Herbs, sometimes suffrutescent at base; stems erect or ascending, up to 30 cm. high or more; stems quadrangular, glabrous or the upper portions minutely puberulous, the hairs upwardly appressed, up to 0.2 mm. long; leaf blades ovate to oblong, up to 12 cm. long and 6 cm. wide, more or less abruptly acuminate (the tip itself acute and some- times curved), rounded or narrowed at base, passing into a winged petiole, moderately firm, entire or undulate, the upper surface spar- ingly puberulous, the hairs up to 0.28 mm. long, the lower surface rather densely and minutely puberulous, the hairs brownish, curved, subappressed, up to 0.2 mm. long, the cystoliths obscure or lacking; petioles up to 5 cm. long, the wing 1 to 1.5 cm. wide at base of blade, gradually narrowed to or below the middle of the petiole, the petiole itself minutely brown-puberulous; flowers solitary or in cymules up to 2 cm. long, several to each node forming a rather loose terminal raceme up to 7 cm. long and 2 to 3 cm. broad at base, the lowermost internode of the inflorescence about 2 cm. long, the others successively shorter, the peduncles 3 to 4 cm. long, these and the branches of the panicle including the pedicels (slender and up to 5 mm. long) minutely hirtellous, the hairs ascending, curved, up to 0.13 mm. long; nodal bracts lance-subulate, up to 8 mm. long and 1 mm. wide near base, the bracts subtending the pedicels similar but smaller, 1.5 to 3 mm. long, 0.25 to 0.5 mm. wide at base, all subglabrous to sparingly and minutely hirtellous with curved hairs; calyx segments linear, 5 mm. long, 0.5 mm. wide at base, rather sparingly and minutely hirtellous; flowers white, the tube (8 mm. long and about 1 mm. in diameter) densely covered without by minute glandular hairs up to 0.05 mm. long, the corolla lobes spathulate, about 5 mm. long, 2 mm. wide above middle, rounded at tip, papillose but less densely so than the tube; stamens included, reaching within 1.5 mm. of the mouth of
the corolla tube, glabrous, the filaments 1.25 mm. long, the anthers 1.5 mm. long, 0.75 mm. broad; staminodes slender, 0.5 mm. long; Ovary glabrous; capsules not seen.
Type in the U. S. National Herbarium, No. 2105340, collected in moist thicket along road between Alto del Tigre and El Socorro, east of Argelia, Department of Antioquia, Colombia, 900 to 1,710 meters altitude, June 1, 1944, by Earl L. Core (No. 819).
Pseuderanthemum thelothrix is near P. chaponense in its relationship and general appearance. The pedicels of P. chaponense, however, do not exceed 1 mm. in length, whereas in this new species they are noticeably slender and are usually as much as 5 mm. long.
The specific epithet is from the Greek, nipple, and hair, in allusion to the papillose puberulence of the corolla.