I’m not sure if this is what he had in mind, but in the book “Science and the Paranormal” (edited by George O. Abell) there is a section devoted to the old claims that plants could respond to music, the loss of a leaf, the death of another plant, etc. The evidence was the result of medical scans taken of the plants. I think that they used “lie detector” devices, attached to the plant’s leaves with electrodes that were spread with conductive contact jelly. They may have used EKGs and EEGs as well, but I’m not sure.
In any event, when you use human test equipment to run tests on plants, which don’t have the same physical features as human beings, you’re going to get weird results. Just because you see something other than a flat line doesn’t mean that the plant is responding. Those apparent “brain waves” or “skin galvanometric response” could simply be (and in many cases apparently was) caused by the drying out of the contact jelly, or changes in humidity, or whatever.
So I can easily believe that hooking up Jello will give you a non-zero signal, but for all I know, it can be your overly-long wires picking up 60 cycle, or the local radio station.