Plants with Weird Leaves: Leaves That Move

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Some leaves really like to shake it up! http://www.oogazone.com & freedesignfile.com

From time to time, I like to write an article about the oddness of some leaves. Here’s yet another, about plants whose leaves actually move.

Leaves Move All the Time

The truth is, leaves that move are not that unusual. They notably move in the wind, or when touched by rain drops or brushed against. However, there are extraneous movements: the plant isn’t moving on its own, it is being moved. That said, many plants do have leaves that move themselves. You’ll learn more about them by reading the following text.

Movement for Protection

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Resurrection fern (Pleopeltis polypodioides) fronds curl up and look dead when dry, but will green up again when the rains come. Source: apalacheehills.com

Many plants have leaves that curl up or roll down under stressful conditions—drought or cold, for example—but recover afterward. The resurrection fern (Pleopeltis polypodioides, syn. Polypodium polypodioides) can survive without a drop of water for many months, even years, then its apparently dead fronds become completely green and functional within 24 hours after a good soaking. Two other resurrection plants are the rose of Jericho (Selaginella lepidophylla) and the alpine gesneriad ramonda (Ramonda spp.).

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These drooping winter rhododendron leaves will straighten up, uncurl and come back to life when warmer weather arrives. Source: http://www.indefenseofplants.com

As for movement to improve cold resistance, the thick leaves of many hardy rhododendrons (Rhododendron spp.) lose most of their moisture and both curl and hang limply all winter, giving their owners quite a scare, yet recover fully when spring returns. It’s thought this habit helps keep frost crystals from forming and damaging leaf cells.

Turning Towards the Sun

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Unless turned regularly, most houseplants will bend in the direction of the light source, Source: Donnie, http://www.houzz.com

On most plants, leaves will turn to face the direction of the sun, at least to some degree. If you transplant or otherwise move a plant—or even if you just cut an overhanging branch that was blocking the sun!—the leaf will adjust, changing its position, usually quite slowly, over days or weeks. This is particularly easy to observe on a forest edge where most light comes from the side or on a windowsill in your home if you don’t give your houseplants the traditional quarter turn regularly: most of the leaves will clearly orient towards the light. This habit of growing towards the source of light is called phototropism. (Remember that term from school?)

Night Moves

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Prayer plant (Maranta leuconeura) leaves move upward at night, like hands in prayer. Source: Aida F., http://www.pinterest.

Other plants have the curious habits of folding their leaves at night, either upward or downward, a phenomenon called nyctinasty. It’s actually very common in some plant families, such as the legume family (Fabaceae) and the oxalis family (Oxalidaceae). You may have noticed this in clover (Trifolium) or false shamrock (Oxalis triangularis), but the best-known nyctinastic plant is the popular houseplant known as the prayer plant (Maranta leuconeura), whose leaves fold up at night like hands in prayer.

This kind of movement is caused by a hinge-like structure at the base of the leaf or leaflet called the pulvinus (plural: pulvini) that is filled with water during the day, but drains at night, so that the resulting lack of turgor causes the leaf to fold.

Scientists still debate why plants do this.

Plants That Dance

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Carefully watch the fire fern (Oxalis hedysaroides ‘Rubra’)—not this photo but a real plant!—and you’ll discover it’s in nearly constant movement. Source: bluepumilio.com

There are plants that, under the appropriate conditions, take the concept of nyctinasty one step further. They too have pulvini and do close at night, but during the day, seem to be constantly readjusting themselves. The fire fern (Oxalis hedysaroides ‘Rubra’), not a fern at all, is a red-leaved oxalis sometimes grown as a houseplant, one of these “dancing plants.”

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The telegraph plant (Codariocalyx motorius) seen using time-lapse photography. You can actually see it move, but not quite that fast! Source: gfycat.com

The telegraph plant (formerly Desmodium gyrans, now Codariocalyx motorius), is another occasional houseplant with seemly motorized leaves.

Both plants will only perform when conditions are fairly warm and humid, but if you sit in from of one and watch patiently, you’ll see each leaf seems to be slowly moving, giving the impression the plant is lazily dancing. The fire fern will also react to touch, at least to a slight degree, but more about touch sensitive plants later.

