Researchers at the University of Washington have developed a method of powering simple circuits using electricity generated solely from living trees. From the article:
A study last year from the Massachusetts Institute of Technology found that plants generate a voltage of up to 200 millivolts when one electrode is placed in a plant and the other in the surrounding soil. Those researchers have since started a company developing forest sensors that exploit this new power source.
The UW team sought to further academic research in the field of tree power by building circuits to run off that energy. They successfully ran a circuit solely off tree power for the first time.
Apparently, the energy being generated by the trees is far to minute to be useful as a usable source of energy for most applications, but it can be used as a low cost method of powering sensors for forest management. I just hope that this will lead to someday being able to plug my TV into a tree so camping trips could hold my interest.