Plants forming dense clones. Roots arising at nodes. Petioles 5--39 cm, hairy. Pinnae 9--35 × 8--39 mm, conspicuously white hairy abaxially, hairy adaxially; hairs often exceeding margin to make pinnae appear white margined from above. Sporocarp stalks erect, frequently branched, attached 2--4.5 mm above base of petiole; unbranched stalks or ultimate branches of stalks 8--17 mm; common trunk of branched stalks 1--5 mm (rarely 2--3 unbranched stalks attached separately to same petiole). Sporocarps strongly ascending, 6--9 × 4.5--5.5 mm, 2 mm thick, elliptic or quadrate in lateral view, covered with matted or twisted hairs; raphe 1.6--2.4 mm, proximal tooth 0.4--0.5 mm, blunt, distal tooth absent or present as slightly raised area. Sori 19--23.

Sporocarps produced nearly year-round (Feb--Nov). On mud and in shallow water, locally abundant in clay soils of ditches, ponds, marshes; 0--400 m; Ala., La., Tex.; Mexico.

Marsilea macropoda is introduced in Alabama and Louisiana.

Marsilea macropoda , with its tall, hairy leaves and branched sporocarp stalks, is a striking and distinctive species, worthy of cultivation. It is sympatric with M . vestita over much of its range, and individuals that appear to be hybrids have been collected in Blanco, Brazos, and Kleberg counties in Texas. These plants combine the large hairy leaves of M . macropoda with the shorter-stalked sporocarps bearing a superior tooth found in M . vestita .