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In many types of plant physiological research, accurate determinations of the physico-chemical properties of expressed plant saps are important. In undertaking such determinations, however, the technical methods of collecting and treating the samples and of expressing the saps have not always been judiciously chosen to meet the requirements of the investigation. Such methods have sometimes been followed according to blind prescription with little or no regard for the limitations imposed upon them by the physiology or anatomy of the tissues. The following comments upon the technique of such determinations, and the attempted critical evaluation of the more generally used methods are based upon the experience gained in an extended investigation upon the physical and chemical properties of expressed leaf saps. The discussion in this paper refers primarily, therefore, to leaves and leaf saps. The earliest investigators of the properties of expressed plant tissue fluids carried out their determinations upon saps expressed from living, untreated tissues. Andre (1) appears to have been the first to observe that fractions of sap successively expressed from an untreated leaf sample showed progressively increasing concentrations of solutes as measured by the depression of the freezing point. Dixon and Atkins (2) obtained results similar to those of Andre when they expressed sap from untreated plant tissues. They explained the progressive increase in the osmotic concentrations of successively expressed fractions of sap by assuming that a larger number of cells burst at the higher pressures, thus permitting solutes to escape to which the protoplasmic membranes of the unbroken cells at the low pressures were impermeable. Since this differential expression of sap from untreated tissues makes it practically impossible to express from them representative samples of sap, Dixon and Atkins introduced the method of freezing the leaves in liquid air before subjecting them to pressure. They presented data to show that the successively expressed fractions of sap from a leaf sample frozen in liquid air did not differ materially in osmotic value. They also found that the volume of sap expressed from frozen leaves was larger than could be expressed from unfrozen leaves, and was freer from debris.

SOME CRITICAL COMMENTS ON THE METHODS EMPLOYED IN THE EXPRESSION OF LEAF SAPS