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DescriptionThis herbaceous plant is a summer annual that has stems about ½–1' long. These stems are usually erect or ascending, although sometimes they sprawl across the ground. The stems are unbranched or sparingly branched, light green to red, bluntly 4-angled (at least above), and short-pubescent (var. teres) to hairy (var. setifera). Pairs of opposite leaves occur along the entire length of each stem. These leaves are up to 1½" long and ¼" across; they are linear, linear-lanceolate, or linear-oblong in shape, entire (toothless) and involute (rolled downward) along their margins, and sessile with prominent central veins. The upper leaf surface is medium green and appressed short-pubescent to glabrous, while the lower leaf surface is slightly more pale and short-pubescent primarily along the central vein. The leaf bases along each stem are joined with merged stipules that form shallow cup-like structures. The exterior of these stipules is green to whitish green and sparsely short-pubescent to hairy. Along the upper rim of each pair of merged stipules, there are long erect bristles up to ½" long; these bristles are light green, white, or red. Either solitary or small clusters of 2-3 flowers are produced from the axils of the middle to upper leaves. Each flower is up to ¼" long, consisting of a tubular-funnelform corolla with 4 spreading lobes, 4 green sepals that are lanceolate in shape, 4 stamens with pale yellow to white anthers, and an inferior ovary with a single white style. The corolla is lilac, pink, or white; its exterior is often finely short-hairy. The sepals are sparsely short-pubescent to hairy; they are shorter than the corolla. The style has a knobby (capitate) tip. Both the stamens and style are included or only slightly exserted from the corolla. The blooming period occurs from mid-summer to early autumn, lasting about 1-2 months. There is no noticeable floral scent. Afterwards, the flowers are replaced by dry fruits (schizocarps) that are obovoid in shape with remnants of the persistent sepals at their apices. Immature fruits are green, while mature fruits are brown. These fruits are sparsely short-pubescent (var. teres) to hairy (var. setifera); their fine hairs are straight and ascending. Eventually, these fruits divide into 2 nutlets each. The nutlets are about 3 mm. (1/8") long, half-obovoid in shape, brown, and more or less covered with persistent fine hairs. The root system consists of a slender taproot with secondary feeder roots. This plant often forms colonies by reseeding itself.