EFFECT OF NARCOTICS 79 



lowered by the application of water cooled with ice ; the 

 response is seen to have been very feeble. The tempera- 

 ture was next raised to the optimum, and then above the 



Fig. 22. Effect of Variation of Temperature on Electric Response 



of Hydrilla 



(a) Feeble response at low temperature, enhanced to (b) at opti- 

 mum and depressed to (c) beyond the optimum temperature. 



optimum. This caused at first an increase, and afterwards 

 a diminution, of the amplitude of response (fig. 22). The 

 electric response of a photosynthetic organ thus exhibits 

 the characteristic changes under physiological variation of 

 temperature. 



Effect of Narcotics 



Ether. — In the last chapter we observed the abolition 

 of photosynthetic activity under the continued action of 

 the narcotic ether. The positive electric response of 

 Hydrilla is also found to be abolished after etherisation. 



Carbon dioxide. — Determination of the effect of C0 2 is 

 of much theoretical importance. It is true that a certain 

 amount of carbon dioxide is necessary for photosynthesis, 

 but its presence in excess must induce a general physio- 

 logical depression. I have found growth to be depressed 

 even to the point of arrest by an excess of carbon dioxide 

 The geotropic curvature of stems is not only arrested but 

 even reversed by the action of this gas. The depressing 



