1. Triosteum angustifolium L. (yellow-flowered horse gentian)

Map 1434, Pl. 337 a, b

Stems 0.2–0.7 m long, moderately to densely pubescent with straight, spreading to somewhat downward-angled, stiff, bristly hairs 1.5–3.0 mm long, these sometimes mixed with shorter, softer hairs that are all or mostly minutely gland-tipped. Leaf pairs not perfoliate, the bases joined only by a small ridge around the stem. Leaf blades 10–19 cm long, 2.0–5.5 cm wide, oblanceolate to narrowly rhombic or narrowly elliptic, tapered at the base, sometimes to an indistinct, broadly winged petiolar base, tapered to a sharply pointed tip, the margins with relatively dense, stiff, ascending hairs, the upper surface moderately pubescent with long, straight, appressed hairs, the undersurface sparsely to moderately pubescent with stiff, spreading hairs along the veins or less commonly moderately to densely and uniformly pubescent with short, soft hairs. Flowers 1 per leaf axil (2 per node). Paired bracts subtending each flower about as long as to somewhat longer than the flower (including the calyx and corolla), narrowly lanceolate to narrowly elliptic. Calyx lobes 9–12 mm long, the margins with dense, stiff, bristly, longer and shorter hairs, the inner and outer surfaces glabrous or with scattered, short hairs. Corollas 13–17 mm long, pale yellow to yellow, rarely orange or red, narrowly funnelform, the mouth noticeably oblique, the outer surface with gland-tipped hairs. Styles not or only slightly exserted (less than 2 mm beyond the corolla lobes). Fruits 5–7 mm in diameter, orangish yellow to pale orange, moderately to densely hairy at maturity. 2n=18. April–May.

Scattered in the Ozark and Ozark Border Divisions (eastern U.S. west to Kansas and Texas; Canada). Bottomland forests, mesic upland forests, bases and ledges of bluffs, and banks of streams and rivers.

Rare plants with the undersurface of the leaves evenly pubescent with soft hairs along and between the veins have been called var. eamesii Wiegand, in contrast to the leaves in var. angustifolium, which have short, stiff hairs along the veins. The differences in hairiness seem trivial. Occasional mutants with red flowers have been called f. rubrum F. Lane and occur sporadically within otherwise yellow-flowered populations.