274 CHAP. XXVIII. GENERAL REVIEW 



Photosynthesis in Absence of C0 2 



In summer the temperature at thermal noon is 43 C. 

 or even higher, which is in sharp contrast with the average 

 temperature of 23 C. in spring. The minimum C0 2 -con- 

 centration for initiation of photosynthesis in spring is 

 about 1 mg. per 100 c.c. ; but in the summer the photo- 

 synthetic evolution of oxygen was found to occur in the 

 total absence of C0 2 , as when the plant was immersed 

 in distilled water. High temperature in summer suggests 

 more active catabolism, greater oxidation and produc- 

 tion of organic acids. As a matter of fact it was found 

 that the juice of plants which was neutral in winter and 

 spring was strongly acid in summer, the acids present being 

 malic and oxalic, the latter in very small quantities. 



On supplying the acid Hydrilla with strengths of malic 

 acid solution increasing from 4 to 16 parts in 10,000 parts 

 of water, the photosynthetic curve was found to be in 

 every way similar to that under increasing strengths of 

 C0 2 -solution. In the photosynthesis of plants in an acid 

 condition the organic acids serve as substitutes for C0 2 



(p. 129). 



The results obtained with acid Hydrilla plants offer a 

 satisfactory explanation of the variations in the assimilatory 

 and respiratory quotients found in different plants. In 

 succulent plants the access of both oxygen and carbon 

 dioxide from the atmosphere is restricted ; organic acids are 

 produced in these plants, which, as in the acid Hydrilla, 

 render them less dependent on the supply of carbon dioxide. 



In normal cases the volume of C0 2 absorbed is about 

 equal to the volume of oxygen evolved: the assimilatory 



quotient -5«_= 1. But in acid plants the denominator C0 2 



is less than the normal, and the value of the quotient is 



C0 2 . , 



greater than I. The respiratory quotient -^- is in normal 



