Small pulses of alternating current have been found to be conducted chiefly in the channels of the cell walls of tubers of potato (Solatium tuberosum), the roots and stems of alfalfa (Medicago sativa), and similar plant tissue. Impedance measurements may usefully detect the state of water relations in these channels when the frequency of the measuring A.C. is low (around 60 c/s), when chlori-dized silver probes are used and when the cell-wall resistance is low compared to the cell membranes in series. Activation energies have been calculated for the conductivity of these tissues. Over the temperature and soil moisture ranges investigated, the log impedance has been found to be approximately linear with the reciprocal of these parameters. Impedance changes have also been found to accompany endogenous and other changes in the plant. In particular, pulses of light or heat caused changes of impedance which reflect a change of internal water relations and possibly transport. Suitable light and potassium fertilizer treatments at temperatures around 4 °C enhanced the impedance of alfalfa and its apparent resistance to frost.