Do Sleeping Plants Dream?

Disclaimer: It amazes me that this essay was written more than 14 years ago. Since then, science has indeed marched on, and an update on nastic movements is now posted over at Foothills Fancies.

My new prayer plant is an early riser. Although the sun is below the horizon and the room is dark, the sky is lightening, and she is already wide awake. How do I know? Well, some plants are dramatic in their daily movements, and her species is one of many that have attracted attention for centuries because they give the appearance of waking and sleeping each day. Pliny the Elder, a Roman scholar circa 75 A.D., remarked on the somnolent plants; much later Linnaeus devoted a famous essay, Somnus Plantarum, to the subject.

When I set out to write this essay, I wanted an appropriate subject to observe, and lo! not one of the 50+ houseplants in my current collection is this visibly hyperactive type. In my search I came first upon a prayer plant (Maranta leuconeura), although I might have found a dainty shamrock (Oxalis spp.), often available this time of year. If I were very lucky, I might even have encountered a sensitive Mimosa, a faddish plant that periodically disappears from the horticulture trade as each generation tires of touching them to watch the leaves collapse. (These days it may take more to amuse us, for I haven’t seen a Mimosa in years.)

Photo of Maranta leuconeura, awake!, by Kurt Stueber. From BioLib.de, Creative Commons, GNU Free Documentation License, accessed from Wikipedia.

But I shall have to get up early indeed if I expect to see this plant in action, her leaves unfolding from her nightly prayers to spread themselves in anticipation of the light. Trying again, I find that she is prepared even before the first hint of light appears to my eye, although she was sound asleep when I went to bed. In his study of plant movements, Charles Darwin observed dozens of plants that raise or lower their leaves in sleep. These plants don’t respond to light alone, he found, but to the cycles of light and dark, the difference between intense light and its absence. Plants need strong light and good moisture conditions to demonstrate sleep behavior. Plants that are stressed or disturbed, as his Maranta was by strong wind one night, remain sleepless for some time thereafter.

Typically thorough, Darwin lists sleeping species in 86 genera belonging to 26 distinct plant families, although the legumes have by far the most. Sleepers include species of lupines, milkvetches, sweet clovers, wild licorice, and many others whose relatives grow wild here in our foothills, as well as tamer ones like hibiscus and cotton, common garden beans and radishes, nasturtiums, four-o’clocks, and morning glories. Sleeping plants...it’s a charming concept that modern scientists tersely labeled nyctinasty (from Greek roots for night and pressing close), then mostly ignored.

Tracing the movements of individual leaves day after day, Darwin concluded that sleep movements are a modified form of the normal—though far less obvious—movements healthy leaves continuously make. How can a plant with neither muscles nor nerves move so quickly? You’ve probably noted that leaves are capable of abrupt position changes when they run low on water. We call it wilting. Most nastic movements are also based on hydraulic changes within the cells. By raising or lowering water pressure on opposite sides of the leaf stem, a leaf can move through angles of 90 degrees or more, and even turn on its axis. Some plants, such as my Maranta, are equipped with specialized joints that control their daily movements. These structures, called pulvini, occur where the leaf blade joins the petiole, functionally a bit like the wrist joint connecting your hand and forearm.

By forcing some leaves to stay “awake,” Darwin also tested whether plants are protected from cold nights by their cozy postures of sleep. It’s not the cold wind that’s the threat, he tells us, but radiation—a subject we tend to overlook. On clear nights, any exposed surfaces will lose heat directly to the open sky—to space, as it were. By folding up or down in sleep, leaves turn their narrowest edges to the sky, protecting their broad flat heat-losing surfaces. According to his experiments, leaves thus protected were less often damaged by frost. If sleeping offers such advantages, why don’t more kinds of plants—especially those in cold areas—sleep?

Well, they do, don’t they? Most of our local plants are deep in a season-long sleep, leaving only tropical houseplants to keep us company through the long deprivations of winter. While these companions keep us in touch with the spirit of plants, they also fill our shuttered houses with good green oxygen and remind us of spring. “Storms and winter weather,” some say, “bring plants and people close together.” We probably have a few storms yet to weather this winter, and can rely on our faithful houseplants to help us get through them.

Although we have an extensive leguminous flora, I’m embarrassed to admit I’ve never caught one of them asleep. So when spring does come, I plan to take some walks at dusk, just to see if I can spot the “jerky movements” Darwin speaks of, and to see how many native cousins of the greenhouse plants he studied also sleep. Darwin noted in his classic understated style, “It is troublesome to observe the movement of leaves in the middle of the night,” although he did so nonetheless. Can such inconveniences explain why the fascinating sleep of plants is so little studied now? In his later years, although he rarely left his home grounds—or sometimes even his study—his curiosity and patience yield us marvels to contemplate more than a century later.

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by Sally L. White, originally published in Upbeat, March 1996, ©2010.