HOW DO SENSITIVE PLANTS DO THAT?
Call them plant motors. Or plant muscles. Tiny bulges of specialized cells in a mimosa plant can fold its feathery leaflets together in seconds, then relax — and do it again.
A new look at these bulges on the Mimosa pudica plant has revealed more details of how a leaf manages its unusually fast folding, says biomechanist David Sleboda of the University of California, Irvine. “I think that these particular organs are really cool because their motion is reversible,” he says. “[W]hen people see plant motion that is reversible, it feels much more similar to animal motion.”
Dr. Sleboda's study is called "Multiscale structural control of hydraulic bending in the sensitive plant Mimos pudica," and you can read it at bioRxiv.
The USDA has updated their classification of this species since I last blogged about it in 2009. They now call this species "Shame Plant" or Mimosa pudica L. var. pudica.
Scientists had already worked out the basic chemistry that drives a little mimosa motor, or pulvinus, he and colleagues write in a paper slated for the Feb. 6, 2023 issue of Current Biology. When a deer hoof or something else scary jostles a leaf, potassium and some other ions shift from one part of a pulvinus toward another. Water follows the swoosh of ions. Cells that lose water deflate and sag while those on the other side bloat. Distortions in multiple pulvini make the halves of a feathery leaf fold toward each other, like an invisible hand gently closing a book.
When one thumps the leaves or flower of this species it quickly closes tight. This is most likely an evolutionary adaptation to discourage predation. This miniature variety goes largely unnoticed by everyone but me. . . and now you.
Instead of studying chemistry, Sleboda and colleagues looked at microscopic structural details in pulvinus cells that help create such useful distortions, he reported January 7 at the annual meeting of the Society for Integrative and Comparative Biology in Austin, Texas. One feature that makes plant-muscle cells bloat more efficiently is reinforcement with microscopic fibrils. They work like corsets, keeping cells from bulging out in all directions. Instead, the corset directs much of the swelling along the axis that will fold up the leaf halves.
Also, pulvinus cells that need to bulge fast have what look like wrinkles of easily expandable tissue for inrushing water, plus special highly porous zones called pit fields. The pits look as if water could sluice through easily in a tickled-leaf emergency. Cell arrangement itself looks specialized for expanding and shrinking. A pulvinus cross section reveals a pattern “like the bellows of a concertina,” Sleboda said.
The widespread M. pudica, or sensitive plant, is one of the better-known leaf flexors. Yet clusters of other plants in the same family, the legumes, also move their leaves, says botanist Thainara Policarpo Mendes of Universidade Estadual Paulista in Botucatu, Brazil. Some relatives close fast like M. pudica, but many are slower. What she also thinks about, though, is why leaves close at all. People have proposed a variety of advantages: discouraging animals from grazing on a plant that suddenly looks more sticklike, or even helping a plant lose less heat on very cold nights. It could also be effective in dislodging insects that might choose to dine on its leaves.
Sleboda too can reel off proposed hypotheses but remains skeptical of all of them. “There’s not a ton of research,” he says. That, however, is fine with him. “My favorite thing about sensitive plants’ leaf closing,” he says, “is that we don’t know why they do it.”
In Florida, the Sensitive Plants (Mimosa microphylla Dryand.; and Mimosa pudica) are starting to bloom. This little plant goes by many names but mostly it is categorized as a noxious weed in Florida. I guess it is all in the eye of the beholder. I find them to be quite whimsical. And Florida's Little-leaf Sensitive Plants are of the fast-closing variety. Its fun to play with these plants and watch them at work.
Meanwhile the other noxious weed called "Mimosa" that is found in Florida, is a tree, and will also be in bloom in Florida by late May. Its a much larger specimen. . . and without the cool rapidly retracting leaves. Most of Florida's Mimosa (below) are actually Albizia julibrissin Durazz. . .also known as Silk Tree.
Albizia spp. are also in the Pea family and also not native to Florida. They are a fast growing tree that is amazingly durable. Despite the current extremely dry conditions they will be in full bloom soon.
Below: A stylized image of Florida Little Leaf Sensitive Plant growing in my neighbors sunny front yard. I put some sky where my neighbor's house should be to make this look like a field of giants. . .when in fact most of the plants are only about 4-inches tall.
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