Plasmalemma and surface electrical biopotentials have been measured in Spinacia oleracea, Phaseolus vulgaris and Chenopodium murale. Both ultradian and circadian oscillations were observed. The structure of the light off induced electrical perturbation seems to be under the control of circadian property of the plasmalemma and could be enslaved by the light/dark photoperiodic cycle. The connection with the flowering process is hypothesized as a model for a coded frequency communication between leaves and apices (flowering evocation).