Mimosa quadrivalvis L.

Sensitive Briar

© SRTurner

Family - Fabaceae/Mimosoideae

Habit - Perennial herbs, with a thickened, woody taproot, armed with prickles.

Stem - Sprawling or clambering over other vegetation, to 3 m, finely ridged toward the base, strongly ribbed above the midpoint, glabrous, yellow to yellowish brown, armed with numerous short, downward-curved prickles, these sparser toward the base, yellow to yellowish brown.

	Stem.
© SRTurner

Leaves - Alternate, twice pinnately compound. Petiole 2-5 cm long, armed with scattered to dense, short prickles similar to those of the stem, lacking glands. Stipules inconspicuous, 2-7 mm long, linear to hairlike. Leaf blades 6-12 cm long, with 3-8 pairs of pinnae, each with 8-16 pairs of leaflets. Leaflets 3-9 mm long, 2-4 mm wide, oblong, oblique at the base, short-tapered to a bluntly pointed or minute, abrupt, sharply pointed tip, with sparse, loosely ascending hairs along the margins.

	Leaf (adaxial).
© SRTurner

	Leaflet response to touch.
© SRTurner

	Leaf rachis.
© SRTurner

Inflorescences - Spherical, headlike clusters, axillary, each 2.0-2.5 cm in diameter (including the stamens), with numerous (usually more than 75) short-stalked flowers (sometimes a few of the inner flowers staminate). Stalks 1.5-4.0 cm long at flowering, becoming elongated to 3-10 cm long at fruiting, armed with scattered to dense, downward-curved prickles similar to those of the stem.

	Inflorescences.
© SRTurner

	Inflorescence.
© SRTurner

Flowers - Calyces minute, to 0.2 mm long, cup-shaped to conical, fused nearly to the tip, with microscopic irregular lobes, glabrous. Corollas 2.5-3.5 mm long, lobed less than 1/2 the way to the base, usually pink to purplish pink (rarely pale cream-colored with pinkish-tinged lobes), the lobes rounded to sharply pointed at the tip. Stamens 8-12, the filaments 7-10 mm long, not fused at the base, pink or lavender pink.

	Stamens and anthers.
© SRTurner

Fruits - Fruits 3-9 cm long, 3-6 mm wide, narrowly oblong, tapered to a nearly sessile base, tapered to a slender beak at the tip, straight or slightly curved, often somewhat flattened but appearing more or less 4-sided, the margins more or less straight and somewhat thickened, the surfaces not constricted between the seeds, densely covered with curved prickles, otherwise glabrous, straw-colored to yellowish brown at maturity, dehiscent by 4 valvelike strips. Seeds 8-30, 3-4 mm long, oblong in outline, slightly flattened, dark brown, smooth, somewhat shiny, sometimes covered with a thin, lighter outer layer that becomes cracked and peeling at maturity.

	Fruit.
© DETenaglia

Flowering - May - October.

Habitat - Glades, upland prairies, savannas, openings of dry upland forests, fields, ditches, railroads, roadsides.

Origin - Native to the U.S.

Lookalikes - None when flowering.

Other info. - This pretty species is found across much of Missouri, though appears to be absent in many northwestern and southeastern counties. It is not particularly common. Beyond Missouri its range comprises a broad band within the southern-central portion of the continental U.S. When in flower the plant is unmistakable from its pink "powder puff" inflorescences. The visible part of these consists of pink stamens, with yellow anthers sometimes visible at the ends. The other flower parts are relatively small and buried within the mass.

The common name "sensitive briar" is doubly appropriate. Stems, leaf rachises, and peduncles all bear curved prickles, accounting for the "briar" epithet. Despite its aggressive defenses, it is a sensitive plant. The slightest touch to a leaflet will cause that and often also nearby pinnae to fold up (see image above). This makes specimens troublesome to press in a pose which preserves open leaves. Prostrate stems typically produce a couple dozen flowering heads, with only a few open on any day, and these releasing pollen for only a single day. The flowers are fragrant but produce no nectar. Visiting bees scrape copious pollen from the heads, each of which they treat as a single flower. The foliage and seeds are consumed by a variety of wildlife. The plant would make an attractive and unusual garden subject, but it can be difficult to propagate.

The plant belongs to the same Fabaceae subfamily as a few other plants with similar flowers, such as the mimosa tree (Albizia julibrissin) and Illinois bundleflower (Desmanthus illinoensis). The group has undergone numerous taxonomic revisions, with this plant formerly known as Schrankia nuttallii (DC. ex Britton & Rose) Standl. Missouri plants are assignable to var. nuttallii, which some authors elevate to species status as Mimosa nuttallii.

Photographs taken off Highway 13, St. Clair County, MO., 7-7-00, and in Big Spring Park, Carter County, MO., 6-7-03 (DETenaglia); also at Shaw Nature Reserve, Franklin County, MO, 5-29-2007 and 6-3-2007, and at Drury-Mincy Conservation Area, Taney County, MO, 4-14-2012 and 5-1-2012 (SRTurner).