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Littleleaf sensitive-briar (scientific name Mimosa microphylla) is a small, woody vine native to the southeastern corner of the United States. It grows in dry woodlands, forests, sandhills and disturbed areas with well drained soils. Its distribution includes Kentucky, Illinois, Virginia, the Carolinas, Georgia, Florida, Alabama, and west as far as Texas. Littleleaf sensitive-briar belongs to the legume family (Fabaceae). Its green, prickly stems sprawl along the ground or wind over other small plants, growing to about 6 feet (2 m) long.
The leaves of littleleaf sensitive-briar are each made up of many (6-16) sub-leaves, called pinnae. These branch off around the leaf’s main shaft. In turn, each pinnae is itself made up of (20-32) tiny leaflets, lined up around a smaller midvein. The overall effect is that the leaves have a feathery look. When it blooms in mid-June, the vine resembles a colorful string of pom-poms. The thumbnail-sized flowerheads are hot-pink balls made up of many tiny spiky flowers sticking out from the center. A dot of yellow pollen at the tip of each tiny flower's anther gives the flowerhead a striking look.
Mimosa microphylla has small, inconspicuous, but sharp thorns along its stem and leaf veins. The seeds grow in thin, round pods, which are also covered with spines. When ripe, the pod dries and splits down its length to disperse the row of seeds harbored inside. Birds, such as quail, northern bobwhite, and other seed foragers eat and disperse the seeds.
Like other legumes, Mimosa microphylla grows nodules on its roots to house mutualistic nitrogen-fixing bacteria. The plants return this nitrogen to the soil, thus improving the soil in which they grow.
Littleleaf sensitive-briar belongs to a group of plants called sensitive plants: they are sensitive to touch or movement. When a leaf is disturbed, tapped, or rubbed, or even if it senses vibrations from disturbance of the next leaf over, the little leaflets quickly fold in against each other along their midrib. At the same time, the midribs curl down past the main stem, lowering the folded up leaves out of the way.
It is believed that sensitive plants curl up their leaves to protect from herbivory. By folding up, the leaves almost disappear, and the sharp thorns along the stems stand out, deployed and unobstructed. At night, the leaves of little-leaf sensitive briar fold in by themselves; since they cannot photosynthesize at night anyway, they might as well have their guard up against predators. Ungulates such as white-tailed deer and cattle do not commonly graze on M. microphylla. The gopher tortoise, however, readily eats it as a large part of its diet, as do snails and some plant-eating insects.
(Eisner, 1981; Flora of the Southeast 2005; Hilton 2004; LBJ Wildflower Center 2015; Knapp 2010; Miller and Miller 2005)