The cold resistance of woody plants has hitherto been studied mainly from the static viewpoint. The problem has been attacked, in most cases, either by means of chemical analysis of whole ground tissues of hardy and tender plants or by microscopic analysis of the tissue structure. A minor amount of study has been directed towards microchemical determination of the distribution of food reserves within the tissues and the estimation of the seasonal changes in such food reserves. Some attention has also been devoted to measuring the amount of colloidal constituents and their degree of dispersion. Life phenomena are, however, dynamic in character, and one of the most fundamental properties of living matter is its reaction to stimuli, or irritability. The fact of irritability may be demonstrated by the application of the most diverse agents, as anaesthetics, electrical shock, and changes in temperature. One of the most delicate means of showing that stimulation is so produced is by measurement of the changes which occur in respiration as evidenced by the rate of consumption of oxygen or of evolution of carbon dioxide. The present writers conceived the idea that respiration might throw some light on the phenomenon of winter hardiness in apple twigs. The present paper is a report of their findings. It seems best to proceed directly with the description of the experiments, and to mention pertinent literature in connection with the discussion of our results.