112 THE FLOWERING PROCESS 



phytochrome absorb some light at all wavelengths in the visible 

 spectrum — see Fig. 7-3 and discussion on pigweed below). 



With the presently available evidence, it may not be possible to 

 distinguish between the two theories. Mohr has recently found an 

 interesting system, however, which supports his theory. Lettuce 

 seedlings produced in the dark have a straight plumule. Irradiation 

 with a small amount of red light causes the cells on one side to grow 

 more than the cells on the other side, and a hook results. Far-red 

 will reverse the effects of red. Thus the hook forms in response to 

 phytochrome (the much studied bean hook opens in response to 

 phytochrome). After the hook has formed, fairly long irradiation 

 periods with blue or with far-red light at relatively high intensities 

 will straighten the hook by causing the cells inside to elongate. It is 

 probably possible to reconcile these observations with a phytochrome 

 theory, but it is quite easy to understand them according to Mohr's 

 concepts. 



Whatever the final disposition of theories, the basic observations 

 are most interesting and important. Obviously plants in nature are 

 exposed to long periods of high intensity light, containing amounts 

 of blue, red, and far-red determined by time of day, weather, filtering 

 of forest canopy leaves, and perhaps other factors. Thus study of 

 these matters could be extremely important in gaining understanding 

 of physiological plant ecology. Unfortunately, we have no ideas 

 about how all of this might relate to the flowering process, but it 

 seems certain that important relationships will be discovered (see 

 section on "Other Light Responses" below). 



The Kinetics of Light Action in Photoperiodism 



(8, 32, 36) 



For many years before the pigment was extracted, attempts had 

 been made to understand something about the way that it must act 

 in the flowering process. Using whole plants, effects of light inter- 

 ruptions of different intensities, durations and qualities, given at 

 different times are studied. Since time is usually a variable in such 

 experiments, this is a kinetic approach. The determination of action 

 spectra, described above, is a good example. Much of what is known 

 about phytochrome action in flowering is based upon such studies, 

 which will now be discussed. 



