PRODUCTS OF CARBON ASSIMILATION 51 



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 formed in the leaves in any appreciable quantity. The 

 amount of sugar increases towards the base of a leaf and con- 

 currently 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. It has been suggested 

 that the reason for levulose being more abundant than dex- 

 trose is that the latter is more used by the leaf in its respiratory 

 processes, the fructose being more suitable for tissue formation. 

 The observations of Robbins f and of Brannon J however, 

 indicate that fructose is not better than glucose for tissue for- 

 mation. In water culture it was found that glucose was the 

 better, whilst fructose may have a temporary toxic effect, 

 especially in the absence of glucose. 



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 



* Parkin : " Biochem. Journ.," 1912, <>» 1. 



t Robbins: "Bot. Gaz.," 1922, 73, 376. 



% Brannon : id., 1923. 75> 37°- 



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



4* 



