Plants forming diffuse or dense clones. Roots arising at nodes. Petioles 2--20 cm, sparsely pubescent. Pinnae 4--19 × 4--16 mm, pubescent to glabrous. Sporocarp stalks erect, unbranched, attached at base of petiole (occasionally up to 3 mm above it), not hooked at apex, 0.5--25 mm. Sporocarps perpendicular or slightly nodding, 3.6--7.6 × 3--6.5 mm, 1.5--2 mm thick, elliptic to nearly round in lateral view, pubescent but soon glabrate, scars left by fallen trichomes often appearing as purple or brown specks; raphe 1.1--1.7 mm, proximal tooth 0.3--0.6 mm, blunt, distal tooth 0.4--1.2 mm, acute, often hooked at apex. Sori 14--22. Sporocarps produced spring--fall (Apr--Oct). Widespread and variable; in ponds and wet depressions and on river floodplains; 0--2300 m; Alta., B.C., Sask.; Ariz., Ark., Calif., Colo., Idaho, Iowa, Kans., La., Minn., Mont., Nebr., Nev., N.Mex., N.Dak., Okla., Oreg., S.Dak., Tex., Utah, Wash., Wyo.; Mexico; South America in Peru. A number of segregate species have been named and recognized in regional floras in North America: Marsilea mucronata A. Braun (less hairy, found east of Rocky Mountains), M . uncinata (glabrous, sporocarp stalks long, distal tooth of sporocarp hooked, south central United States), M . tenuifolia (pinnae very narrow, central Texas), and M . fournieri (small plants and pinnae, southwest). The features upon which these species are based intergrade into one another. The species are therefore best treated as conspecific with M . vestita (D. M. Johnson 1986). Putative hybrids between Marsilea macropoda and this species are discussed under the former.

PETIOLES: 1–35 cm long, glabrous or sparsely appressed-hairy. LEAFLETS: 5–22 mm long, wedge-shaped to broadly wedge-shaped, broadly rounded to nearly truncate at the tips, often slightly asymmetrically cuneate, glabrous to more commonly moderately pilose on both surfaces, the lateral margins straight or more commonly 1 margin shallowly concave. SPOROCARPS: solitary, appearing more or less perpendicular to the peduncle produced from the tip of a short peduncle attached at or rarely just above the petiole base, (3.6–)4–7 mm long, 3–6 mm wide, oblong-quadrangular to broadly oval in outline, the tooth distal to the stalk tip well-developed, 0.4–1.2 mm long, sharply and narrowly triangular (Fig. 2B). MEGASPORES: 450–515 μm in diameter (excluding the apical papilla). MICROSPORES: 65–80 μm in diameter. 2n = 40. NOTES: Shallow water and margins of ponds, lakes, stock tanks, reservoirs, and streams: Cochise, Mohave, Pima, Pinal, Santa Cruz, Yuma cos. (Fig. 1B); 450–1650 m (1400–5400 ft); WA to MN, s to CA. REFERENCES: Windham, M. D. and G. Yatskievych. 2009. Vascular Plants of Arizona: Marsileaceae. CANOTIA 5 (1): 30–33.