Source: http://modernfarmer.com/2014/06/inside-j...

To measure plant water levels continuously, every plant has to be equipped with its own personal water sensor — and that’s exactly what the Cornell team went for. Lakso and Abraham Stroock, associate professor of chemical and biomolecular engineering, as well as Vinay Pagay, a Ph.D student at the time, created an electronic microchip water sensor that can be inserted right into the plant. In building the chip, they used the plants’ very own architecture for water management, basing the design on plant physiology.

Plant leaves have tiny pores filled with water brought up from the roots; that water evaporates through the leaves’ membranes. The chip also has a cavity filled with water and a membrane through which water evaporates. As the plant’s water depletes, so does the sensor’s. When the level dips, the chip sends a signal to a data logger via a wire or a wireless transmitter to be interpreted and stored. The farmer is tipped off that it’s time to water the plants. When crops are quenched, the chip will replenish its moisture, too, and the cycle will begin anew.