Pilea fontana (Lunell) Rydb.

Lesser Clearweed

Pilea fontana plant

Family - Urticaceae

Habit - Annual taprooted forb, monoecious or dioecious, unarmed, glabrous, the stems and leaf petioles translucent.

Stem - To 50 cm, erect or ascending, occasionally from a spreading base, usually unbranched, stout, glabrous, translucent, and slightly succulent.

Pilea_fontana_stem.jpgStem.

Leaves - Opposite, long-petiolate, stipulate. Blades 2-12 cm long, elliptic to ovate, broadly angled to rounded at the base, tapered at the tip, the margins bluntly toothed to nearly scalloped or occasionally sharply toothed, with 3 main veins.

Pilea_fontana_leaf1.jpgLeaf adaxial.

Pilea_fontana_leaf2.jpgLeaf abaxial.

Inflorescences - Small axillary clusters, these often arranged into small panicles, the staminate and pistillate flowers usually on different branches of the same panicle. Bractlets not forming an involucre.

Flowers - Staminate flowers with 4 sepals, these 0.7-1.1 mm long, loosely cupped around the stamens. Stamens 4. Pistillate flowers with 3 free sepals, these 0.8-1.2 mm long, equal or occasionally 1 somewhat enlarged and hoodlike. Style absent, the stigma capitate, not persistent at fruiting.

Fruits - 1.3-1.8 mm long, flattened, ovate in outline, the surfaces irregular and tuberculate, dark purple to nearly black.

Pilea_fontana_infructescences.jpgInfructescence.

Pilea_fontana_fruits.jpgFruits.

Flowering - July - October.

Habitat - Fens, very wet soils.

Origin - Native to the U.S.

Lookalikes - Pilea pumila; more broadly, other members of the Urticaceae such as Urtica dioica and Boehmeria cylindrica.

Other info. - This species was first discovered in Missouri by Aaron Floden and Meg Englehardt in August 2019, from a fen in Lafayette County. The species is much less common than its sibling P. pumila, growing in scattered locations in the upper Midwest and along the eastern U.S. coast. The occurrence in Lafayette County is considerably disjunct from previously known populations, and the plant should be searched for in other Missouri locations, particularly in the north.

In appearance this species is very similar to the very common P. pumila, which could partially account for the paucity of Missouri specimens. The most reliable differentiating character is the appearance of the fruits. In P. fontana these have a pebbled surface and, at maturity, are evenly dark (not striped or spotted) in color. The leaf surfaces are also typically less shiny than those of P. pumila, and the herbage less translucent, but these differences are subtle and best discerned in a side-by-side comparison.

Little is known about the plant's behavior in Missouri, but it is certainly restricted to wet areas and may be a fen obligate.

Photographs taken at Hicklin Fen, Lafayette County, MO, 10-1-2019 (SRTurner).



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