The World Flora Online

Plants small, floating aquatics. Stems creeping, branched, bearing hairs but no true roots. Leaves in whorls of 3, with 2 leaves green, sessile or short-petioled, flat, entire, and floating, 1 leaf finely dissected, petiolate, rootlike, and pendent. Submerged leaves bearing sori that are surrounded by basifixed membranous indusia (sporocarps); sporocarps of 2 types, bearing either megasporangia that are few in number (ca. 10), each with single megaspore, or many microsporangia, each with 64 microspores. Spores of 2 kinds and sizes, both globose, trilete. Megagametophytes and microgametophytes protruding through sporangium wall; megagametophytes floating on water surface with archegonia directed downward; microgametophytes remaining fixed to sporangium wall.

Leaves in whorls of three of which two are entire and floating, and the third submerged and finely dissected into root–like segments, which are thickly covered with hairs; floating leaves papillose Small floating herbaceous plants Rhizome horizontal, branched, siphonostelic, without roots Small floating herbaceous plants Rhizome horizontal, branched, siphonostelic, without roots

Plants aquatic, forming floating mats; roots present (Azolla) or lacking (Salvinia); stems horizontal, protostelic, vegetative reproduction rapid due to fragmentation. Fronds with exposed upper surface covered with water-repellent papillae, fronds in whorls of 3: two fronds floating, green, lamina orbicular to oblong, small (8-25 mm), third one submersed, finely dissected and rootlike (Salvinia), or fronds alternate, distichous, minute (ca. 1 mm), 2-lobed: one lobe floating, with a cavity near base containing blue-green algae (Anabaena), other lobe submersed, only 1 cell thick (Azolla). Sporangia contained within sporocarps, these inserted on submersed frond and externally uniform (Salvinia) or in pairs with a globose microsporocarp and a smaller ovoid megasporocarp (Azolla); spores of two kinds (plants heterosporous), microspores globose, trilete, megaspores large, spore germination endosporic.

Floating aquatic plants with slender branched siphonostelic rootless rhizomes Leaves dimorphous, borne in whorls of three, two floating, oblong to orbicular, entire, variously papillate on the aerial surface, the third submerged, finely dissected, hairy and rootlike Leaves dimorphous, borne in whorls of three, two floating, oblong to orbicular, entire, variously papillate on the aerial surface, the third submerged, finely dissected, hairy and rootlike