THE YIELD OF ORGANIC MATTER 



37 



content of sunflower leaves kept in the dark, and that of similar leaves 

 which had been exposed to light for 3-5 hours, and compared the amounts 

 of synthesized carbohydrates with the quantities of carbon dioxide con- 

 sumed by the illuminated leaves, he found "carbohydrate deficiencies" 

 of 5-35%. Similarly, Krasheninnikov (1901) was able to identify as 

 carbohydrates only 50-75% of the dry matter synthesized by the 

 leaves of bamboo, cherry laurel, sugar cane, linden and tobacco. These 

 results could be taken as indications of a rapid transformation of the 

 primary product of photosynthesis into compounds other than carbo- 

 hydrates; this conclusion was supported by the observation of Ruben, 

 Hassid and Kamen (1939) who found that, after one hour of photo- 

 synthesis by barley leaves in radioactive carbon dioxide, only 25% of 

 the assimilated radioactive carbon could be recovered in water-soluble 

 carbohydrates, and not more than 10% in insoluble material (cellulose). 

 Smith (1943), on the other hand, was able to recover, in the form of 

 carbohydrates, practically all carbon assimilated by sunflower leaves in 

 illumination periods of 1-3 hours (c/. Table 3. III). According to this 



Table 3.III 



Determination of Carbohydrates in the Photosynthate 



OF Sunflower Leaves (after Smith) 



" Assumed to have the elementary composition of cellulose. 



table, more sucrose and less monosaccharides (and less starch) are ob- 

 tained at 10° C. than at 20°. The proportion of monosaccharides may 

 rise to as much as 35% if several hours of respiration in the dark are 

 allowed to pass between illumination and analysis. (This amylolysis in 

 the dark may be caused by water deficiency, c/. page 333). 



The difference between the observations of Smith and those of the 

 earlier investigators may be attributed to improved methods of analysis. 

 However, the results may also depend on the plant species used and on 

 the conditions of the experiment (duration and intensity of illumination, 

 temperature, etc.). An explanation remains to be found for the failure 

 of Ruben and coworkers to recover more than one quarter of assimilated 

 radioactive carbon in the carbohydrates photosynthesized by barley 

 leaves. 



