1. Marsilea quadrifolia Linnaeus, Sp. Pl. 2: 1099. 1753.
Rhizomes creeping, with internodes up to 4.8 cm, but internodes occasionally shorter and fronds then appearing tufted; roots arising both at nodes and 1 or 2 on internodes, short shoots 1-3 mm occasionally formed at nodes; main rhizome sparsely pubescent to glabrate. Fronds with stipes green to straw-colored, 10-18 cm, slender, glabrate; pinnae obtriangular to fan-shaped, 0.8-1.6 × 0.7-1.5 cm, margin entire, glabrate above, sparely appressed-pubescent to glabrate below. Sporocarps (1 or)2-5(-7) per frond, attached at base of stipe where stipe joins rhizome or up to 25 mm up stipe, sometimes persistent after decay of frond and then appearing to arise directly from rhizome; peduncles 5-12.5 mm (when 2 or 3 sporocarps on a frond these attached to stipe by a common peduncle 2-8 mm), plus an additional 1.5 mm adnate to sporocarp, glabrate; sporocarp brown to black, elliptic in lateral view, 3.5-4 × 2.9-3.2 × ca. 2 mm, softly leathery, convex on lateral sides, obtuse to rounded on perimeter, usually marked with a distinct tooth 0.3-0.4 mm above end of peduncle, densely appressed-pubescent when young, becoming thinly pubescent to glabrate with age.
Gansu, Hebei, Heilongjiang, Henan, Jilin, Liaoning, Nei Mongol, Ningxia, Qinghai, Shaanxi, Shandong, Shanxi [Japan, Korea; Europe; introduced in NE North America].