1. Pilea pumila (L.) A. Gray (clearweed)

P. pumila var. deamii (Lunell) Fernald

Pl. 572 j, k; Map 2673

Plants annual, unarmed, glabrous, the stems and leaves relatively translucent, usually with a short taproot. Stems 7–50(–70) cm long, erect or strongly ascending, occasionally from a spreading base, usually unbranched, stout and slightly succulent. Leaves opposite, long-petiolate, stipulate. Leaf blades 2–12(–15) cm long, elliptic to broadly elliptic or ovate, broadly angled to rounded at the base, occasionally more narrowly angled, tapered at the tip, the margins bluntly toothed to nearly scalloped or occasionally sharply toothed, with 3 main veins; cystoliths linear. Inflorescences axillary, small 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. 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 (papillose and appearing bushy, not persistent at fruiting). Fruits 1.3–1.8 mm long, flattened, ovate in outline, the surface smooth or nearly so, green to straw-colored, often with fine purple streaks. 2n=24, 26. July–October.

Scattered to common throughout the state (eastern U.S. west to North Dakota and Texas; Canada; Asia). Bottomland forests, banks of streams and rivers, margins of ponds and lakes; also roadsides and moist, disturbed areas.

Steyermark (1963) noted that some plants in Missouri with more narrowly angled leaf bases and margins with denser, sharper teeth were referable to var. deamii, a variant that occurs mostly in the northeastern portion of the species range. However, he discussed the numerous intermediates and chose not to formally recognize this variety, a practice followed by most subsequent botanists (Boufford, 1997b).

Steyermark (1963) also noted that clearweed has sometimes been used as a teaching tool in biology classes to demonstrate the passage of substances through a plant from its roots into the aboveground portions. Because the stems are relatively translucent, water that has been dyed with a stain such as eosin can be seen to be uptaken by roots or cut stems and transported into the upper stems and leaves.

Botanists in northern Missouri should watch for the closely related P. fontana (Lunell) Rydb., which has a broad range in the eastern and northern United States that includes the southern half of Iowa and adjacent portions of Illinois and Nebraska. The two species can be very difficult to distinguish in the field. Compared with P. pumila, P. fontana tends to have slightly smaller, more opaque, less shiny leaves. The main technical differences are in the achenes. Those of P. fontana are dark purple or dark olive to black, except for narrow, pale, marginal bands, with a noticeably pebbled surface (Boufford, 1997b).