IntroductionThe resistance of plant tissues to low temperatures and the causes thereof have been the subject of extensive research by many investigators. Since there have been comprehensive reviews of both phases of the subject written by CHANDLER (4, 5), ROSA (22), AKERMAN (1), MAXIMOV (15), LEVITT and SCARTH (13), and SCARTH and LEVITT (23), this paper refers only to those investigations which are specifically related to the immediate problem.In all but very few of the previous studies of ice formation in living tissue, the material was frozen by various means, and then after thawing, was examined either grossly or microscopically. What had occurred during freezing was postulated from the appearance of the thawed tissue. Of the few investigators who observed freezing directly, apparently MOLISCH (17) was the first to see ice formation as it occurs in living cells. His microscope was put into a specially constructed ice cliest with openings to give access to the controls. The apparatus was inconvenient to use, and the temperature could not be regulated, but MOLISCH was able to use it to watch freezing in Amoebae and in several plant tissues.WIEGAND (26) investigated the condition of buds during subzero weather. He used a microscope and sectioning apparatus out-of-doors and noted where ice was formed in the buds. Ice was found as a single continuous layer always in the mesophyll of the leaf or scale, never between the scales. The cells on either side were in a state of collapse and often occupied less space than the ice sheet. Ice formation was observed in som-e species at -180 C.,