THE MOLECULAR FACTOR 91 



at an intermission of 10 seconds than at one of 5 seconds. 

 As already said, the processes of absorption and respiration 

 are relatively slow, hence short intermission such as that 

 of 3 seconds would produce little or no retardation. 



(2) As regards the molecular factor, the physiological 

 momentum may be expected to last for only a very short 

 time, after which the molecular strain will begin to dis- 

 appear, and the photosynthetic efficiency will decrease 

 with the duration of the intermittent darkness. Thus, to 

 take a concrete example : 



If we expose the plant for, say, 160 seconds to (a) con- 

 tinuous and (b) intermittent light, the total duration of 

 exposure in the latter case will be 80 seconds. If the number 

 of bubbles given out under continuous light (zero inter- 

 mission) for 80 seconds be N, and under intermittent light 

 of the same total duration be N', then— 



Photosynthetic rate per hour under continuous light will be 



A r ■-= — x 60 X 60 



80 



Photosynthetic rate per hour under intermittent light will be 



N' 

 Ay = o- x 6o x 60 



1 80 



If now it be found that N' = N, it follows that photosyn- 

 thetic efficiency is not affected by intermission ; but if it be 

 found that N' < N, then clearly intermission has caused a 

 decline of activity which becomes more marked the longer 

 the periods of intermission, and may therefore be attributed 

 to molecular recovery. 



The retardation induced by increasing periods of inter- 

 mittent darkness will not, however, increase indefinitely. 

 For increase in the period of intermission of darkness and 

 light would also mean exposure to the action of light for 

 longer periods, which would be more effective. Hence 

 we would expect a turning-point in the curve which 

 represents photosynthetic efficiency at various rates of 



