POTATO juice was shown by Hanes1 to contain a phosphorylase (P-enzyme) by the agency of which the long, largely unbranched chains of the amylose component of starch were built up from glucose-1-phosphate. The main component of potato starch is, however, the branched-chain component (amylopectin), and this is not synthesized by P-enzyme acting alone on glucose-l-phosphate. An investigation instituted by Haworth, Peat and Bourne2 of the enzyme system of the potato resulted in the isolation of a second enzyme (Q-enzyme) which, in conjunction with P-enzyme, effected the synthesis of amylopectin from glucose-l-phosphate3,4. The cross-linking enzyme (Q-enzyme) possesses a dual function. In addition to its synthetic activity, it also functions as an amylase in effecting the hydrolytic, as opposed to the phosphorolytic, degradation of amylose4. The synthesis and degradation of starch by the agency of P- and Q-enzymes is schematically represented in the paper by Bourne and Peat3.