tinuous water columns in the xylem, and experimental evidence that cohesive tensions of sufficient magnitude may develop in tall trees and lianas in a period of transpiration, has been furnished by experiments of potometer type (14, 21, 22, 23, 24, 25). One may assume that leaf shedding does not per se break any such tension, as it has been shown that the walls of the vessels and tracheids will not let through a gas-water interface (29). With exception of the grapevine, and probably some other species which have air-filled vessels in the winter (28), it is known that the water content at all levels in the xylem of several species of trees remains as high or nearly as high in winter as in summer (7, 9, 15, 16, 30). When water freezes, dissolved gas is forced out as bubbles. This happens whether water freezes in bulk or in glass capillaries (27). It has been shown that the sap of the grapevine is fully saturated with dissolved atmospheric nitrogen (28), and the same very likely holds also for other species. One must, therefore, assume that gas bubbles are formed whenever freezing takes place in sap-filled vessels and tracheids, and a great many of these would hence become emptied and eliminated for the transport system, were the sap under tension at the time of thawing. The only mechanism which might conceivably repair such breaks, would be a sap pressure high enough to redissolve the gas. In the case of the grapevine, which empties its vessels during the winter, the root pressure in the springtime is so high that it may readily refill the conducting system (28). But such pressures are at present known only in a few hardwood species (3, 8, 13, 17, 32). Sub-zero temperatures with freezing of the sap, therefore, are difficult to fit into the cohesion theory, but this difficulty will not arise if the sap remains supercooled throughout the winter. A few degrees of supercooling has been produced experimentally in different species of trees, but this was observed for a brief period only (20). In the present investigation the status of the sap has been studied in various species of trees at subzero temperatures. Materials and Methods