PRODUCTS OF CARBON ASSIMILATION 37 



formed in the leaves in any appreciable quantity. The amount 

 of sugar increases towards the base of a leaf and concurrently 

 the ratio of the sucrose to the hexoses diminishes. The sucrose 

 increases rapidly in amount after sunrise, reaches a maximum 

 and diminishes during the night, the percentage amounts of 

 the hexoses of the leaf, however, remain fairly constant. As 

 the season advances, the proportion of sucrose to the hexoses 

 decreases, the latter being formed at the expense of the former. 

 Parkin confirms Brown and Morris in the observation that the 

 levulose is, as a rule, more abundant than the dextrose and 

 supports the contention that sucrose is the first recognizable 

 sugar in carbon assimilation, the hexoses- being formed from 

 it by inversion. The reason for levulose being more abundant 

 than dextrose is that the latter is more used by the leaf in its 

 respiratory processes. 



Davis, Daish, and Sawyer,* in the instances of the potato 

 and the mangold, agree that sucrose is a primary sugar of 

 carbon assimilation and from it the hexoses arise. The cane 

 sugar and hexose are at their maximum at mid-day and the 

 amounts increase with the season. Unlike Brown and Morris 

 they found pentose to be present whilst maltose was consist- 

 ently absent, both by day and by night, from the leaves men- 

 tioned. In view of the presence of starch in the potato leaf, 

 it is surprising that the presence of maltose was not estab- 

 lished : there is good reason for supposing that maltase is 

 commonly present in plants in which starch occurs and is 

 digested : this enzyme is rather indiffusible, is destroyed by 

 alcohol and chloroform under ordinary conditions, and is 

 very susceptible to heat, a temperature of 50 C. having a 

 marked adverse effect upon its activity, whilst at 55 C. it may 

 be destroyed. It is these properties of maltase which account 

 for the varying statements regarding the occurrence of mal- 

 tose in green leaves : if the procedure of experiment is such 

 as to destroy the maltase, maltose will be found ; if, on the 

 other hand, the enzyme is not destroyed, the maltose is con- 

 verted into hexose sugars. f 



Gast I examined the carbohydrate content of the leaves of 



* Davis, Daish, and Sawyer : " Journ. Agric. Sci.," 1916, 7, 225. 

 f Davis: " Biochem. Journ.," 1916, 10, 31, 49, 56. 

 JGast: " Zeitsch. Physiol. Chem.," 1917,99, i. 



