CHAPTER VIII 



CHANGE IN PHOTOSYNTHETIC ACTIVITY UNDER STIMULUS, 

 ANAESTHETICS AND POISONS 



Effect of stimulus, minimal and maximal, on photosynthesis — Effect of 

 chloroform, ether and alcohol — Effect of poison. 



I will in the present chapter describe in detail the varia- 

 tions of photosynthetic activity induced by varying in- 

 tensities of stimulus and by the action of different chemical 

 agents. 



Effect of Stimulus 



I have already referred to the fact that the effect of 

 stimulus, generally speaking, is modified by its intensity. 

 Thus a feeble stimulus enhances the rate of growth, but 

 strong stimulus retards or inhibits it ; the same is true of 

 transpiration by the leaves. 



In investigating the effect of electrical stimulus on photo- 

 synthesis, two contacts were made by means of silver wires 

 insulated except at the points where they touched the tip 

 and the base of the plant. These wires were led out of 

 the plant- vessel through the india-rubber cork, care being 

 taken that the cork did not allow any leakage of air into 

 the plant-vessel. The two electrodes were connected with 

 the secondary of an electric induction-coil, the intensity of 

 the shock being adjusted from the minimum to a maximum 

 by the gradual approach of the primary coil towards the 

 secondary. 



In the following experiments the temperature was 20°C. ; 

 the source of illumination was sky light, the constancy of 

 which was assured by frequent readings of the Electric 

 Photometer. The normal period of bubbling was found to 



