Carbohydrate Transformations in the Leaf. 1 29 



either hexoses or sucrose as the first sugar produced in assimilation 

 is scarcely adequate for discussion. In any case Lundegardh (1914) 

 regards the transformation of sugar into starch and the reverse 

 process as a very complicated one, depending not merely on the 

 concentration of the sugar in the cytoplasm, but also on the quantity 

 of an enzyme, the concentration of which depends on factors at 

 present unknown. 



The evidence that hexoses are in excess of other sugars in the 

 conducting tissue of the plant seems definite enough. From this it 

 is concluded that sucrose is converted into hexose by means of 

 invertase in the conducting cells of the plant, and is translocated as 

 hexose sugars. The percentage of sucrose in the petiole is less than 

 in the midribs of the leaf, from which one could expect a diffusion 

 of sucrose away from the leaf. Such a state of affairs would, of 

 course, be produced if the sucrose were inverted in the manner 

 suggested by Davis, Daish and Sawyer in the vascular bundles of the 

 leaf and stem, and they cite in support of this view the observation of 

 Robertson, Irvine and Dobson (1909) that invertase is abundant in 

 the leaf and stem of the beet, although absent from the root. The 

 migration of sucrose, therefore, from a place of higher concentration 

 in the leaf to a place where its concentration is kept constantly 

 lower by the invertive action can be readily understood. Also it is 

 likely that the simpler monosaccharides would diffuse through 

 the plant more rapidly than the more complex disaccharide. The 

 difficulty arises from the fact that the published analyses of Davis, 

 Daish and Sawyer show generally a higher percentage of hexoses 

 in the petioles than in the leaf veins, and one would therefore expect 

 a flow of hexoses towards the leaf and not away from it. On the 

 other hand there is no definite information as to the 

 concentration of sugars in the actual conducting cells, and as the 

 hexoses are elaborated into sucrose in the root, it would seem that 

 the concentration of hexoses in the cells of the leaf must rise above 

 that in the root, so that diffusion of hexoses towards the root will 

 take place. There is no doubt that the mechanism of translocation 

 is complex, depending probably on differences of enzyme concen- 

 tration and possibly also on permeability changes, of which we are 

 at present not merely ignorant of the causes, but also of the nature. 



But the form in which carbohydrates are translocated has 

 little bearing on the question of carbon assimilation itself. The 

 inversion of sucrose into hexoses for purpose of translocation, 

 is regarded by Davis, Daish and Sawyer as evidence that 



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