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Why does a sensitive briar faint?

October 25, 2001

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The sensitive briar (Mimosa quadrivalvis) [mi mo' sa quad ri val' vis] is native to the Americas with a number of varieties ranging from Brazil to Canada. Linnaeus named the genus Mimosa from the Latin mimus or mime since the plants mimic the movement of animals. This low creeping briar is at home in old fields, pastures, and roadsides where it is subject to the predations of herbivores. But the plant has two tricks to defend itself against predators.

The sensitive briar is capable of thigmonastic movement, motion in response to external stimulus. The compound leaves made up of numerous leaflets, appear to faint with even the slightest touch. A specialized organ called a pulvinus is at the base of each leaflet.

The pulvinus contains vascular tissue, phloem and xylem, surrounded by a bulky cortex of thin-walled cells. The thin walls are interconnected and called the apoplast. The apoplast absorbs sugar from the phloem and the sugar appears to drive the response to touch. The pulvini can rapidly change the turgor (water pressure) of the leaf.

There is probably a difference in electrical potential between the cells and the cell walls. When touched, positively charged potassium ions jump from the cells into the sugar loaded apoplast. Most of the water in the cells follows the potassium ions into the cell walls. The cells of the leaf collapse. The plant does not lose any water; the water is merely dispersed throughout the cell walls.

After the leaves have collapsed, the second line of defense is noticeable. The sensitive briar is covered with prickles. The prickles hook the skin and break off. A browsing herbivore is faced with a plant whose leaves have suddenly "disappeared" and lots of prickles to snag the nose.

The Missouri Wildflower Guide has two excellent photographs of Mimosa quadrivalvis.

Click here to view these photographs