90 PHOTOSYNTHESIS 



higher to a lower concentration, the leaf shows a primary increased rate 

 of carbon dioxide emission. Conversely, when the carbon dioxide-content 

 of the air changes from a lower to a higher concentration the leaf shows 

 a reduced rate of carbon dioxide emission. In the method just described, 

 the leaf in the vessel containing barium hydroxide would therefore emit 

 carbon dioxide at a higher rate than the leaf in the vessel containing only 

 water. This would result in an apparently higher rate of respiration than 

 is actually the case in the vessel containing a higher concentration of carbon 

 dioxide. There are a number of modifications of this method, but the 

 principle and errors involved are the same. 



4— Comparison of leaves with different chlorophyll-content. Hereby 

 the gaseous interchange of leaves of the same plant which are unequally 

 green is measured, on the assumption that the two varieties dififer only as 

 to photosynthetic rates. One difficulty in this method is to obtain leaves of 

 the same age and yet dififering in their chlorophyll-content. Willstatter 

 and StoU *' have shown that there is no direct proportionality between 

 chlorophyll-content and photosynthetic activity. Also Plester ** has shown 

 that the light-green or aurea varieties, with low chlorophyll-content have a 

 low respiratory activity as compared with the normal varieties. 



By means of these methods Bonnier and Mangin determined the photo- 

 synthetic quotient of a large number of plants. Following their notation, 



'- the values obtained for the photosynthetic quotient were usually 



vol. CO2, 



greater than 1, and ranged between 1.1 and 1.3. 



In the 40 years since the publication of the work of Bonnier and 

 Mangin a number of publications have appeared on the subject of the 

 photosynthetic quotient notably by H. Jumelle ^^ and Th. Schloessing 

 fils.^° With the exception of Willstatter and Stoll, whose work will be 

 considered later, no contributions of note have been made to the problem 

 of measuring separately the respiratory and photosynthetic activities. 

 Maquenne and Demoussy,^^ employing essentially the first method of 

 Bonnier and Mangin, made an extensive investigation of the quotients 

 of gas interchange. Their results of the respiratory and photosynthetic 

 quotients are given in Table 16. They used a closed chamber and the 

 composition of the gas was determined after exposure to light and darkness. 



In 29 of the 34 plants studied the photosynthetic quotient was equal 

 to or very slightly less than the respiratory quotient. After making cer- 

 tain corrections Maquenne and Demoussy conclude that the volume of 

 oxygen emitted is equal to the volume of carbon dioxide decomposed. 



"Willstatter and Stoll, Untcrsuchuiigcn ucber die Kohlensaeiireassilation, p. 86. 

 Berlin (1918). 



** Plester, Bcitragc zur Biol. d. PHanzcn. 11, 249 (1912). 



"Jumelle, Compt. rend., 112, 888 (1891); 113, 920 (1891); Rev. gen. hot., 4, 

 49 (1892). 



"Schloessing, Compt. rend., 115, 881, 1017 (1892) ; 117, 756, 813 (1893). 



" Maquenne and Demoussy, Echanges Gaseux des Plantes Vertes avec fAtmos- 

 sphere. Paris, 1913. 



