4. Pilea fontana (Lunell) Rydberg, Brittonia. 1: 87. 1931.
Adicea fontana Lunell, Amer. Midl. Naturalist 3: 7. 1913
Herbs , annual, 1-7 dm. Stems simple or slightly branched, erect. Leaf blades elliptic to ovate, paired blades equal, 1-10 × 0.6-4.5 cm, margins dentate. Inflorescences crowded or lax. Flowers ca. 1 mm across. Achenes uniformly black except for very narrow, pale, often inconspicuous, marginal band, compressed, teardrop-shaped, 1.3-1.7 × 1-1.5 mm, conspicuously pebbled or warty with raised bosses.
Flowering late summer-fall. Mixed woods, along streams, swamps, seepages, and marshes; 0-300 m; Ont.; Ala., Conn., Fla., Ga., Ill., Ind., Iowa, Md., Mass., Mich., Minn., Nebr., N.J., N.Y., N.C., N.Dak., Ohio, Pa., R.I., S.C., S.Dak., Vt., Va., Wis.
Pilea fontana and P . pumila are separated primarily by differences in their mature achenes. In addition, leaves of P . fontana are often more opaque and less shiny than those of P . pumila . A few collections of P . pumila from Bourbon, Owen, and Robertson counties, Kentucky, and Macon County, Tennessee, have the black achenes of P . fontana , but without the bosses, and show striations on the younger achenes as in P . pumila . I have seen only two mixed collections (Chisago County, Minnesota, and Richland-Ransom county line, South Dakota), which probably indicates that these two very similar species seldom occur together, even though their ranges overlap completely.