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The carambola (Averrhoa carambola) has leaves that move all on their own. Source: biogeodb.stri.si.edu

The tropical fruit carambola or starfruit (Averrhoa carambola), in the Oxalidaceae, likewise has leaflets that both close up at night and move visibly, although slowly, during the day, all on their own … if you watch them patiently!

Response to Touch

Plants that react to touch are certainly the weirdest of all plants with leaves that move. This phenomenon, known as thigmonasty or seismonasty, occurs when something touches or shakes the leaf. And some will also react when you hold a match up to them. This can be incredibly rapid and is certainly visible. Again, all these plants close up at night and, again, it’s pulvinus at the leaf or leaflet’s base that empties rapidly, causing the leaf folding. Studies show that there is even an electrical current that runs between the pulvini on many of these plants, almost like nerves in animals, plus there is also a chemical reaction involved.

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Sensitive plant (Mimosa pudica). Source: worldoffloweringplants.com

The best known thigmonastic plant is the sensitive plant (Mimosa pudica), a legume also known as sleepy plant, dormilona, touch-me-not or shy plant, a decent if usually short-lived houseplant easy to grow from seed … and also a pernicious and quite prickly weed in tropical countries. A light touch will cause a single leaflet of the bipinnately compound leaf to fold inward, a firmer touch will lead to the whole leaf drooping and shaking the plant will cause all its leaves to collapse. If you run a finger down the leaf, the leaflets will close like dominoes, as in the photo below. Yet if you leave the leaf alone, it will recover in just 15 to 30 minutes.

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Mimosa pudica leaf closing. Source: Mimosa_Pudica Hrushikesh, Wikimedia Commons

It’s thought this quick reaction to touch helps prevent foraging by grazing animals. I mean, wouldn’t you stop eating if you thought you were biting into a luscious plant, then the leaves all collapsed after your tongue touched the first one, leaving the plant looking barren, unappetizing and full of (previously hidden) thorns?

M. pudica is the most commonly grown sensitive plant, but there are some 400 other species in the genus Mimosa, both herbs and shrubs, all sensitive to touch to at least some degree. There is even a hardy sensitive plant (zone 5) that can be grown as a perennial, M. nuttallii.

Note that these are true mimosas, not the trees and shrubs often called mimosas and which are actually very different, non-sensitive plants with similar pinnate leaves such as Albizia julibrissin (silk tree) and several acacias, including Acacia dealbata (blue wattle or mimosa).

There are also several species of “aquatic sensitive” (Neptunia spp.) with leaves much like those of the sensitive plant that react to touch in a similar fashion. As the common name suggests, they grow in water or at least under very boggy conditions.

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Little tree plant ((Biophytum sensitivum) has leaves that move. Kenraiz, Wikimedia Commons

Less well known is the little tree plant (Biophytum sensitivum), a small herbaceous houseplant in the Oxalidaceae that looks like a tiny palm tree and is sometimes used as a tree substitute in terrariums and fairy gardens. It is modestly touch sensitive … but its leaves move all on their own much of the time, albeit quite slowly.

Finally, the partridge or sensitive pea (Chamaecrista fasciculata), a fairly common annual species of legume native to the eastern United States, also has pinnate leaves that close at night … and are slightly sensitive to the touch during the day.

Touchy Feely Carnivores

The other group that includes plants sensitive to touch are carnivorous plants or, more correctly, insectivorous plants.

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Venus flytrap (Dionaea muscipula) with its leaf traps. To learn how to grow this capricious plant, read No Hamburger for the Venus Flytrap. Source: Citron / CC-BY-SA-3.0, Wikimedia Commons

The best known of these is the Venus flytrap (Dionaea muscipula), often offered as a houseplant, although rarely very long-lived in the average home environment. I already wrote a bit about this plant in 5 Plants with Weird Foliage. It’s bear trap-shaped leaves are dotted with tiny hairs. If an insect touches one hair, nothing will happen. This is believed to be a protection to keep leaves from closing for inopportune reasons, such as when a raindrop or a fallen leaf touches it. However, if the hair is touched a second time within 20 seconds, or if a second hair is touched within the same time limit, the cause is probably a wandering arthropod and the trap closes rapidly, in one tenth of a second. After that, the insect is slowly digested, then the trap opens again. It takes 5 to 14 hours for the trap to reopen after a false alert, while actually digesting an insect can take 10 days or more.

