Acorus L., usually included in Araceae, is here transferred to the monogeneric family Acoraceae. Ray (1987) already provided evidence for the exclusion of Acorus from Araceae, the absence of a spathe being the most notable distinguishing feature. His viewpoint was subsequently confirmed by molecular phylogenetic research (Chase 2004, Angiosperm Phylogeny Group III 2009). On the same grounds the Lemnaceae family is included in the Araceae.
Very useful additional information on the Araceae (s.str.) is provided by the International Aroid Society on: http://www.aroid.org/. Similarly, interesting information on Lemnaceae is available here: http://waynesword.palomar.edu/1wayindx.htm#difficult.
1. Plant aquatic, floating or submersed. Leaves pubescent or glabrous === 2
1. Plant (sub-) terrestrial (but sometimes confined to wetlands). Leaves glabrous === 6
2. Leaves pubescent, in dense rosette, 2-20 cm long === Pistia
2. Leaves glabrous, never in rosette, usually less than 10 mm long (if slightly longer than leaves submersed) === 3
3. Fronds rootless, globular, at most 1,6 mm long (native) === Wolffia
3. Fronds always rooted, flat, usually at least 1,6 mm long === 4
4. Roots 1 per frond. Veins 1-3(-7). Lowerside of fronds never entirely purplish (but sometimes in part with purple pigmentation) === Lemna
4. Roots numerous, up to 20 per frond. Veins usually more numerous, 5-16. Lowerside of fronds often entirely purplish === 5
5. Fronds 1,5-2x as long as wide, 1-3 mm wide. Veins usually 5-9. Roots 2-10 === Landoltia
5. Fronds 1-1,5x as long as wide, 3-6 mm wide. Veins usually 7-15. Roots 5-21 (native) === Spirodela
6. Leaves pedately divided in 7-15 lobes or leaflets === Sauromatum
6. Leaves undivided === 7
7. Mature leaves hastate-sagittate at base, often dark-spotted and/or white-veined. Spathe greenish === Arum
7. Mature leaves rounded-cuneate to cordate at base, concolourous. Spathe white or yellow, or intermediate === 8
8. Spathe white, 3-7 cm long, not enclosing the spadix. Spadix much longer than its stipe. Leaves appearing before flowering, cordate at base, at most 15 cm long (native) === Calla
8. Spathe white, yellow or yellowish white, 10-35 cm long, enclosing the stipe of the spadix. Spadix much shorter than its stipe. Leaves appearing at or soon after flowering, cuneate-rounded at base, often exceeding 100 cm === Lysichiton
APG III (2009) An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants. APG III. Bot. J. Linn. Soc. 161(2): 105-121.
Chase M.W. (2004) Monocot relationships: an overview. Amer. J. Bot. 91(10): 1645-1655.
Daubs E.H. (1965) A monograph of Lemnaceae. University of Illinois Press, Urbana: 118 p. [available online at: https://www.ideals.illinois.edu/bitstream/handle/2142/26697/monographofl...
Ray T.S. (1987) Leaf types in the Araceae. Amer. J. Bot. 74: 1359-1372.