Published in 1973, The Secret Life of Plants was written by Peter Tompkins and Christopher Bird. It is described as "A fascinating account of the physical, emotional, and spiritual relations between plants and man."
Essentially, the subject of the book is the idea that plants may be sentient, despite their lack of a nervous system. This sentience is observed primarily through changes in the plant's conductivity, as through a polygraph.
ISBN 0060915870