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The trap leaves of bladderworts (Utricularia spp.) do their job underwater, so it’s not easy to see them catch their prey. Source: wetland-plants.co.uk

Less well-known than the Venus flytrap, bladderworts (Utricularia spp.) are even faster. Their bladder-shaped trap is small modified leaf, so designed that when it is “set,” a vacuum forms inside the bladder. If a water flea or other small invertebrate touches the sensitive hair on the outside, the trap opens, instantly sucks in the creature, then closes. The whole process only takes ten to fifteen thousandths of a second.

Gardeners won’t likely find this trap as fascinating as that of the Venus flytrap, as all of this action takes place more or less out of sight underwater or even underground in soggy soil, as bladderworts are bog or aquatic plants.

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Some sundews (here, Drosera capensis) have leaves that will (slowly) wrap around the insects they have caught. Source: Noah Elhardt, Wikimedia Commons

Other insectivorous plants show some leaf movement. Some sundews (Drosera spp.) have leaves that will slowly wrap around their prey once it is glued to the sticky glands that cover them, but this happens so slowly you’d need a time-lapse camera to notice. Butterworts (Pinguicula spp.) leaves also roll up slightly when they trap a prey item, but their movement is even less impressive than that of sundews.


Leaves that move: one of Mother Nature’s little surprises!20180211A www.oogazone.com & freedesignfile.com.jpg

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The Sensitive Plant Likes to Shake It Up

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Mimosa pudica

Plants are not the immutable objects people usually think they are. In fact, they all move, although too slowly for the human eye to notice.

Growth, for example, is movement: stems grow upward and toward the light source. Leaves also turn towards the light, imperceptibly changing position as the sun wanders. And a plant that only receives sun from one side will actually bend in that direction. The buds of the sunflower (Helianthus annuus) do a half circle each day, facing the east in the morning and the west in the afternoon, then do another half moon back to the east at night. Also, many plants have leaves or flowers that close at night and open the day: this kind of circadian movement is called nyctinasty. Finally the twining stems and tendrils of climbing plants wrap around their support: movement once again.

Only a few plants move quickly enough for people to see them without slow-motion filming. You’ve probably seen a Venus flytrap (Dionaea muscipula), a carnivorous plant, on TV or even in person as its trap closes around an insect. And there are dozens of other plants that have sudden and very visible movements in reaction to touch, an effect called thigmonasty.

The Plant with the Hottest Moves

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Sensitive plant reacting to touch.

But the undisputed master of moving plants is the sensitive plant (Mimosa pudica), also called touch-me-not, shame plant or humble plant (one enterprise is now marketing it to kids a “TickleMe Plant”). Its compound leaves are incredibly sensitive to touch. Just brush against a single leaflet and it will quickly fold down. If you run your finger along the leaf stem, the leaflets on each side will fold one after the other, like dominoes, as in the video image. And if you shake the entire leaf, not only will all the leaflets collapse downward, but the entire leaf will bend down as if to escape your “attack”.

It is sensitive to heat as well: bring a lighted match close to the plant and all its leaves will close up.

Unless another something else interferes, the leaves will remain closed for about 15 minutes, then re-open again.
 Moreover, the leaves of the sensitive are also nyctinastic: they close at night also, like the leaves of so many other plants.

There are also other thigmonastic plants (plants that react to the touch) in other genera, such as Oxalis, Biophytum, Codariocalyx , Drosera and Neptunia, including “water mimosa”, N. oleracea, sometimes used in water gardens.

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The “mimosa” (Acacia dealbata) is not a sensitive plant.

