Pilea with 600–715 species is the largest genus in the plant family Urticaceae and one of the larger genera in the Urticales and Eudicot Rosids.
It is distributed throughout the tropics, subtropics, and warm temperate regions (with the exception of Australia and New Zealand).
The majority of species are succulent shade-loving herbs or shrubs, which are easily distinguished from other Urticaceae by the combination of opposite leaves (with rare exceptions) with a single ligulate intrapetiolar stipule in each leaf axil and cymose or paniculate inflorescences (again with rare exceptions).
Pilea is of little economic importance; four species are of minor horticultural importance (P. cadierei, P. involucrata, P. microphylla, and P. peperomioides) and one species is used in Chinese traditional medicine (P. plataniflora). The genus has attracted little monographic attention since Weddell (1869), and the majority of taxonomic contributions have come from floristic treatments. To date, 787 species names have been published (International Plant Names Index, 2003) and estimates for the species number range from 250 to 1000 (C. D. Adams, BM, personal communication). Based on previous floristic treatments ca. 30% of the species from regions not yet covered by contemporary floristic treatments may be undescribed.
The genus name is derived from Latin pileus, "felt cap", because of the calyx covering the achene.
- Weddell H. A. (1869). Pilea. In A. De Candolle [ed.], Prodromus systematis naturalis regni vegetabilis 16, 104–163. Victoris Masson, Paris, France.
- P. D. Strausbaugh and Earl L. Core. Flora of West Virginia. 2nd ed. Seneca Books Inc., 1964. ISBN 0-89092-010-9, pages 318-319
- Ann Fouler Rhoads and Timothy A. Block. The Plants of Pennsylvania. University of Pennsylvania Press, 2000. ISBN 0-8122-3535-5, page 694
- Nathaniel Lord Britton and Hon. Addison Brown. An Illustrated Flora of the Northern United States and Canada. In three volumes. Dover Publications, 1913, 1970. ISBN 0-486-22642-5, page 634
- Hortus Third, pages 872-873
- Monro, A.K. (2006). The revision of species-rich genera: a phylogenetic framework for the strategic revision of Pilea (Urticaceae) based on cpDNA, nrDNA, and morphology. American Journal of Botany 93:426-441. HTML
- USDA U.S. Dept. of Agriculture Plants Database
- Chen, C. J. (1982). A monograph of Pilea (Urticaceae) in China. Bull. Bot. Res., Harbin 2: 1-132.
- Monro, A.K. (2006). The revision of species-rich genera: a phylogenetic framework for the strategic revision of Pilea (Urticaceae) based on cpDNA, nrDNA, and morphology. American Journal of Botany 93:426-441.