I remember reading somewhere a long time ago that they gave some sort of traceable substance (maybe it was cesium?) to a plant in one spot and several days later it could be detected in plants up to twenty feet away.
First off, I think this is super cool.
Secondly, I can't seem to find the link now. Can someone find it and post it here?
One study used radioactive iodine and found a fungal induced transference of 100m plant to plant, and the receiver plant was a child of the plant which was the test subject, they even had entwined roots and shared the same ectomycorrhizae hyphae.