Consider how the sunflower turns to the sun. It detects light and adjusts its structure to maximise how much energy it catches. This is an example of plant intelligence. Ok, it should still be an insult to call someone as dumb as a stump. But that stump has (or, more precisely, the living tree before had) some hidden, slow intelligence that, though it may not be obvious, is intelligence nonetheless.
The question that reveals the sense in which plants can be considered intelligent is pretty simple: How do plants choose where they grow? They don't just blindly put down roots and hope for the best. Different plants do this in different ways, but there are clearly some computations going on when a plant decides which way to send roots, stems, leaves, so that resource acquisition is maximized. In doing so the plant computes pros and cons and "decides" which way to go, and it's right in its calculations in surprisingly sophisticated ways. Plants know how to find light, water, and the kind of soil they need.
This is the gist of one of the essays in the recently reviewed book The Deep Structure of Biology. And I'm not sure about the term "intelligence" really. "Computation," sure. But intelligence seems a higher standard. Yet, which gardener among us hasn't felt outsmarted by weeds?
At the very least, talking about plant intelligence should help us define what intelligence is not. If plants have a rudimentary intelligence, it is built around resources (light, water, soil) and not around communication. So don't talk to your plant. Unless it makes you feel better. But do water it.
Without really taking a stand on the prospects for a full-blown "plant intelligence," I'd like to jump to conclusions and draw a heedless parallel or two. I'm reminded of 2 Kings, when Elisha and his servant are in a city under siege by the Samarian Army. The servant is understandbly worried. Elisha tells him to look up, and the servant suddenly sees an army of angels all around, filling the hillside. You can take this story in two wrong ways: you can run with it to try and figure out how angelic armies work, or you can allegorize it and try to explain what it means, even though you believe the angels weren't really there, they just symbolized something. Both of those strategies undermine the passage, but I'm interested in what it says. It says there are unseen intelligences hidden around us. The intelligence of plants is one way (and ONLY one way) in which intelligence might be hidden around us. Clearly, plants are not angels, and if we start eliding the two we're gonna drift into animism. But both are represented as creations, with different levels of intelligence, put here by a creator. I'll have to think a while to unpack what that means. It implies a rich and responsive creation, I'll tell you that.
Maybe an image of what it means would be better than a proposition. Think of the end of Prince Caspian, when the trees come alive and march onto the battlefield. Or the March of the Ents in The Lord of the Rings. I think the writings of Lewis and Tolkien about nature may actually accord somehow with research into plant intelligence. IF and only if it turns out to hold water scientifically. I am no judge of that, having never taken a botany course.
There's nothing conclusive here. It could be that plants are unintelligent, that an apple tree is no more intelligent than an Apple II computer. If so, we're left with the wonder of watching a sunflower turn its face toward the sun, how at the very least that's a wonderful and beautiful process. I'm OK with that too -- the world is rich and responsive as we understand it now. If there's something to be gained from scratching the surface of plants and peering into their inner workings, let's take a look and see.