Published in 1880, The Power of Movement in Plants by Charles Darwin explored hundreds of experiments done by him and his assistants on why plants move the way they do. Without benefit of cameras or computers, the Darwin team made elaborate notes and drawings of how broccoli seedlings moved in their first days of life, how broad bean roots reacted to soil obstructions, how oxalis plants fold their leaves at night as if to sleep, and dozens of other perplexing questions for which Darwin sought answers.
By then he was an old man, plagued with health issues, and 20 years had passed since the publication of his blockbuster, On The Origin of Species. The Power of Movement in Plants was Darwin’s last book, and he offered this summary of his research: Plants have a spontaneous tendency to revolve or circumnutate, or to move in arcs and circles.
Today, time lapse photography has validated this observation, along with many of Darwin’s other thoughts on how plants move. This article contains links to several videos that should not be missed. I like to think that if Darwin were alive today, seeing them would make him a very happy man.
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Each day, young sunflower plants turn from east to west and then back again overnight
Following the Sun
It is sometimes said that the most basic economy on Earth runs on photosynthesis as plants turn sunlight into life-giving energy. All plants have their ways of using light, and some twist and turn to increase their light supply like self-regulating solar collectors. Called heliotropic movements, as shown in this video of British mallow a number of popular garden plants start their day facing east, twist toward the west in the afternoon, and are back facing east by morning.
Sunflowers, dandelions, poppies, tulips, and even tomatoes are famous for following the sun, which has been traced to their ability to grow new cells on alternate sides of their main stem, as shown in this animated video. Their efficiency with light makes heliotropic plants fast growers. As they reach maturity, movements slow down and they settle in to face east.
Sunlight and circadian rhythms also can affect how plants hold their leaves. If you visit your garden at night and it seems that your peas and beans look droopy, it’s because the plants have closed up shop for the day and are taking a rest. As shown in this video of bean seedlings, by the time the plants are two weeks old, they fold their leaves down at night, and extend them in the morning when the sun returns. These movements may help prevent moisture loss and offer some protection from pests.
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Cucumber tendrils adjust their tension with stretchy springs
Confident Climbers
Do plants have a sense of touch? Almost certainly some do, particularly vines that must tether themselves to support. As soon as a seeking tendril finds something to hang onto, the growth rate slows on the side where the plant is touched but the other side keeps going, causing a tendril to curl or a vine to twine.
Back to Darwin’s circles. Peas are big into circles, and the seedlings start rotating soon after they emerge from the ground. As the tendrils develop, they become like tentacles that wave about, desperately seeking moorage. When bean plants are ready to climb, they use similar circular movements as they cast about in search of support.
But the masters of tendril tethering may be the cucumber family, which throw out their seeking tendrils, establish a tight wrap, and then go back and tighten the lines into stretchy springs. Cucumbers, squash and melons perform this miracle in an almost nautical fashion. Check out this video of a gourd plant securing its tendrils and adjusting their tension.
Plant roots have a negative sense of touch, responding to problems by turning away. The crooked carrot that grew around a stone in your garden showed this talent, turning to take the path of least resistance to better growing conditions. Root tips are hugely sensitive to potential problems, branching and turning as needed to avoid obstacles as they work with gravity to find nutrients and water.
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Strawflower blossoms close at night to protect their precious pollen
Safeguarding Pollen
Some plants seem to go to sleep at night, folding their petals or sometimes their leaves, which reopen anew when the sun returns in the morning. Called nyctinasty, the sleeping response can be triggered by darkness or foul weather combined with the plant’s natural circadian rhythms. Circadian rhythms also can be keyed to darkness with night bloomers like four-o-clocks, nicotiana, and moon vine, which stay closed until late in the day.
Flowers that close at night or during rainy weather are often short-lived blossoms, serving their purpose in a couple of days, like squash, morning glories, crocus, dandelions, poppies and daylilies. Closing at night protects the pollen for a second day, and then the bloom is spent.
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Tulip blossoms open and close with the weather and turn toward the sun
Then there are the repeat openers including cacti, gazania, strawflower, tulip, daffodil, portulaca, and many daisies, that close up at night and during rainy weather, not once or twice but every day. Often the petals of these blossoms are lightweight bracts that need minor effort from motor cells to move them up and down. Viewed in time lapse, the exquisite opening and closing of cactus blossoms look like fireworks. Charles Darwin would surely be impressed.
