1 Plant a floating aquatic (or stranded), the individual thalloid leaves < 2 cm long; [subfamily Lemnoideae].

  2 Fronds rootless; fronds without nerves; reproductive pouch 1, terminal.

    3 Fronds thick, globoid, < 2 mm long

    3 Fronds flat, elongate and curved, 4-14 mm long

  2 Fronds with roots; fronds with 1 or more nerves; reproductive pouches 2, lateral.

      4 Roots 1 per frond; fronds with 1-5 (-7) nerves

      4 Roots (1-) 2-21 per frond; fronds with (3-) 5-21 nerves.

        5 Roots (1-) 2-7 (-12) per frond; fronds with (3-) 5-7 nerves; fronds 1.5-3× as long as wide; all of the roots perforating the scalelike leaflet

        5 Roots 7-21 per frond; fronds with 7-16 (-21) nerves; fronds 1-1.5× as long as wide; only some of the roots perforating the scalelike leaflet (borne on the underside)

1 Plant terrestrial, rooted in wetlands, or a floating aquatic (if a floating aquatic – Pistia – the individual leaves > 2 cm long).

          6 Plant a floating aquatic, with gray-green, velvety, cabbage-like leaves; [subfamily Aroideae, tribe Pistieae]

          6 Plant rooted (even when growing in water), the leaves various, but not as above.

             7 Leaves compound (or sometimes very deeply 3-lobed, with only <3 mm leaf tissue connecting the lobes).

               8 Plant a liana, with many leaves along the stem; sap milky; [subfamily Aroideae, tribe Caladieae]

               8 Plant an herb, with 1-5 basal leaves; sap clear; [subfamily Aroideae, tribe Arisaemateae].

                 9 Spadix free of the spathe; spathe unisexual (with either female flowers only or male flowers only) or bisexual (and then the female and male flowers not separated by a sterile gap); leaves either palmately compound with 3-5 leaflets or pedately compound with 5-15 leaflets, the central leaflet as large as or smaller than the adjacent leaflets; bulblets lacking on the petiole; [native, common].

                 9 Spadix fused basally along one side (for several cm) to the spathe; spathe bisexual, with a basal section of female flowers, a sterile gap, a section of male and female flowers on the free side; leaves either palmately compound with 3 leaflets (juvenile leaves sometimes simple or very deeply 3-lobed) or pedately compound with 5-7 (-9) leaflets, the central leaflet as large as or larger than the others; bulblets either absent or present at base and summit of the petiole; [exotic, rarely naturalized]

             7 Leaves simple.

                   10 Leaves both peltate and cordate-hastate; [subfamily Aroideae, tribe Colocasieae]

                     11 Primary lateral veins and midrib of abaxial leaf surfaces with wax glands; mature fruit orange or red and odorless, some containing a few large seeds

                     11 Primary lateral veins and midrib of abaxial leaf surfaces without wax glands; fruits inconspicuously colored and odorous, with many small mucilaginous seeds

                   10 Leaves not peltate, either cuneate, rounded, cordate, or hastate.

                       12 Spathe present, surrounding the spadix, at least at its base; leaf blade 1-2.5× as long as wide, either hastate at the base (Arum, Peltandra, Syngonium, and Xanthosoma), or rounded (Symplocarpus), or cordate (Calla), broadly ovate in outline.

                          13 Sap milky; [rare exotic, n. FL southwards]; [subfamily Aroideae, tribe Caladieae]

                          13 Sap clear; [collectively widespread].

                            14 Spathe white; leaves cordate; plants from elongate rhizomes; [MD northward]; [subfamily Calloideae]

                            14 Spathe green or white; leaves hastate or rounded at base; plants from fibrous roots, a short thick rhizome, tuber, or a corm; [collectively widespread].

