Carbon Assimilation. 
231 
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 veins 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, and it is 
more 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 actual 
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 whether carbohydrates are translocated as hexoses or as 
sucrose has little bearing on the question of carbon assimilation 
itself. The inversion of sucrose into hexoses for purpose of 
translocation, Davis, Daish and Sawyer regard as evidence that 
