1. Lycopodium dendroideum Michx. (round-branched ground-pine) Pl. 18a,b; Map 39

L. obscurum L. var. dendroideum (Michx.) D. Eaton

Rhizomes 5–15 cm below the surface of the soil. Aerial stems erect, with many spreading branches. Ultimate branches round in cross-section. Leaves 3–5 mm long, needlelike, spreading with ascending tips, in 6 ranks along the ultimate branches. Strobili 1–7 at the tips of the main aerial stems, 12–50 mm long, the tips blunt, not attenuate, the leaves bearing the sporangia, 3.0–3.5 mm long, dense, broadly triangular. July–October.

Known from a single drainage in Ste. Genevieve County (northern U.S. and Canada, south to Virginia; South Dakota, Wyoming; Asia). Moist ledges of shaded sandstone bluffs.

The Missouri population is about 500 km disjunct from the closest populations in northern Illinois. It has been interpreted as a relict from the Pleistocene Ice Age, when the cooler climate in southern Missouri allowed species to colonize the area that presently are restricted to more northern climates. Such relicts were trapped in isolated microhabitats as the climate warmed and the surrounding areas grew relatively inhospitable.