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.

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• 3
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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.

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• 4
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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

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• 1
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Small floating herbaceous plants

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• 1
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Rhizome horizontal, branched, siphonostelic, without roots

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• 1
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Floating aquatic plants with slender branched siphonostelic rootless rhizomes

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• 2
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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

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• 2
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