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A self-powered plant-health-monitoring sensor powered by a battery that is activated by the sap of a vascular plant has been developed as an application for wireless biosensor networks. The battery consists of a galvanized iron nail inserted into a tree trunk and a stainless-steel electrode planted in the soil nearby. It outputs on the order of 1 μW of power. An intermittent power supply circuit with a dividedpower-line structure, which separates the power line of a storage capacitor while it is discharging, makes it possible to charge the capacitor with an input power of around 1 μW and to supply power to a wireless transmitter. To verify the effectiveness of the circuit scheme, a prototype sensor was fabricated and tested on a potted pachira tree. The sap-activated battery acted as a current source (3 μA) with an open voltage of 1 V, and the sensor had a 10-mF storage capacitor. This setup enabled the sensor to transmit a signal about every 40 minutes over a distance of 5 m.I.