Michael Pollan has a new piece in the New Yorker and, while it's long, I highly recommend every gardener read it.  Pollan reports on fascinating (and scientifically sound, although controversial) studies that suggest the possibility of plant senses and behaviors we didn't learn about in school.  Some of the scientists Pollan spoke to even use terms like "plant intelligence" and talk about plants learning.

I was most interested in possible plant senses outside the ordinary.  For example, multiple time-lapse videos show that bean plants grow directly toward a metal trellis, seeming to know exactly where the support is located.  Pollan reported that the scientist "speculates that the plant could be employing a form of echolocation. There is some evidence that plants make low clicking sounds as their cells elongate; it’s possible that they can sense the reflection of those sound waves bouncing off the metal pole."  In another study, the same scientist "found that plant roots would seek out a buried pipe through which water was flowing even if the exterior of the pipe was dry, which suggested that plants somehow 'hear' the sound of flowing water."

I'd heard rumblings about this next study, but found Pollen's interpretation fascinating there as well.  He wrote about how a scientist used radioactive carbon isotopes in a forest to show how trees exchange nutrients using mycorrhizal fungi.  In the study, "mother trees" specifically nourished their offspring, and were seemingly able to recognize that familial relationship.  In addition, evergreen and deciduous trees of different species shared food at different times of the year --- "the evergreen species will tide over the deciduous one when it has sugars to spare, and then call in the debt later in the season."

Finally, a very controversial study suggests that plants may be just as conscious as certain animals.  One study showed that "plants can be rendered unconscious by the same anesthetics that put animals out."  Pollan continued, "What’s more, when plants are injured or stressed, they produce a chemical—ethylene—that works as an anesthetic on animals. When I learned this startling fact from Baluška in Vancouver, I asked him, gingerly, if he meant to suggest that plants could feel pain.... 'If plants are conscious, then, yes, they should feel pain,' [Baluška] said. 'If you don’t feel pain, you ignore danger and you don’t survive. Pain is adaptive.' I must have shown some alarm. 'That’s a scary idea,' he acknowledged with a shrug. 'We live in a world where we must eat other organisms.'"

Interesting reading if you're willing to imagine plants as different from the passive organisms most of us consider them to be.  Or check out the embedded videos to hear directly from a couple of the scientists in question.