Saxifragaceae
The familial limits of the Saxifragaceae family have long been uncertain, mostly due to a morphologically more or less intermediate position in traditional classification schemes. Molecular studies have considerably improved our understanding of the family (Morgan & Soltis 1993; Soltis & al. 1993, 2001). Many genera are now accommodated in other families, the woody and shrubby ones in e.g. Hydrangeaceae (e.g. Deutzia, Hydrangea, Philadelphus), Grossulariaceae (e.g. Ribes) or Escalloniaceae (e.g. Escallonia), others in e.g. Parnassiaceae (Celastraceae s.l.). The remaining genera form a firm clade that clearly falls apart in two major groups: the genus Saxifraga (that is polyphyletic in its traditional circumscription) and the so-called heucheroid-clade that encompasses all other genera.
At present Saxifragaceae counts ca. 38 genera and ca. 600 species (Soltis 2007, Fortson Wells & Elvander 2009). Only two genera are native to Belgium, Chrysosplenium and Saxifraga (Lambinon & Verloove 2012). In addition to the genera included in this account, another one has also been encountered in Belgium: Astilbe. Assessing the identity of these plants, however, requires further study. Moreover, in most cases it seems to be nothing else than a mere relic of cultivation.
1 Leaves palmately compound === Rodgersia
Leaves simple (although sometimes deeply lobed), never compound === 2
2 Leaves peltate === Darmera
Leaves never peltate === 3
3 Petals absent. Sepals 4 (native) === Chrysosplenium
Petals present. Sepals 5 === 4
4 Stamens 3 or 5 === 5
Stamens 10, twice as many as petals === 6
5 Stamens 3, petals 4 === Tolmeia
Stamens 5, as many as petals === Heuchera
6 Petals pinnately divided in narrow lobes === Tellima
Petals entire (but sometimes very narrow!) === 7
7 Carpels 2, very unequal === Tiarella
Carpels 2, equal === 8
8 Rhizomatous perennial, rhizomes stout and scale. Leaves large and thick, the larger at least 10 cm across. Petals pink or red, never white === Bergenia
Annual to perennial (sometimes stoloniferous but usually not rhizomatous or rhizomes short and thin). Leaves usually much smaller. Petals white, sometimes yellow or pink/red (only in cultivars) === Saxifraga
Literature
Fortson Wells E. & Elvander P.E. (2009) Saxifragaceae. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 8. Oxford University Press, New York-Oxford: 43-146. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=10800]
Lambinon J. & Verloove F. (avec coll. Delvosalle L., Toussaint B., Geerinck D., Hoste I., Van Rossum F., Cornier B., Schumacker R., Vanderpoorten A. & Vannerom H.) (2012) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Sixième édition. Jardin botanique national de Belgique, Meise: CXXXIX + 1195 p.
Morgan D.R. & Soltis D.E. (1993) Phylogenetic relationships among members of Saxifragaceae sensu lato based on rbcL sequence data. Ann. Missouri Bot. Gard. 80: 631-660.
Soltis D.E. (2007) Saxifragaceae. In: Kubitzki K., The Families and Genera of Vascular Plants, vol. 9. Springer: xiii + 509 p. [available online at: https://www.researchgate.net/profile/Douglas_Soltis/publication/26200615...
Soltis D.E, Morgan D.R., Grable A., Soltis P.S. & Kuzoff R. (1993) Molecular systematics of Saxifragaceae sensu stricto. Ann. Missouri Bot. Gard. 80: 631-660. [available online at: https://www.researchgate.net/profile/Douglas_Soltis/publication/25027038...
Soltis D.E., Kuzoff R.K., Mort M.E., Zanis M., Fishbein M., Hufford L., Koontz J. & Arroyo M.K. (2001) Elucidating deep-level phylogenetic relationships in Saxifragaceae using sequences for six chloroplastic and nuclear DNA regions. Ann. Missouri Bot. Gard. 88: 669-693.