THE NATURE OE PHOTOSYNTHESIS 119 



tion and not simply on the basis of equal time. With high illumination 

 intensity equal quantities of radiant energy reduced more carbon dioxide 

 when the light was intermittent than when it was continuous. The ex- 

 cess of photosynthesis with intermittent illumination was about 100 per 

 cent when the alternations of the rotating sector were 8000 per minute, 

 and only al)out 10 per cent when the alternations were 4 per minute. 

 With low light intensity there is no difference between intermittent and 

 continuous illumination. It is important that in a period of time which 

 is long in comparison with the length of time occupied by a single flash 

 of light in the intermittent illumination, the amount of carbon dioxide 

 reduced is the same whether the light is continuous or intermittent. 



There are two possible explanations for the effect of intermittent 

 illumination. Either photosynthesis continues at an undiminished rate 

 during the periods of darkness, which would be in accord with an older 

 conception of Tswett,^"^ or photosynthesis is interrupted during the 

 periods of darkness and is doubled during the periods of illumination. 

 It would seem that the latter is the more probable explanation. Thus, 

 during the dark periods carbon dioxide would have an opportunity to 

 enter the centers of photosynthetic activity and synthesized material move 

 away, both of which would tend to increase the rate of photosynthesis. 

 When the chloroplast is then again illuminated there are available higher 

 concentrations of dissociable material. With light of low intensity, in 

 which there is little difference between equal periods of illumination of 

 the continuous and intermittent type, the reduction of the concentration 

 of carbon dioxide in a given length of time of illumination is less ; the rate 

 of replacement is therefore also less and the influence of the periods of 

 darkness is less pronounced. This is virtually the same explanation 

 that Willstatter and Stoll "^ give for the results of Brown and 

 Escombe ^°^ with intermittent light. 



c. Partial Pressure of Carbon Dioxide. 



The early observations on the influence of various concentrations of 

 carbon dioxide were naturally of a qualitative nature.^"* Thus it has 

 been known for a long time that leaves which ordinarily produce only 

 soluble carbohydrates but no starch can be induced to form the latter 

 by exposing them to an atmosphere enriched in carbon dioxide. More- 

 over Kreusler ^°^ and others clearly demonstrated that an increase in the 

 partial pressure of the carbon dioxide resulted in increased photosynthetic 

 activity. The object of these investigators was to establish the optimal 



101 ■ 

 102 



'Tswett, Zcit. phxsik. Chcm., 76, 413 (1911). 



'Willstatter and Stoll, "Untersuchungen ii. die Ass. der Kohlens," Berlin, 1918, 

 p. 240. 



'"'Brown and Escombe, Proc. Roy. Soc. London, 76 B, 86 (1905). 

 •" Godlewski, Flora, 56, 378 (1873) ; 60, 215 (1877). Schimper, Bot. Zeitg., 43, 

 737 riS8S) 



"'Kreusler, Landiv. Jahrb.. 14, 951 (1885). 



