CHAPTER XI 



PHENOMENON OF PHOTOS YNTHETIC INDUCTION 



The relation between the quantity of light and the amount of photosynthesis 

 under continuous light — ' Uphill ' or positive work — Cessation of 

 photosynthesis on stoppage of light — Molecular strain under photic 

 stress — Partial undoing of positive work on cessation of light — In- 

 duction-period and its prolongation under increasing periods of previous 

 darkness — The cyclic curve — Effect of physiological inertia. 



The rate of photosynthetic evolution of oxygen is uniform 

 under constant intensity of light ; I have shown in a previous 

 chapter that the amount of photosynthesis is proportional 

 to the quantity of incident light. This law of photo- 

 synthetic action is strictly applicable under the action of 

 continuous light. The question now arises as to whether 

 this quantitative relation holds good in the case of inter- 

 mittent light. This would appear to be hardly probable 

 from the theoretical considerations given below. 



Photosynthesis is, as we have seen, a process of building 

 up, the work being ' uphill,' the evolution of oxygen coming 

 to a stop after the cessation of light. We may compare this 

 with the rolling of a stone uphill by a continuous push, the 

 stoppage of which is followed by a cessation of movement. 



But the stoppage of uphill movement on the cessation 

 of the push is temporary, for the stone may roll ' downhill,' 

 thus undoing the positive work that had been accomplished. 

 In photosynthesis likewise there is a possibility of positive 

 work being partially undone during the cessation of light. 

 The photosynthetic production of carbohydrate is brought 

 about by a series of chemical dissociations and combina- 

 tions. In the antecedent dissociation the molecules have 

 to be put in a state of strain as a preliminary to rup- 

 ture. We may visualise the process by observing the 



