Trees = batteries ?

''Trees seem capable of providing a constant voltage of anywhere between 20 and a few hundred millivolts – way below the 1.5 volts from a standard AA battery and close to the level of background electrical noise in circuits, says Babak Parviz, an electrical engineer at the University of Washington in Seattle. "Normal circuits don't run from very small voltages, so we need ways to convert the small voltages to something that is usable," he says.'' [source]