Call them plant engines. Or plant muscles. The mimosa plant’s tiny bulges of specialized cells can fold their feathery leaves together in seconds, then relax — and do it again.
A new look at these bulges on the plant Mimosa pudica revealed more details about how the leaf manages its extremely rapid folding, says biomechanist David Sleboda of the University of California, Irvine. “I think these particular organs are really cool because their movement is reversible,” he says. “When people see the reversible movement of plants, they more closely resemble the movement of animals.”
Scientists have already worked out the basic chemistry that powers the mimosa’s little engine, or pulvinus, he and his colleagues write in a paper due Feb. 6 Current Biology . When a deer’s hoof or something else scary pushes a leaf, potassium ions and some other ions move from one part of the pulvin to another. Water follows the ion tick. Cells that lose water swell and sag, while cells on the other side swell. Deformations in many pulvini cause the halves of the feathery leaf to fold together like an invisible hand that neatly closes a book.
Instead of studying the chemistry, Sleboda and his colleagues looked at the microscopic structural details in the pulvinus cells that help create such useful distortions, he reported Jan. 7 at the annual meeting of the Society for Integrative and Comparative Biology in Austin, Texas. One of the features that make plant muscle cells swell more efficiently is the strengthening of microscopic fibrils. They work like corsets, preventing cells from protruding in all directions. Instead, the corset directs most of the swelling along an axis that bends the leaf halves.
In addition, the pulvinus cells, which must expand rapidly, have what appear to be folds of tissue that expand easily to allow water to enter, as well as special highly porous areas called pit fields. The pits look like water could easily seep through them in a leaf-tickling emergency. The cell structure itself appears to be specialized for expansion and contraction. A cross-section of the pulvina reveals a pattern “similar to concertina bellows,” Sleboda said.
Widespread M. pudica , or sensitive plant, is one of the most famous leaf benders. However, clusters of other plants from the same family, the legumes, also move their leaves, says botanist Tainara Policarpo Mendez of the Universidade Estadual Paulista in Botucatu, Brazil. Some relatives close quickly, like M. pudica , but much slower. But she also thinks about why she leaves close at all. People have suggested a number of benefits: preventing animals from grazing on the plant, which suddenly becomes more stick-like, or even helping the plant lose less heat on very cold nights.
Sleboda can also refuse the proposed hypotheses, but is skeptical about them. “There’s not a ton of research,” he says. This, however, suits him. “I like the fact that sensitive plants close their leaves,” he says, “that we don’t know why they do it.”
