Amongst a host of environmental stimuli plants' roots sense are gravity, light and sound. That's right, new research points to plants being able to hear water inside pipes and move towards it. And don't even get me started on how they communicate with each other over long distances. Even Darwin was enamored of them, writing on the concept of plant intelligence and had no problem using the "b" word.
“It is hardly an exaggeration to say that the tip of the radicle . . . having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense organs and directing the several movements.” -Darwin, "The Power of Moving Plants", 1880,
I remember a short story about a race of aliens living in a radically sped-up dimension of time arrive on Earth and, unable to detect any movement in humans, come to the logical conclusion that we are “inert material” with which they may do as they please. The aliens proceed ruthlessly to exploit us. - Stefano Mancuso, plant molecular biologist