124 CHAP. XVI. PHOTOSYNTHESIS WITHOUT CARBON DIOXIDE 



those of acid plants in summer, we find : (i) that in the 

 former photosynthesis is initiated at a minimum C0 2 -con- 

 centration of I mg. per ioo c.c., while in the latter it occurs 

 in the absence of C0 2 ; and (2) that the point of reversal 

 in spring-specimens is at 12 mg., while in acid specimens 

 it is at the lower value of 9 mg. 



Table XVI. — Showing the Effect of Increasing C0 2 -Concentration 

 on Photosynthesis of Acid Specimens of Hydrilla 



These facts show that in acid plants the organic acids 

 serve as a partial substitute for C0 2 ; consequently the 

 characteristic points in the photosynthetic curve are 

 transposed backwards. 



Electric Response of Acid Plants 



The conclusion arrived at above, that the organic acids 

 present in the plant may, under certain conditions, serve as 

 a substitute for carbonic acid, finds a very interesting 

 confirmation in the results of a different line of investiga- 

 tion on the electric response. It has been shown (p. 78) 

 that the anabolic process of building up which occurs in 

 active photosynthesis has an electrical concomitant in an 

 electric response of galvanometric positivity. As acid 

 plants exhibit photosynthesis even in the absence of an 

 external supply of carbon dioxide, I expected to obtain a 

 marked positive electric response from these plants. This 

 was actually observed in Cicer arietinum, the leaves of 



