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| 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; [alien, 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 alien, 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; [alien, 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” |
............ 7 Inflorescence tightly packed with flowers, an elongate, golden yellow spadix; leaves blue-green, “unwettable” |
| ......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 strongle 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 |