94 PHOTOSYNTHESIS 



of organic acids results from the incomplete oxidation of sugars. In 

 the light these acids disappear, partly on account of a greater oxygen- 

 supply due to photosynthesis, and also due to the direct photolysis of 

 the acids as described by Spoehr.''^ There are therefore several compli- 

 cations which arise in determining the photosynthetic quotient. The 

 organic acids (e.g. malic acid in the cacti) break down in the light with 

 the liberation of carbon dioxide. Thus it happens, that, as Richards 

 records, carbon dioxide is given off by the plant when exposed to diffuse 

 light or direct sunlight. The oxygen also varies greatly under these 

 circumstances ; in diffuse light there is an absorption of oxygen while in 

 intense illumination oxygen is emitted. It must also be realized that in 

 the interior of the plant the breaking down of the acids serves as a source 

 of carbon dioxide, which in the light is used in photosynthesis. 

 When this oxygen escapes from the plant it results in a greater vol- 

 ume of oxygen being emitted than carbon dioxide is absorbed, i.e., 



CO2 absorbed , . , • • , , r , , , 



— — < 1 ; m some cases this ratio has been found to be less 



O2 emitted 



than 0.5. With long continued exposure to light the ratio increases and 



slowly approaches unity. These processes have the general significance 



that in plants which are protected against great water loss the gaseous 



exchange is slow and carbon dioxide is dealt with most economically. 



CO2 

 Kostytschew ^^ has brought out the very interesting fact that the — — — 



O2 

 ratio during illumination varies with time. The initial carbon dioxide 



content in his experiments was about 6 per cent, i.e. considerably above 



normal air. He found that during photosynthesis the leaves absorb 



initially a great deal more carbon dioxide than oxygen is given off ; about 



one third the absorbed carbon dioxide is fixed without oxygen emission. 



After a short time these conditions are reversed, more oxygen is given 



COo 

 off than carbon dioxide is absorbed, and finally the — — ratio attains a 



constant value of 1. Kostytschew found that these relations maintain 

 for aquatic plants as well as for leaves of land plants. These observations 

 are found in accord with those of Spoehr and McGee ^^ on the absorptive 

 capacity of leaf material for carbon dioxide. They are especially impor- 

 tant in their bearing on the methods of measuring photosynthetic activity. 

 If this behavior is universal it would follow that photosynthetic activity 

 should be measured by the amount of carbon dioxide absorbed rather 

 than by the amount of oxygen emitted. It is also of direct bearing on 

 the theory of a primary chemical reaction of carbon dioxide with an 

 absorbing substance as the first step in photosynthesis. 



"'Spoehr, Biochcm. Zeit., 57, 95 (1913). 

 • '" Kostvtschew, Ber. hot. Gcs.. 39, 319 (1921). 

 "Spoehr and McGee, Amer. Jour. Bot., 11, 493 (1924). 



