It means even on the lower levels of life, there is a profound consciousness or awareness that bonds all things together. 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 and a brain. This sentience is observed primarily through changes in the plant's conductivity, as through a polygraph, as pioneered by Cleve Backster. The book also contains a summary of Goethe's theory of plant metamorphosis.
That said, this book is about much more than just plants; it delves quite deeply into such topics as the aura, psychophysics, orgone, radionics, kirlian photography, magnetism/magnetotropism, bioelectrics, dowsing, and the history of science. It was the basis for the 1979 documentary of the same name, with a soundtrack especially recorded by Stevie Wonder.