36 THE SYNTHESIS OF CARBOHYDRATES 



in the dark, but it takes place also by day and it is only in 

 sugar leaves that the undue concentration of -soluble carbo- 

 hydrates, owing to hindrances to rapid translocation, or to a 

 high assimilatory efficiency, is likely to prove an important 

 factor in carbon assimilation. In this respect the observations 

 of Broock * are of considerable interest and value. He found 

 that the leaves of the sugar beet on bright sunny days showed 

 a rapid increase in their dry weight up to mid-day, at which 

 hour there was a sudden decrease: this loss in weight was 

 more or less uniform until about midnight. The sudden drop 

 in the carbon assimilation at noon indicates that the process 

 was brought to a standstill owing, in all probability, to the 

 closure of the stomates resulting from the accumulation of 

 materials which were produced at a greater rate than that at 

 which they could be translocated. 



With regard to the oxygen evolved, since carbon assimila- 

 tion by green leaves is unaffected by various concentrations of 

 oxygen in artificial atmospheres, and since the oxygen pressure 

 of the internal atmosphere of plants is a very variable quantity, 

 it would not appear that the amount of oxygen is a limiting 

 factor of any great significance. 



THE ORGANIC PRODUCTS OF CARBON ASSIMILATION. 



The ultimate products of carbon assimilation are carbo- 

 hydrates : Brown and Morris,f in their classical work on the 

 chemistry and physiology of foliage leaves, identified sucrose, 

 dextrose, levulose, and maltose in the leaves of Tropceolum, 

 the sucrose being in greatest abundance. These results were 

 generally accepted, reinvestigations of the subject being of 

 recent date. Parkin { in his investigation of the sugars of the 

 leaf of Galanthus nivalis, selected for its convenience as a sugar 

 leaf, found that a considerable quantity of sugars, 20 to 30 per 

 cent of the dry weight, occurred in the active leaves ; sucrose, 

 dextrose, and levulose were recognized but maltose was never 

 found, a result which was to be expected, since starch is not 



* Broock: " Uber tagliche und stundliche Assimilation einiger Kultur- 

 pflanzen. Halle: 1892. See Thoday : " Proc. Roy. Soc.," Lond. B., 1910, 82, 



443- 



t Brown and Morris: " Journ. Chem. Soc.," Lond., 1893, 63, 604. 

 J Parkin; "Biochem. Journ.," 1912, 6, i. 



