Sp. Pl. 2: 1099. 1753; Gen. Pl. ed. 5, 485, 1754.
Plants aquatic or amphibious, forming diffuse or dense colonies. Roots arising at nodes, sometimes also on internodes. Leaves deciduous in temperate regions, heteromorphic, floating leaves averaging larger than land leaves. Petiole filiform, stiffly erect or procumbent in land leaves, lax in floating leaves. Blade palmately divided into 4 pinnae. Pinnae cuneate or obdeltate, pulvinate at base, frequently with numerous red or brown streaks abaxially in floating leaves. Sporocarps borne on branched or unbranched stalks at or near bases of petioles, aboveground (except in Marsilea ancylopoda), attached laterally to stalk apex (attached portion called raphe), tip of stalk often protruding as bump or tooth (proximal tooth), some species also with tooth distal to stalk apex (distal tooth); sporocarps densely to sparsely hairy, less so with age, dehiscing into 2 valves.
Species identification is virtually impossible without fertile material. The common name water-clover refers to the resemblance of the leaves to those of clover (Trifolium spp., Fabaceae); pepperwort refers to the sporocarp, which approximates a peppercorn in size and shape.
Ca. 45 species (6 in the flora with 5 native, 1 introduced).
Species Ca. 45 (6 in the flora with 5 native, 1 introduced)
|1||Roots both at nodes and sparsely (1–3) along internodes; stalks of sporocarps frequently branched.||Marsilea quadrifolia|
|1||Roots only at nodes; stalks of sporocarps branched or unbranched.||> 2|
|2||Distal tooth of sporocarps 0.4–1.2 mm, acute.||Marsilea vestita|
|2||Distal tooth of sporocarps absent or to 0.4 mm and blunt.||> 3|
|3||Sporocarps 6–9 mm, strongly ascending; stalks usually branched.||Marsilea macropoda|
|3||Sporocarps 2.4–6 mm, perpendicular to strongly nodding, i.e., stalk curved or bent; stalks unbranched.||> 4|
|4||Stalks recurved or prostrate, often hooked again at base of raphe; sporocarps borne underground or below stem level.||Marsilea ancylopoda|
|4||Stalks erect, never hooked at base of raphe; sporocarps borne aboveground or above stem level.||> 5|
|5||Sporocarps 2–3 mm wide, proximal tooth of sporocarp 0.2 mm or absent.||Marsilea mollis|
|5||Sporocarps 3.6–4 mm wide, proximal tooth of sporocarp 0.2–0.6 mm and curved away from sporocarp.||Marsilea oligospora|