Molecular phylogenetic research supports the inclusion of the genus Hydrocotyle (sometimes set aside in a distinct family, the Hydrocotylaceae) in the Araliaceae (Chandler & Plunkett 2004). It was traditionally placed in Apiaceae (see also Lambinon & al. 2004). Stace (2010) adopts an alternative taxonomy and accepts Araliaceae and Hydrocotylaceae as distinct families.
1. Low herbaceous plants, rooting at the nodes. Leaves peltate or petiolate from a deep basal sinus. Inflorescence inconspicuous, always shorter than the subtending leaf === Hydrocotyle
1. Woody plants, climbers, shrubs or small trees. Leaves never peltate or petiolate from a deep basal sinus (at most cordate at base). Inflorescence often very conspicuous and much longer than the subtending leaf (except sometimes in Hedera) === 2
2. Plant climbing or sprawling, with numerous adventitious roots. Evergreen. Inflorescence a simple umbel === Hedera
2. Plant erect, without adventitious roots. Deciduous or evergreen. Inflorescence compound === 3
3. Deciduous. Plants with or without spines. Leaves to 3-pinnately compound, leaflets papery to membranous, not leathery === Aralia
3. Evergreen. Plants without spines. Leaves palmately lobed, not pinnately compound, leathery === Fatsia
Literature
Chandler G.T. & Plunkett G.M. (2004) Evolution in Apiales: nuclear and chloroplast markers together in (almost) perfect harmony. Bot. J. Linn. Soc. 144: 123-147.
Frodin D.G. & Govaerts R. (2003) World checklist and bibliography of Araliaceae. Royal Botanic Gardens, Kew: 444 p. Hussner A. (2007) Zur Biologie des aquatischen Neophyten Hydrocotyle ranunculoides L. f. (Apiaceae) in Nordrhein-Westfalen. Flor. Rundbr. 40: 19-24.
Jonsell B. & Karlsson T. (eds.) (2010) Flora Nordica, vol. 6. The Bergius Foundation, Stockholm: XIII + 298 p. Jury S.L. (2004) The Euro+Med treatment of Hedera (Araliaceae) – recent studies and a new name. Willdenowia 34: 351-352.
Lambinon J., Delvosalle L., Duvigneaud J. (avec coll. Geerinck D., Lebeau J., Schumacker R. & Vannerom H. (2004) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Cinquième édition. Jardin botanique national de Belgique, Meise: CXXX + 1167 p.
Lawrence G.H.M. (1959) The cultivated genera of Araliaceae. Baileya 7: 133-140.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.