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