Oliver Sacks at the New York Review of Books:
We all distinguish between plants and animals. We understand that plants, in general, are immobile, rooted in the ground; they spread their green leaves to the heavens and feed on sunlight and soil. We understand that animals, in contrast, are mobile, moving from place to place, foraging or hunting for food; they have easily recognized behaviors of various sorts. Plants and animals have evolved along two profoundly different paths (fungi have yet another), and they are wholly different in their forms and modes of life.
And yet, Darwin insisted, they were closer than one might think. He wrote a series of botanical books, culminating in The Power of Movement in Plants (1880), just before his book on earthworms. He thought the powers of movement, and especially of detecting and catching prey, in the insectivorous plants so remarkable that, in a letter to the botanist Asa Gray, he referred to Drosera, the sundew, only half-jokingly as not only a wonderful plant but “a most sagacious animal.”
Darwin was reinforced in this notion by the demonstration that insect-eating plants made use of electrical currents to move, just as animals did—that there was “plant electricity” as well as “animal electricity.”