During sucrose uptake by the cotyledons of castor bean seedlings excised from their endosperms, small amounts of glucose and fructose accumulate in the medium. When these hexoses were supplied separately, it was found that the rate of fructose uptake exceeded that of glucose at concentrations up to 0.5 m. Sucrose uptake exceeded that of both hexoses combined, particularly at concentrations greater than 0.1 m. Only minute amounts of labeled hexoses were recovered from the cotyledons after exposure to glucose-U-14C or fructose-U-14C; sucrose contained virtually all of the 14C in the sugar fraction. The fructose and glucose moieties of the extracted sucrose were equally labeled when glucose-U-14C was supplied, and the glucose moiety contained 41% of the 14C when fructose-U-14C was the substrate.
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