AbstractThe nature, seasonal variation and mobilization of reserves inMalus × domesticahave been studied by means of14C, carbohydrate analyses and extractions of xylem sap.Following exposure to14CO2in the autumn, the majority of the14C absorbed is found in the root. During the winter and in particular the spring the amounts of14C in the top and root are reduced to approximately 40 per cent of the autumn values; in the root the amount of dry matter was also considerably reduced. In the tops, most of the14C absorbed was found as methanol (80 %)‐soluble14C which also showed the greatest seasonal reduction; sorbitol, sucrose or glucose in particular are responsible for the decrease in concentration within this fraction. Maximum values for methanol‐insoluble14C were found in March. In the root, the highest values for absolute changes were found for methanolinsoluble14C. Hydrolysis of this fraction showed considerable activity in glucose. In the root there was also considerable activity in a precipitated fraction of the methanol extract.Eluates of xylem sap from apple branches contained primarily sorbitol, the highest concentration of which was found at the beginning of March.For a tree with a dry matter weight of about 300 g, the utilization of reserves from the tree in the spring was calculated to be at least 13 g of dry matter. However, only a minor part (less than 25 per cent) of the latter appears to serve as building material for new gr