                              15 Leaves ovate, rounded or subcordate at the base; spathe purple, or purple flecked with white; [subfamily Orontioideae, tribe Symplocarpeae]

                              15 Leaves hastate at the base (somewhat arrowhead-shaped); spathe green or white; [subfamily Aroideae].

                                16 Larger leaf blades > 5 dm long; longer petioles 10-20 dm long; [subfamily Aroideae, tribe Caladieae]

                                16 Larger leaf blades < 5 dm long; longer petioles < 7 dm long.

                                  17 Plant from a horizontal tuber; leaves variegated; [exotic, of moist soils]; [subfamily Aroideae, tribe Areae]

                                  17 Plant from fibrous roots; leaves not variegated; [native, of wetlands]; [subfamily Aroideae, tribe Peltandreae]

1 Leaves peltate.

  2 Leaf blades oval in shape, ca. 1.5-2× as long as wide, at maturity floating on the water’s surface; underwater portions of fresh plant coated in transparent mucilage

  2 Leaf blades orbicular in shape, ca. 1× as long as wide, at maturity floating on the water’s surface, emersed, or submersed; underwater portions of fresh plant not mucilaginous (though possibly with green algae, etc.).

    3 Leaves small, < 8 cm in diameter, at maturity emersed or submersed

    3 Leaves large, > 20 cm in diameter, at maturity floating on the water’s surface or emersed

1 Leaves not peltate, the petiole attached at a cuneate, cordate, or sagittate base.

      4 Leaf blades cuneate, rounded, or truncate at the base.

        5 Basal leaves lanceolate, 3-6 mm long

        5 Basal leaves variously shaped, > 10 mm long.

          6 Main veins palmate from the leaf base and also diverging from the midvein, the secondary and tertiary veins then reticulating; inflorescence a spike; [Eudicots]

          6 Main veins either parallel or palmate from the leaf base with cross-veins at nearly right angles to the main veins; inflorescence either a diffuse raceme or panicle of white flowers, or a linear spadix of tightly packed golden-yellow flowers; [Monocots].

             7 Inflorescence diffuse, a raceme or panicle with whorled branches or pedicels, the flowers widely spaced and white; leaves green, “wettable”

      4 Leaf blades cordate or sagittate at the base.

               8 Leaf blades sagittate, the two lobes distinctly acute; leaf apex acute; leaf blade 1.3-3× as long as wide

               8 Leaf blades cordate, the two lobes rounded or sub-acute; leaf apex rounded or apiculate; leaf blade 0.8-8× as long as wide.

                 9 Leaf blade margins serrate, dentate, and/or incised

                 9 Leaf blade margins entire or obscurely crenate.

                   10 Flowers 4-5-merous (sepals 4-5, petals 4-5, stamens 4-5); [Eudicots]

                     11 Leaves emersed or submersed, on stout, stiff petioles (the submersed and winter leaves cuneate-based and lanceolate); flowers 4-merous (sepals 4, petals 4, stamens 4); inflorescence a spike

                     11 Leaves floating, on slender, flexuous petioles (all leaves cordate-based); flowers 5-merous (sepals 5, petals 5, stamens 5); inflorescence an umbel

                   10 Flowers 3-merous or many (>5-) -merous; [Basal Angiosperms or Monocots].

                       12 Perianth parts numerous (usually showing differentiation into sepals and petals, though often with some intergradation), borne in a spiral; stamens numerous; leaves usually > 10 cm long or > 10 cm wide, or both (a few northern species of Nymphaea with leaves as small as 2.5 cm × 2.5 cm); [Basal Angiosperms]

                       12 Perianth parts 3-6 (either differentiated or not into sepals and petals); stamens either 3, or 9-12 (-18), or >20; leaves 1-10 cm long, 1-10 cm wide; [Monocots].

                          13 Perianth strongly differentiated, the 3 sepals green, leathery, and persistent, the 3 petals yellow or white, delicate, deciduous; stamens 20 or more

                          13 Perianth not strongly differentiated, the sepals petaloid and similar in texture and color to the petals; stamens 10 or fewer.

                            14 Flowers unisexual, white, the sepals and petals separate; stamens 3, 6, 9, 12, 15, or 18; leaves either with a central area of spongiform cells (most easily seen on the lower leaf surface) (Limnobium), or without spongiform cells (Ottelia)

                            14 Flowers bisexual, white to blue, the perianth segments united below into a perianth tube 3-45 mm long; stamens 3; leaves lacking a central area of spongiform cells