS — Research in Photoperiodism 



necessary for photoperiodic induction, Hamner (1942, 1944) has pre- 

 sented what he calls "a simple hypothesis," for the support of which a cer- 

 tain amount of experimental data have been secured with these plants. 

 Therefore, it seems to be based on inferences which have already the 

 weight of some evidence in its favor. To quote Hamner: "It appears 

 that determinative reactions take place during both the light and dark phases 

 of the cycle and also that there is an interaction among them. For con- 

 venience the changes or conditions which arise owing to exposure to light 

 may be designated as A, those owing to darkness as B, and the possible 

 summation or resultant changes related to A and B may be referred to as 

 C. Thus, an interaction between A and B results in C. Such a postulation 

 necessarily implies a carry-over of the effects produced during the photo- 

 period into a subsequent dark period, or a carry-over of the effects pro- 

 duced during a dark period into a subsequent photoperiod, since such must 

 be the case in order for the interaction to take place. It is assumed that 

 through the medium of C the observable effects, such as differentiation of 

 floral primordia, flower development, and the like are manifested." 



While the symbols A, B and C have been used for convenience to ex- 

 press possible steps in the photoperiodic responses of Biloxi and Xanthium 

 plants, Hamner emphasizes that they may represent a whole series of 

 interlocked reactions involving many substances. The experimental data 

 would seem to suggest that A is a relatively stable substance, which is 

 formed during exposure to light and evidently depleted during the sub- 

 sequent dark period because of reaction with B to form C. During the 

 exposure to darkness B probably increases in amount till a threshold value 

 is reached when an interaction with A takes place. Whatever its nature, 

 B apparently is destroyed by as brief an exposure to light as 1 minute. C 

 appears to be relatively unstable also for the amount formed during one 

 photoperiodic cycle is not usually carried over to another, if an intervening 

 non-photoperiodic cycle is set between them. The action of C evidently 

 is increased by light intensity obtaining after the photoinductive cycle. 

 It should be emphasized that there is no confirmation of this hypothetical 

 scheme. 



Since long-day plants do not seem to require an exposure to darkness, 

 the above hypothesis cannot be applied to them. Neither is it suitable for 

 the so-called "neutral" plants nor applicable to the action of the light period 

 on modification of vegetative organs and other photoperiodic effects. 



The chemical nature of the flowering hormone(s) is unknown. Sev- 

 eral investigators have shown, by grafting experiments, that it is the same 

 whether formed by the action of long or short photoperiods. It has been 

 effectively transmitted from annuals to biennials and interchangeably from 

 short to long or neutral plants and vice versa, in various combinations 

 (Kuijper and Wiersum, 1936; Moshkov, 1937; Melchers, 1937, 1938; 

 Stout, 1945). 



The idea that, instead of a special hormone, plant growth substances, 

 specifically the auxins, in some way may be responsible for initiation of the 

 flowering state has been proposed by some investigators of whom the main 

 representative is Cholodny (1939). The present evidence seems to be 

 largely against it. But some indirect support to this assumption may be 

 presented from the interesting experiments on flower induction in the 



