We’re all familiar with the idea of the “fight or flight” reaction that animals display in threatening situations. We don’t usually think about plants’ response to threats; but, from the perspective of natural selection, your average Brussels Sprout does not want to become dinner any more than you do. Plants, of course, generally do not have the “flight” option available, so fighting is their only choice, and their weapon of choice is generally chemical warfare. Many of the nastier poisons that occur in nature are manufactured by plants: atropine, nicotine, digitalis, hydrocyanic acid, and strychnine, to name just a few.
All of this is pretty elementary evolutionary biology, but a recent article in the New York Times, by Natalie Angier, discusses some recent research that illustrates that plant responses to threats are not only more complex, but can happen much faster, than we might have thought.
“I’m amazed at how fast some of these things happen,” said Consuelo M. De Moraes of Pennsylvania State University. Dr. De Moraes and her colleagues did labeling experiments to clock a plant’s systemic response time and found that, in less than 20 minutes from the moment the caterpillar had begun feeding on its leaves, the plant had plucked carbon from the air and forged defensive compounds from scratch.
In addition to producing toxic chemicals, plants can also emit chemical signals that, for example, attract the natural predators of the insects that are chewing on their leaves. These signals can also alert other plants of the same species to begin defensive measures, even if they have not yet been attacked.
Some species of plants have evolved remarkably complex strategies to protect themselves against predators:
… when a female cabbage butterfly lays her eggs on a brussels sprout plant and attaches her treasures to the leaves with tiny dabs of glue, the vigilant vegetable detects the presence of a simple additive in the glue, benzyl cyanide. Cued by the additive, the plant swiftly alters the chemistry of its leaf surface to beckon female parasitic wasps. Spying the anchored bounty, the female wasps in turn inject their eggs inside, the gestating wasps feed on the gestating butterflies, and the plant’s problem is solved.
We’ve already begun to see, in many cases, that our assumptions about a vast gulf in intelligence between humans and “dumb animals” may be far too flattering to ourselves. So I guess we should not be too surprised to find out that we underestimated plants, too.