I wonder, however, whether the inspiration for both of these crackpots came from a piece of fiction in the September 17, 1949 issue of The New Yorker--Roald Dahl's "The Sound Machine," which is reprinted in numerous short story collections, including his volume Someone Like You (1973). In this tale, a man named Klausner, obsessed with sounds beyond the ability of human beings to hear, builds a machine to convert higher pitches into human-audible sounds. He discovers, to his horror, that plants and trees shriek with pain when cut.
Does anyone know of any documented references from Hubbard or Backster to Dahl? Or is there another common ancestor I've missed?
My title includes a reference to the Seattle-area grunge band, Screaming Trees, whose Wikipedia entry doesn't comment on the origin of their name--but Dahl's story seems a likely inspiration there, too.
UPDATE (6 February 2013): It looks like the Hubbard photo pre-dates Backster, and was likely taken in 1959 or 1960! It prompted a feature titled "PLANTS DO WORRY AND FEEL PAIN." in the December 18, 1959 Garden News.
UPDATE (10 February 2013): David Hambling's "The Secret Life of Plants" in the December 2012 issue of Fortean Times (p. 18) points out that Charles Darwin's 1880 The Power of Movement in Plants suggested that plants have something like a nervous system, and that Jagadish Chandra Bose published a 1907 paper on the electrophysiology of plants. He puts Backster before Hubbard, making the same mistake of dating Hubbard's claims by the Life magazine photo caption.
Backster, by the way, was inspired by Bose's work. He says that he started his work with plants on February 2, 1966, as reported in the introduction of his "Evidence of a Primary Perception in Plant Life," International Journal of Parapsychology, Vol. X, No. 4, Winter 1968, pp. 329-348.