Review: What a Plant Knows

As denizens of planet earth, we keep looking for intimate companions on our journeys through space and time. Gods, aliens, or chimps dressed in tuxedos—we outfit them with traits and feelings that resemble our own. Every once in a while a book comes along that even assures us there are close emotional and spiritual relationships between plants and humans (for example, 1973’s The Secret Life of Plants). Another book of the same era argued that playing soft, gentle music to plants enhanced their growth while hard rock created botanical monsters.

Scientists naturally cringe at such experimentally deficient nonsense. But we share much DNA with plants, and valid comparisons across the taxonomic kingdoms may work to the benefit of each. Daniel Chamovitz, in his new book What a Plant Knows: A Field Guide to the Senses, seeks to define the barriers and bridges between us. Chamovitz, a biologist and director of the Manna Center for Plant Biosciences at Tel Aviv University in Israel, shows that the reality of what plants “know” and do far surpasses the fervid suppositions of the “secret life” proponents. “Clearly, my use of the word ‘know’ is unorthodox,” he writes. “Yet different parts of a plant are intimately connected, and information regarding light, chemicals in the air, and temperature is constantly exchanged between roots and leaves, flowers and stems, to yield a plant that is optimized for its environment.”

Judiciously manipulating similes with dashes of anthropomorphism, Chamovitz introduces each of the vital human senses (all except taste) and explains its meaning for humans as contrasted with its function in plants. There are no noses or eyes as such in the plant world, but there are organs and responses that mimic our physiology. The author recounts how willows, attacked by caterpillars, send airborne pheromones to neighboring willows. Warned by these gaseous signals (or “smells”) of a nearby infestation, the neighbors begin manufacturing increased levels of toxic chemicals to render their leaves unpalatable to the caterpillars.

The author recalls that his grandmother had little plant biology but lots of practical know-how. She knew she could soften an avocado by putting it in a brown paper bag with a ripe banana. Magic? Well, she had no idea why it worked but simply knew from experience handed down from the past that it did. Elsewhere, that sort of reliance on hand-me-down knowledge caused consternation.

“In the early twentieth century, farmers in Florida would ripen citrus in sheds heated by kerosene,” Chamovitz writes. “These farmers were sure that the heat was what induced the ripening. . . .  You can imagine their dismay, then, when they plugged in some electric heaters near the citrus and found that the fruit didn’t cooperate at all.”

Scientists learned that kerosene smoke contains a molecule called ethylene. Treating any fruit with pure ethylene gas ripens it. All fruits contain ethylene, which is the universal plant hormone that induces fruit to ripen. “When we put a ripe banana in a bag with a hard pear,” Chamovitz writes, “the banana gives off ethylene, which is ‘smelled’ by the pear, and the pear quickly ripens. The two fruits are communicating their physical states to each other.”

It may come as a surprise to many readers that a great deal of what we know about plants today can be traced back nearly a century and a half to Charles Darwin. Experiments on plant senses that he carried out in his home gardens and greenhouses still hold up, or provide a solid launching point from which to solve old botanical questions. Darwin was among the first to explore in depth what plants “feel” in his 1875 treatise, Insectivorous Plants. He became a world authority on the subject through studies on the Venus flytrap, a carnivorous plant that responds to an insect of the right size alighting on it. The plant snaps shut its leaves, then employs chemicals to “digest” the tiny animal and make use of its protein.

Plants feel the slightest touch but not pain. “A plant is free from these subjective constraints because it lacks a brain,” Chamovitz writes. “But plants feel mechanical stimulation, and they can respond to different types of stimulation in unique ways.”

Darwin also attacked the conundrum of plants and music. A modestly talented bassoonist, he serenaded a mimosa plant at length. But its leaves failed to close, prompting him to give up his “fool’s experiment.” Modern scientists have fared no better in their efforts to confirm the belief of some New Age spiritualists that plants exposed to soft classical music, even Muzak, thrive, while plants and humans bombarded by Led Zeppelin become stunted or worse.