SUMMARY 8l 



As regards the effect of C0 2 on the anabolic electric 

 response of Hydrilla, the following experiment demonstrates 

 the depressing effect of long-continued action of an excessive 

 quantity of this gas. The normal positive electric response 

 of Hydrilla in tank-water containing the normal proportion 

 of the gas is shown in the accompanying record (fig. 24, a). 

 Water charged with excess of C0 2 was next introduced 

 into the photo-electric cell, with the result of the gradual 

 abolition of response. 



It will be shown in Chapter XV. that an excess of 

 carbon dioxide also induces a depression of the photosyn- 

 thetic activity of the Hydrilla plant. 



Summary 



The leaf of Mimosa under excitation undergoes a fall ; 

 this excitatory D reaction is detected by a simultaneous 

 electromotive change of galvanometric negativity. The 

 electric response is independent of the mechanical response, 

 and persists even when the motile response is inhibited by 

 external or internal restraint. 



The electric response of leaves on exposure to light is 

 most conveniently obtained by the device of the photo- 

 electric cell in which the two plates are leaves or plants. 



In the electric response of leaves the predominant 

 negative D often masks the positive A. On the cessation of 

 light, A becomes unmasked and is exhibited as a positive 

 after-effect. 



The A effect may also be unmasked by the phenomenon 

 of ' overshooting.' 



The electric response of the actively assimilating 

 Hydrilla plant is positive, indicative of the predominant 

 anabolism A. 



The positive electric response of Hydrilla is appropriately 

 modified under physiological variations. Rise of tempera- 

 ture to an optimum enhances response ; temperature above 

 the optimum induces a depression. 



