Touch a sensitive plant (Mimosa pudica) and it will react quickly. Photo: Needpix.com.
Plants are not the stationary 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 toward 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.
That said, 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 That Really Gets a Move On
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. 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 above. And if you shake the entire leaf, not only will all the leaflets collapse, but the entire leaf will bend down as if to escape your “attack.”
This plant is sensitive to heat as well: bring a lighted match close to a sensitive plant and all its leaves will close up.
Unless another something else interferes, the leaves will remain closed for about 15 minutes, then reopen. 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.
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 (USDA zone 5) that can be grown as a perennial in temperate climates: Nuttall’s sensitive briar (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 petiole. It’s the rapid loss of that water that causes the leaf to move so quickly, beginning within a tenth of a second. 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 much of the mechanics of how the leaves and leaflets react, but not all the details.
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 predators.)
Also, the “sensitivity” of the plant may 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
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 if you buy a started plant, 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 in seed catalogs. A rapid Internet search should lead you to a source. Also, the seedlings grow very quickly. For the cost of a pack of seeds, 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.
The very hard seeds of the sensitive can be 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. (Yes, boiling water, straight out of a kettle!) 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 21-24˚C/70-75˚F) and germination should occur within a week. At cooler temperatures, germination can take 3 or 4 weeks.
The sensitive plant needs good light, if possible with lots of direct sun, decent atmospheric humidity and regular watering. Normal room temperatures 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. At any rate, 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 seed pods 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 the longest-lived houseplant, growing it remains nonetheless a fascinating experience for people of all ages, from 5 to 105. 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!