Erect succulent herbs up to 60 cm. high, stems subquadrangular, nitid, sparingly puberulous with curved septate hairs up to 0.24 mm. long, the lower portions of the stem glabrate, the cystoliths straight, parallel, up to 0.11 mm. long, inconspicuous and scattered; leaves of a pair noticeably unequal, the larger leaves alternate with the smaller of the succeeding pair, the blades oblong-elliptic, up to 21 cm. long and 7.5 cm. wide, short-acuminate (the tip itself obtuse), acute to obtuse at base and decurrent on the petiole, rather thick, entire or undulate, the upper surface nitid, sparingly puberulous, the hairs up to 0.19 mm. long, septate, appressed, the cystoliths inconspicuous, up to 0.08 mm. long, the costa and lateral veins (about 8 pairs) obscure, the lower surface puberulous, the hairs confined chiefly to costa and veins, these and the coarsely reticulated veinlets conspicuous, the cystoliths inconspicuous; petioles (unwinged portion) up to 4 mm. long, with the winged portion up to 3.5 cm. long, the wings gradually enlarged from base to 1 cm. wide at foot of blade, the petiole sparingly hirtellous; panicles terminal, about 10 cm. long and 5 cm. broad, each node producing a large and a small branch, these alternating through- out the panicle, the flowers as many as 4 or 5 at each node, the peduncle 8.5 cm. long, subquadrangular, about 2 mm. thick, finely striate, rather densely hirtellous, the hairs retrorsely curved, up to 0.2 mm. long, septate, the lowermost internode of the panicle 3 cm. long, the others successively shorter toward tip of the inflorescence, all hirtellous, the hairs spreading or ascending, septate and up to 0.2 mm. long; bracts triangular, up to 5 mm. long, 1.5 mm. wide at base, gradually narrowed to a blunt tip, carinate, hirtellous, the hairs ascending, mostly marginal and dorsal, septate; bractlets triangular, up to 3 mm. long, 1.25 mm. wide at base, acute, hirtellous; pedicels slender, up to 3 mm. long, densely hirtellous with ascending hairs up to 0.13 mm. long; calyx up to 4 mm. long, the segments subulate, 3 mm. long, 0.5 mm. wide near base, nitid, minutely and sparingly hirtellous, the hairs about 0.06 mm. long, some of them gland-tipped; flowers showy, light purple, 16 mm. long, sparingly hirtellous, the hairs up to 0.1
mm. long, some of them gland-tipped, the tube 1 mm. broad at base, narrowed at 2 mm. above base to 0.75 mm., thence gradually enlarged to 2 mm. at throat, the lobes ovate, 8 mm. long, 4 mm. wide, rounded at tip; stamens reaching the mouth of the corolla tube, the filaments 0.75 mm. long, glabrous or bearing a few minute hairs at base, the anthers 1.75 mm. long, 0.5 mm. broad, glabrous; staminodes linear, 0.5 mm. long; ovary glabrous; capsules not seen.
Type in the U. S. National Herbarium, No. 1995094, collected in a wet forest glade along the Río Nuqui, Intendencia of Chocó, Colombia, about 300 meters altitude, January 25, 1947, by Oscar Haught (No. 5489).
Pseuderanthemum diachylum is characterized by its large leaves, attractive light purple flowers, and succulent texture. The plant from which the type was taken was the only one seen by the collector. The specific epithet is from the Greek, succulent.