36 • Photoperiodism: Attempts at Analysis 



Uninterrupted controls had a mean flowering stage of 6.0; 10 

 seconds red gave a value of about 4.8, 20 seconds brought it to 

 about 2.5, and 30 seconds, to 0. One minute of sun-source far-red 

 was sufficient to reverse the effects of two minutes of red if given 

 immediately after, returning the value to 6, but twelve minutes of 

 far-red brought it down again to nearly 4; such "overreversals," in 

 which long exposures to far-red act more like red, occur in other 

 plants as well, and will be discussed later. 



Downs next studied the effect of interposing a brief period be- 

 tween the red and far-red treatments. In one experiment, far-red 

 immediately after red gave a value of 6.5 compared with the un- 

 interrupted controls of 7.0. With a 20-minute dark period before 

 the same far-red treatment, the value was only 3.8, and with a 

 40-minute dark period, 0.5. Thus the far-red treatment had to be 

 given soon after the red to be effective; the simplest explanation is 

 that when most of the pigment is in the far-red-absorbing form 

 (after the red), a series of reactions inhibitory to induction is started 

 and reaches such a stage after 40 minutes that even changing the 

 pigment will no longer change the result. If the plants are held at 

 5° C during the intervening dark period, this "escape from photo- 

 chemical control" occurs much more slowly. With a 40-minute 

 dark period, for example, the red effect was still almost completely 

 reversible at this temperature, precisely as would be expected under 

 the explanation given. The escape from photochemical control also 

 explains w T hy, under ordinary conditions, repeated reversals cannot 

 be carried on indefinitely and the red effect eventually predomi- 

 nates. 



Downs's results typify the kind of control exerted by the red, 

 far-red system in photoperiodism, but by no means exhaust the 

 subject. Evidence was obtained, first in lettuce seed (Borthwick 

 et al., 1952a) and later elsewhere, that the conversion from the 

 far-red-absorbing to the red-absorbing form takes place not only on 

 exposure to far-red but also, more slowly, in darkness by some 

 thermal (temperature-dependent) process. This revises the relation 

 previously written to: 



red 



far-red P F . 



^dark, thermal"" 