Despite its botanical name, Mimosa, the sensitive plant is not very closely related to any of the trees commonly called mimosas, such as Albizia julibrissin or Acacia dealbata, neither of which are sensitive to touch. On the other hand, there are some 400 species of Mimosa and all react to touch, although usually less quickly than M. pudica. There’s even a hardy sensitive plant (zone 5) that can be grown as a perennial, M. nuttallii.

What Makes the Sensitive Plant So Reactive?

The sensitive plant’s curious reaction to touch has long attracted the attention of scientists. Darwin himself studied the sensitive plant, but despite over 200 years of study, we still don’t know exactly what allows the plant to react so quickly. We know that there is a nodule (“pulvinus”) full of water at the base of each leaflet and also at the base of each petiole. It is the rapid loss of that water that causes the leaf to move so quickly. There is even an electrical current that runs between the pulvini, almost like nerves in animals. Plus there is also a chemical reaction involved. So we know bits and pieces of how the leaves and leaflets react, but not all the details.

Why?

Of course, if the sensitive plant reacts so quickly to touch, there must be a reason. Scientists postulate three.

First, it may be a defense mechanism against insects and grazing animals. Indeed, when a herbivore bends to crunch the beautiful leaves of the sensitive plant, all the leaves withdraw, leaving an apparently naked stem and indeed one which is very thorny: nothing very appetizing! (Cut stems give off an acrid smell, also apparently to discourage potential predaors.)

Also, the “sensitivity” of the plant may be help protect it against fire. A brush fire passes very quickly and leaves of the sensitive, having already closed at the first indication of a rapid rise in temperature, could possibly come out unscathed.

Finally, it is possible that this movement may also help protect the plant from violent rain and strong winds.

Growing Your Own Sensitive Plant

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Mimosa pudica

Let it be said right away: although the sensitive plant is generally sold as a houseplant, it’s not the best one, at least not if you judge the success of a plant by its longevity. Indeed, under average home conditions, you’ll be unlikely to keep a purchased sensitive plant going for more than 5 to 6 months (8 or 9 months if you grow it from seed). And even in the wild, it is considered an annual or short-lived perennial. But during the months you do grow it, you’ll be able to test the plant’s rapid reflexes over and over. This is an excellent subject for the budding botanist!

You can sometimes find plants of Mimosa pudica in local garden centers. If not, seed is quite widely available, both in stores and by mail (try www.stokeseeds.com, for example) and the seedlings grow very quickly. For about $ 2.50 plus a few recycled pots and a bag of soil, you’ll be able to supply the whole neighborhood with sensitive plants! And you can also sow the seeds outside in the summer, in which case the plant really will act as an annual.

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Young seedling.

The very hard seeds of the sensitive can slow to germinate unless you give them a heat treatment. Place the seeds in a sieve and pour boiling water over them before sowing them. Alternatively, let the seeds soak for 48 hours in a thermos of hot water. Then sow seeds in a pot of damp potting soil, barely covering them with mix. Keep the pot in a warm spot (about 70-75˚F/21-24˚C) and germination should occur within a week. At cooler temperatures, germination can take 3 or 4 weeks.

The sensitive plant needs good light with at least some direct sun, decent atmospheric humidity and regular watering. The normal temperatures of our homes suit it perfectly. You won’t need to fertilize this plant: it probably won’t live long enough to take advantage of the added minerals. Anyway, in its native Latin America, it’s a weed that grows in poor soil, a good sign it doesn’t need much in the way of nutrients.

Sensitive plants bloom readily, often in less than three months. It’s rounded feathery purplish pink inflorescence are interesting, but not necessarily striking. The flowers, however, will produce seedpods which, when brown, can be harvested, allowing you to start new plants.

Aging plants become susceptible to insects, notably spider mites and thrips. If so, trying rinsing the plant with water, as the leaves tend to be sensitive to insecticides, even such mild ones as insecticidal soap. If you do try an insecticide, test it by spraying it on a leaf or two before treating the entire plant. If the leaves blacken, you can rule that product out!

Although the sensitive plant is not is the easiest houseplant, growing it remains nonetheless a fascinating experience for people of all ages, from 5 to 90. And if you’re an educator, this is an excellent choice for the classroom. Try it and see: you’ll be delighted with the results!