KINETIC ANALYSIS OF PHOTOPERIODISM 459 



We will now proceed with the various experimental evidences for 

 the 8-point basis for classification given in Table II. There have been 

 extensive investigations of the action spectra of all types of red induc- 

 tion responses in higher plants, and they all lead to the conclusion that 

 this is a homogeneous group. Activation is either the same or is 

 triggered by a very similar photoreceptor system. Withrow (1941) 

 suggested that the photomorphogenic seedling reactions which had 

 earlier been observed and studied extensively by Priestley and Ewing 

 (1923) were due to the same photochemical stimulus as the photo- 

 periodic reactions. These conclusions were based upon studies with 

 light filters. More complete investigations with spectrographic technics 

 by Parker et al. (1945, 1950) have greatly extended the range of 

 observation to include both long- and short-day plants, light-induced 

 seed germination (Borthwick et al, 1952) and certain other photo- 

 morphogenic responses. A detailed spectral analysis of the induction 

 of the opening of the hypocotyl hook in bean and the photoinactiva- 

 tion of the process reveals that the induction peak is at 660 m/i. and 

 that the reversal region has two peaks at 710 and 730 m/x. These 

 curves are very similar to those reported by Mohr (1956) for fern 

 spores. We can generally assume that the action spectra of all these 

 processes have the same general characteristics and that these phe- 

 nomena are capable of being photoreversed by the far red. 



Reciprocity also completely fails for both classes of phenomena for 

 all but the shortest of exposures. The efficiency of any given quantity 

 of radiant energy in inducing various growth reactions is greatest 

 when applied continuously over a long period of time. Any form of 

 intermittent irradiation is less effective. This is clearly demonstrated in 

 Fig. 8 for the hook-opening photomorphogenic response. In the solid 

 line is plotted the angle of opening for various durations of the irradia- 

 tion period, the intensity being inversely proportional to time. The 

 total energy in each case was 250 mj'/cm^ It will be noted that as the 

 time of exposure increases up to a total of the 20-hr period of the 

 experiment, the effectiveness of the constant value of energy increases. 

 If now the angle of hook opening is extrapolated over the log-response 

 curve to give the equivalent energy required for a 20-hr exposure, the 

 dashed curve is obtained. This curve is directly related to the photo- 

 chemical effectiveness of the applied energy. It shows that the relative 



