Most plants developing under very favorable conditions for growth are tender and exhibit little resistance to frost damage. Certain plants, however, are capable of developing the capacity to resist freezing and are then said to be hardened. While exposure to low temperature is the principal natural and artificial agency producing such an effect, apparently any condition unfavorable to growth may contribute to some degree of hardening (5,19,31,32). The work of Rosa (31) and Hooker (16) indicated that the hardiness of plants was associated with an increase of hydrophilic colloids. Newton and Gortner (29) proposed a cryoscopic method for the measurement of such colloids, believing they reflected the amount of water bound by the tissue in such manner as to protect the plant from frost damage. A vast amount of data has accumulated in support of this general view; namely, that colloidal material in the plant may retain water in the liquid state at temperatures well below the normal freezing point and thus protect the plant. The implication here is that hardiness is related to bio-colloids by virtue of their water-retaining capacity and that the water thus retained is held so firmly to colloidal surfaces that it is unavailable for purposes of solution or for ice formation. Such water has been designated as "bound," The assumption of a correlation between bound water and hardiness in plants elevates the subject to an important position in plant physiology. Since the variation in the response of plants to low temperature is a decisive factor in plant distribution and agricultural practices, the r?le that bound water may play in the temperature effect is a subject of considerable interest. The principle involved in the determination of bound water by the cryoscopic or other comparable methods is based upon the thermodynamic properties of the solution comprising the sap. It is a well known fact that any solute has a definite effect upon the freezing point depression, vapor pressure lowering, or osmotic value of pure water ; and the numerical values of these three colligative properties are interrelated through the applicability of the gas laws to solutions. Few, if any, solutes yield the theoretical values implied by the gas laws but non-electrolytes approach the ideal value. In certain instances, notably in the sap of hardened plants, the freezing point depression has been found to be greater than expected. To account for this