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 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 absent or obscure; leaf blade 2.5-5× as long as wide, cuneate at the base, lanceolate or narrowly elliptic; leaf venation parallel; [subfamily Orontioideae, tribe Orontieae]

                       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 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 palmately-pedately 5-foliolate (the lateral 2 leaflets on each side borne on a common Y-shaped stalk).

  2 Leaflets toothed; sap clear

1 Leaves palmately compound (all the leaflets attached at a single point).

    3 Leaves > 6 dm wide

    3 Leaves < 3 dm wide.

      4 Leaflets with entire margins; [non-natives].

        5 Plants deciduous woody climbers (but not twining, or specialized climbing structures); leaflets usually 5 per leaf (occasionally 3 or 7); flowers maroon-colored, lacking petals but with 3 prominent sepals, the inflorescences axillary

        5 Plants evergreen shrubs or trees (occasionally partially epiphytic); leaflets usually 7-9 per leaf (occasionally 5 or 10); flowers yellowish-green or red, petals present, lacking showy prominent sepals, the inflorescences (most often) terminal

      4 Leaflets with serrate margins; [natives and non-natives].

          6 Stems armed with prickles scattered in the internodes

          6 Stems unarmed or with paired nodal spines.

             7 Inflorescence an umbel; leaves evergreen, glossy; stems often with paired nodal spines

             7 Inflorescence a panicle; leaves deciduous, dull; stems unarmed