Pilea

Herbs, shrubs, or subshrubs, annual or perennial, glabrous. Stems simple or branched, erect, ascending, or repent. Leaves opposite; stipules present. Leaf blades paired, equal or unequal, ovate, margins dentate or entire; cystoliths linear, ± conspicuous. Inflorescences axillary, compact to lax cymes. Flowers unisexual, staminate and pistillate flowers in same cyme; bracts deltate to linear. Staminate flowers: tepals 4; stamens 4; pistillode conic. Pistillate flowers: tepals 3, equal or sometimes 1 tepal enlarged and hoodlike; staminodes 3, opposite tepals, under tension and ejecting mature achene; style and tufted stigma deciduous. Achenes sessile, laterally compressed, ovoid to teardrop-shaped, free from perianth at maturity, partly covered by hoodlike tepal. x = 12, 13.

Distribution

Mostly tropical and subtropical regions worldwide except Australia and New Zealand.

Discussion

Species ca. 400 (5 in the flora).

Pilea should be further revised.

Selected References

Chen, C. J. 1982. A monograph of Pilea (Urticaceae) in China. Bull. Bot. Res., Harbin 2: 1-132.

Fernald, M. L. 1936. Pilea in eastern North America. Contr. Gray Herb. 113: 169-170.

Hermann, F. J. 1940. The geographic distribution of Pilea fontana. Torreya 40: 118-120.

Lower Taxa

Pilea fontana, Pilea herniarioides, Pilea microphylla, Pilea pumila, Pilea trianthemoides

Key

1	Leaf margins dentate; paired blades equal.	> 2
1	Leaf margins entire; paired blades unequal.	> 3
2	Achenes uniformly black except for very narrow, pale, marginal band, surface conspicuously pebbled with raised bosses.	Pilea fontana
2	Achenes uniformly light colored or streaked with purple, surface smooth or purple striations sometimes slightly raised.	Pilea pumila
3	Stems repent or prostrate.	> 3
3	Stems erect or diffusely ascending.	> 4
4	Herbs; leaf blades spatulate to obovate.	Pilea microphylla
4	Shrubs or subshrubs; leaf blades of 2 types, obovate and orbiculate to orbiculate-cordate.	Pilea trianthemoides