19. Pterostegia Fischer & C. A. Meyer, Index Seminum (St. Petersburg). 2: 48. 1836.
Woodland threadstem [Greek pteron, wing, and stege, covering, alluding to winged bract]
James L. Reveal
Herbs, annual; taproot slender. Stems arising directly from the root, sprawling and spreading, solid, not fistulose or disarticulating into ringlike segments, thinly pubescent. Leaves persistent, cauline, opposite; petiole present; blade broadly elliptic to fan-shaped, margins entire or lobed. Inflorescences terminal, cymose, uniparous due to suppression of secondaries distally; branches dichotomous, not brittle or disarticulating into segments, round, thinly pubescent; bracts absent. Peduncles absent. Involucral bracts 1, erect, 2-winged, reticulately veined, lobed or notched, slightly gibbous with age, invaginated adaxial surface hyaline. Flowers 2-3 per involucral cluster; perianth pale yellow to pink or rose, campanulate when open, urceolate when closed, sparsely pubescent abaxially; tepals (5-)6, connate for ca. 3 their length, monomorphic, entire apically; stamens 6; filaments adnate to perianth, glabrous; anthers yellow, oval. Achenes included, yellowish brown to brown, winged, globose, glabrous. Seeds: embryo straight. x = 14.
Species 1: w United States, nw Mexico.
Pterostegia and the Baja California, Mexico, endemic Harfordia are the only genera of the tribe Pterostegieae. In both, the involucre is highly modified, becoming a slightly to markedly gibbous, reticulated, winged structure that encloses the mature achene. In Pterostegia, the two wings are slightly enlarged, but in Harfordia the wings are greatly inflated, which apparently aids in achene dispersal.
Reveal, J. L. 1989c. Remarks on the genus Pterostegia (Polygonaceae: Eriogonoideae). Phytologia 66: 228-235.