Herbs up to 1 meter high; stems terete or the upper portions sub- quadrangular, glabrous or puberulous in 2 lines, the hairs brownish, up to 0.15 mm, long, spreading or retrorsely recurved, the internodes 1 to 4 cm. long; leaf blades lanceolate, up to 9 cm. long and 1.5 cm. wide, acuminate (the tip itself obtuse), narrowed at base, rather thin, entire or undulate, glabrous or the costa of the lower surface sparingly hirtellous, the lateral veins (5 or 6 pairs) obscure, the cystoliths of both surfaces numerous and conspicuous under a lens, up to 0.24 mm. long; petioles up to 0.5 cm. long, more or less minutely hirtellous; spikes terminal and lateral, up to 3 cm. long and 1.5 cm. broad, rather lax at maturity, the peduncles about 0.5 mm. long, hirtellous, the hairs septate, up to 0.46 mm. long, the rachis glabrous or sparingly hirtellous, the cystoliths usually conspicuous under a lens; bracts rhombic-ovate, up to 13 mm. long and 7 mm. wide, acute or sub- acuminate, narrowed at base, thin, entire, sparingly ciliate with hairs up to 0.75 mm. long, otherwise glabrous or nearly so, the cystoliths conspicuous under a lens but less numerous than on the leaf blades; bractlets narrowly lanceolate, up to 8 mm. long and 1 mm. wide near base, slenderly acuminate, ciliate toward tip, otherwise glabrous; calyx 4 mm. long, deeply segmented, the segments subulate, about 0.5 mm. wide near base, gradually narrowed to a slender tip, glabrous or sparingly hirtellous and ciliolate; corolla white, about 1 cm. long, 1 mm. broad at base and mouth, narrowed to 0.5 mm. above base, the lips about 2 mm. long, the upper lip crect, entire, the lower more or less spreading, 3-lobed at tip, the stamens exserted beyond the mouth of the corolla tube but not exceeding the corolla lips, the an- thers 1.5 mm. long, 0.25 mm. thick, superposed, vertically attached to connective; capsule 7 mm. long, 2 mm. broad, 1 mm. thick, minutely hirtellous toward tip, becoming glabrous; retinacula about 3 mm. long, flat and rounded at tip; seed ovate, flattened, about 3 mm. long and 2 mm. broad, glabrous.
Type in the herbarium of the New York Botanical Garden, col- lected on the Río Esmeralda, between Las Dantas and Puerto Canoa, Department of Bolívar, Colombia, 400 to 800 meters altitude, February 27, 1918, by Francis W. Pennell (No. 4529).
The specific epithet is from the greek Aerrós, thin or delicate, and xλauís, bract.