These are smooth watery-appearing annuals with translucent stems, often forming dense colonies where abundant seeds have been deposited in a previous season.

The tropical perennial Pilea microphylla (L.) Liebm. was collected in 1958 in the "Nursery between Plant Science Greenhouses and Baker Woodlot" on the Michigan State University campus. Even if not actually planted there, it surely cannot overwinter, is unlikely to re regularly re-introduced, and is thus not considered an element of the flora. 

1. Mature achenes mostly 1.1-1.4 mm wide, deep purple- or olive-black with narrow pale margins, warty on the sides with low irregular raised bumps.

P. fontana

1. Mature achenes 0.7-1 (-1.1) mm wide, pale yellowish or greenish to light brown, often marked with darker ± elongate but scarcely raised speckles.

P. pumila