COMPARISON OF THE HEATING AND FREEZING METHODS OF KILLING PLANT MATERIAL FOR CRYOSCOPIC DETERMINATIONS

In a study of the changes in the sap concentration of Larrea tridentata reported by the writer (1), the leaf and twig tissues were killed by heating them in closed vessels in boiling water for a period of 30 minutes before the sap was extracted for the cryoscopic determinations. This is a new departure in cryoscopic studies in this country, where plant tissues have been killed by subjecting them to freezing temperatures. The heating method, however, has been used extensively by Walter (3). The method is described in detail in the paper just cited (1). On theoretical grounds the heating method of rendering plant tissues permeable to the cell contents is open to criticism. Both enzymatic changes and other chemical reactions may be speeded up two to three times for every 10° C. rise in temperature; that is, the hydrolysis of sugars and other cell contents may occur which would change the osmotic concentration of the cell sap from its natural condition. In order to establish the practical importance of such changes in relation to a determination of the Ov of any particular species, comparative studies were made between the heating and freezing methods. The purpose of this paper is to report the results of these experiments. In cooperation with Dr. Matlock and Mr. Hobart of the agronomy department of the University of Arizona, a comparative study was conducted using field cotton. This experiment also served the purpose of comparing results obtained by the use of a Beckmann thermometer with those found when a Drucker-Burian thermometer was employed. The material used for this experiment was cotton grown on experimental plots on the university farm near Tucson. Two sets of leaf samples were collected simultaneously from each of two plots. Each plot was divided roughly into four sections, each set of samples consisting of one sample from each section and each sample containing about twelve leaves picked at random from plants within a given section. One set of samples from each plot was collected by Matlock and Hobart, each collecting two samples, while the other two sets were collected by the writer. One set of samples from each plot was frozen in an ice-salt mixture and one set from each plot was heated in boiling water for 30 minutes. All of the osmotic value determinations for plot no. 7 were made by Matlock and Hobart, using a Beckmann thermometer, and they also made the determinations for the set of samples which they collected on plot no. 6. The osmotic values for the other set of samples