STUDIES OF THE HARDINESS OF PLANTS: A MODIFICATION OF THE NEWTON PRESSURE METHOD FOR SMALL SAMPLES

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Abstract
A method is described by means of which small samples of wheat leaves can be tested for hardiness according to the principle outlined by Newton. The modification of the original method consists in absorbing the expressed juice in ashless filter pads and determining the quantity of sap by measurements of electrical conductivity after extraction of the pad and the residual tissue with distilled water. Extraction of the pad, alone, gives an indication of the relative volume of sap expressed. Extraction of the pressed tissue, itself, appears to measure the degree of crushing injury in the press. In each case, extracts of tender varieties have higher electrical conductivities than those from hardier plants. The large varietal differences in extraction after pressing are shown to correlate very well with the differences in extraction of frozen samples, according to the method of Dexter et al. These differences are not due to correspondingly great differences in "soluble minerals" in the samples. Extraction of plants subjected to freezing or pressure served to differentiate the varieties on a hardiness basis, while extraction of plants injured by heat, ether, or grinding did not. It would seem probable that much of freezing injury is due to the mechanical injury of pressure of the ice formed.