Shrubs; stems glabrous sor the uppermost internodes puberulous, the cystoliths numerous, minute, subpunctiform, conspicuous under a lens; leaf blades oblong-elliptic, up to 13.5 cm. long and 5.5 cm. wide, acute or subacuminate (the tip itself blunt), gradually narrowed to base and decurrent on the petiole, thin, entire or undulate, the upper surface glabrous, the costa impressed, this and the lateral veins (6 to 8 pairs) scarcely prominent, the lower surface glabrous with a few minute hairs borne on the costa, the venation rather prominent, the cystoliths of both surfaces numerous, linear, about 0.25 mm. long, con- spicuous under a lens; petioles up to 5 mm. long, sparingly puberulous; racemes narrow, terminal, spikelike, up to 10 cm. long, peduncled, the flowers rather numerous, crowded in sessile or short-peduncled fasci- cles, the peduncle up to 6.4 cm. long and, as is the rachis, rather densely pubescent with whitish, more or less ascending hairs up to 0.25 mm. long, the lower internode about 4 cm. long, the others becoming suc- cessively shorter toward tip of raceme, the uppermost flowers becom- ing contiguous; bracts narrowly linear, the lower pair 5.5 mm. long and 1 mm. wide at base, the others successively smaller toward tip of raceme; bractlets narrowly lanceolate, 1.5 to 2 mm. long, acute or acuminate; calyx about 4 mm. long, the segments narrowly lanceolate, 0.5 mm. wide at base, ciliolate, the pedicels up to 1 mm. long, minutely puberulous; corolla white or pale lilac, minutely puberulous, the tube 12 mm. long, 1.5 mm, broad at base, 2.5 mm, broad at mouth, the lobes subequal, spreading, narrowly ovate, about 1 cm. long and 3.5 mm. wide, rounded or shallowly emarginate at tip; stamens inserted at about the middle of the corolla tube, glabrous, the filaments 1.5 mm. long, slender, the anthers 1,5 mm. long, the staminodes filiform, about 1 mm. long; ovary about 2 mm. long, puberulous; style reaching the mouth of the corolla tube, puberulous, the stigma minute, capitate; capsules not seen.
Type in the Herbario Nacional Colombiano, collected in a wet forest along the Río Putumayo at Puerto Porvenir, Comisaría of Putumayo, Colombia, 230 to 250 meters altitude, November 17, 1940, by J. Cuatre- casas (No. 10629). Fragment of type in the U. S. National Herbar- ium, No. 1995427.
Cuatrecasas' No. 11086, collected in wet forests of Quebrada de la Hormiga, Río San Miguel, Putamayo, 200 meters altitude, December 16, 1940, is also of this species (Col).
The leaf blades of Pseuderanthemum potamophilum are large for the genus and, under a lens, the cystoliths are noticeably numerous and conspicuous on both surfaces of the blades. The species is further marked by the dense, crowded sessile or short-pedunculate fascicles of white or whitish flowers, these forming a narrow, spikelike raceme. The specific epithet is derived from rоrauós, river, and piλéw, to love.