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.

Lvs long-petiolate, the blade mostly floating or emergent, with 4 approximate lfls (2 juxtaposed pairs) regularly spreading from the petiole-tip and suggesting a 4-lvd clover; lfls dichotomously and reticulately veined, and with an intramarginal connecting vein; sporocarp longitudinally ±bilocular and with many transverse partitions, finally dehiscent by 2 valves, somewhat compressed, ±ovate or elliptic, in many spp. (including ours) attached laterally to the distal part of the peduncle, which forms a raphe that may project as an "inferior" tooth; a "superior" tooth often also present on the upper side of the sporocarp a little above the base; sori several or many, each with central megasporangia and lateral microsporangia on an elongate receptacle. 45, cosmop.

Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.

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