92 



PHOTOSYNTHESIS 



of photosynthesis. Of course the differences in the composition of the 

 gas to be analyzed are very much smaller than in the closed chamber 

 method, but these differences can be determined with modern methods of 

 gas analysis. For this purpose Willstatter and Stoll employed conditions 

 of maximal photosynthetic activity, so that respiration was 1/20 to 1/30 

 of photosynthesis. Thus slight variations in respiration, as occasioned, 

 for instance, by the differences in respiration in light and in the dark, had 

 but an insignificant effect on the determination of the photosynthetic 

 quotient. Carbon dioxide-concentrations of 5-6.5 volume per cent and 

 light intensity of about 45,000 lux were used. Under these conditions the 



photosynthetic quotient —^ at 10 to 35° was found to be constant and 



exactly 1. This applied to a variety of plants including Ilex aquifolimn, 

 Samhucus niger, Pelargonium sonale, and Aesculus hippocastanum. Even 

 under conditions of photosynthetic inhibition through the accumulation 

 of products (see below) the value of 1 was maintained. 



The importance which attaches to the value of the photosynthetic 

 quotient can be readily seen from the following considerations. Just as 

 from the value of the respiratory quotient definite information can be 

 gained as to the nature of the material which is oxidized, the photosynthetic 

 quotient indicates the nature of the material formed in the decomposition 

 of carbon dioxide. It has been found that the respiratory quotient 



vol. COo emitted , . , ,. r i/^/^ r i /^ 



— of annuals on a diet of 100 per cent fat and per cent 



vol. O2 absorbed, 



carbohydrate is 0.707, and when on a diet of i>er cent fat and 100 per 



cent carbohydrate is 1.00. Quotients intermediary between 0.707 and 



1.00 indicate that mixtures of the two materials are being used. This 



principle has become one of the most useful tools in the hands of the 



animal physiologist and has been elaborated to include a large variety of 



substances and conditions. 



o 



Similarly the photosynthetic quotient may yield some information 

 relative to the first product formed in photosynthesis. Between carbonic 

 acid and carbohydrates there are a number of possible reduction products. 

 A great deal of speculation has centered about the question as to which 

 is the first substance formed in the process of photosynthesis. In the 



