Marsilea quadrifolia L.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Species Description - Rhizome rooting between as well as the nodes; petioles to 30 cm; lfls glabrous or nearly so, those of emergent lvs 7–30 mm long and wide, those of floating lvs avg a little larger; peduncle attached 1–12 mm above the base of the petiole, 3–20 mm, simple or often branched 1–4 mm above its base to bear 2 or 3 sporocarps; sporocarp 4–5.5 × 3–4 mm, 2.5 mm thick, hairy when young, soon glabrate, the inferior tooth mostly wanting, the superior one wanting or to 0.2 mm; sori 10–17 per sporocarp, with ca 12 microsporangia and 3–7 megasporangia per sorus. In quiet shallow water; native of Europe, locally escaped and established in our range.