Jeannette Cooperman in The Common Reader: Tempted as I am to lavish consciousness on everything around me, I was fascinated to learn that tobacco and tomato plants click when they are stressed. The frequency is too high for us to hear these distress calls, but mice and moths do. As a plant dehydrates, the clicking speeds up, as though they are nervously cracking their knuckles. The beach evening primrose reacts to the sound of bees buzzing by secreting a sweeter nectar. Deprived of water, plants avoid dehydration by tightening the pores in their leaves. Roots avoid salt in the soil by inching toward areas that are less salty. Grains of starch shift with gravity, telling a plant which way is up. Flowers plan ahead, turning toward the sun before it rises and timing their pollen production to be ready when a pollinator shows up. Plants release volatile chemicals when they are eaten or infected or mowed down (that summertime freshly-mown-grass smell we all rhapsodize about). Neighboring plants receive this communication and take, when possible, defensive measures.
Most astounding, plants learn. The “sensitive plant,” Mimosa pudica, folds up if you touch a single leaf—but if you keep touching, the stimulus ceases to frighten, and the plant stays open. The same thing happens in studies where the plant is dropped, an extreme stimulus that causes it to fold up instantly. Drop it again and again, though, and it will realize there is no danger and stop folding its leaves. A solid month later, you can drop the plant, and it will remember that it need not fold.
Paco Calvo, a philosopher of plant behavior, believes plants possess a form of intelligence that lets them remember, learn from the past, and anticipate the future. In Planta Sapiens, he notes that a plant “always has to make a compromise among different things. It needs some kind of valence, a higher-level perspective. And that’s the entry to sentience.”
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