Saxifraga L.
Saxifraga is the most species-rich and taxonomically complex genus of Saxifragaceae and counts about 440 species. It is best represented in temperate to polar climatic zones and of montane to alpine vegetation belts in mountain ranges of the Northern Hemisphere (Tkach & al. 2015). Particularly rich are European mountain areas, western North America, Himalayan-Tibetan area and eastern Asia. The genus is very popular in horticulture (see e.g. Gornall 1995 and Jäger & al. 2008 for accounts for the genus in cultivation in Europe). Gornall l.c. keys out 167 species and numerous infraspecific taxa. Since several of these ornamentals easily reproduce from seed, the natural distribution range of some species has become unclear. In Belgium two species are doubtlessly native (S. granulata L. and S. tridactylites L.; Lambinon & Verloove 2012), while residence status of two similar species is critical: S. rosacea Moench subsp. sponhemica (C.C. Gmel.) D.A. Webb and S. hypnoides L. Both are commonly grown as ornamentals and Belgium is located outside of their main distribution in Europe. The former probably has always been considered native in Belgium (e.g. Durand 1899, Webb 1957, Lambinon & Verloove 2012) although it is mostly confined to damp rocks near old castles and similar places. It is best known at present from the Reinhardstein castle in Waimes-Ovifat where numerous ornamentals have been introduced in the past, including several species of Saxifraga (e.g. S. xgeum and S. xurbium). S. hypnoides, on the contrary, was believed to be non-native in Belgium for quite a long time (e.g. Durand 1899). Webb (1957), however, argued that – although located far outside the species’ main distribution range in Europe – it could be considered native since its Belgian habitat closely resembles that seen elsewhere in Europe, for instance in the British Isles (see also Duvigneaud 1992 and Kerger & al. 1994). In this account both are considered native, which is in allignment with contemporary floras (Lambinon & Verloove 2012). However, the issue requires further study. Both these species also easily hybridize with S. exarata Villars, producing complex hybrids that usually are referred to as S. xarendsii complex (Gornall 1995).
Recent molecular phylogenetic analyses (Tkach & al. 2015) support the recognition of at least 13 sections and 9 subsections within Saxifraga. These in part agree well with previously recognised infrageneric groupings, whereas others are differently delineated.
Several online sources provide useful information about Saxifraga, especially with respect to the ornamental species. One of the most valuable certainly is: The Saxifrage Society at: http://www.saxifraga.org/default.asp. There also is a vast literature on Saxifraga. Some major references are Harding (1970), Köhlein (1984), Webb & Gornall (1989) and Harding (1992).
1 Annual === 2
Perennial, with or without sterile shoots === 3
2 Petals yellow. Stem decumbent to suberect. Leaves orbicular, with shallow lobes or nearly entire. Ovary superior === Saxifraga cymbalaria
Petals white. Stem erect. Leaves oblanceolate, the lower deeply 3-5 lobed. Ovary semi-inferior (native) === S. tridactylites
3 Flowers strongly zygomorphic, the lower petals at least twice as long as the others === S. stolonifera
Flowers actinomorphic, all petals more or less alike === 4
4 Ovary superior (sepals arising from underneath it; hence, absence of a clearly differentiated hypanthium). Leaf blade more or less orbicular with coarsely dentate margins === 5
Ovary (semi-)inferior (sepals arising from its side or top; hence, presence of a clearly differentiated hypanthium). Leaf blade either deeply lobed or orbicular with dentate margins (in S. granulata only) === 7
5 Stem leafy. Sepals erecto-patent. Petals 6-11 mm long. Leaf petiole glabrous === S. rotundifolia
All leaves in a basal rosette. Sepals reflexed. Petals 4-5 mm long. Leaf petiole hairy on margins === 6
6 Leaf petioles usually distinctly shorter than leaf blade === S. xurbium
Leaf petiole usually at least as long as leaf blade === S. xgeum
7 Stolons (sterile shoots) absent. Stem erect. Basal bulbils usually present in leaf axils. Leaves orbicular in outline, with scalloped margins (native) === S. granulata
Stolons (sterile shoots) present. Mat- or cushion forming. Basal bulbils absent. Leaves deeply lobed === 8
8 Petals pink or red, exceptionally white === S. xarendsii group
Petals white === 9
9 Summer-dormant leafy buds present in the leaf axils of at least some sterile shoots. Flower buds pendent (possibly native) === S. hypnoides
Summer-dormant leafy buds absent. Flower buds erect (native) === S. rosacea subsp. sponhemica
References
Damboldt J. (1968) Zur Cytotaxonomie der Gattung Saxifraga L., 3., Willdenowia 4: 43-52.
Durand T. (1899) Phanérogames. In: De Wildeman E. & Durand T., Prodrome de la flore belge. A. Castaigne Editeur, Bruxelles: 1112 p.
Duvigneaud J. (1992) Trois particularités botaniques de la basse vallée de la Lesse. Natura Mosana 44(4): 73-80.
Engler H.G.A. (1872) Monographie der Gattung Saxifraga L. 292 pp. [available online at: http://www.biodiversitylibrary.org/bibliography/15612#/summary]
Gornall R.J. (1986) Trichome anatomy and taxonomy of Saxifraga (Saxifragaceae). Nordic J. Bot. 6: 257-275. [available online at: https://www.researchgate.net/publication/230042535_Trichome_anatomy_and_...
Gornall R.J. (1987) An outline of a revised classification of Saxifraga L. Bot. J. Linn. Soc. 95: 273-292. [available online at: https://www.researchgate.net/publication/230541708_An_outline_of_a_revis...
Gornall R.J. (1988) Saxifraga L. Plant Crib 1988: 57. [available online at: http://bsbi.org/identification]
Gornall R.J. (1990) Making sense of saxifrages. J. Saxifrage Group 1: 7-15.
Gornall R. (1994) Some aspects of hybridisation in the genus Saxifraga. Saxifrage Mag. 2: 17-22.
Gornall R.J. (1995) Saxifraga. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 4. Cambridge University Press, Cambridge: 251-280.
Harding W. (1970) Saxifrages. The genus Saxifraga in the wild and in cultivation.
Harding W. (1992) Saxifrages. A gardeners’ guide to the genus.
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
Kerger M.-T., Parent G.H. & Thoen D. (1994) Matériaux pour la troisième édition de l’Atlas de la flore belge et luxembourgeoise (observations de 1982 à 1991). Adoxa 2: 13-20.
Köhlein F. (1984) Saxifrages and related genera.
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.
McGregor M. & Harding W. (1998) Saxifrages: a complete list of species: ii + 176 pp.
Tkach N., Röser M., Miehe G., Muellner-Riehl A.N., Ebersbach J., Favre A. & Hoffmann M.H. (2015) Molecular phylogenetics, morphology and a revised classification of the complex genus Saxifraga (Saxifragaceae). Taxon 64: 1159-1187.
Webb D.A. (1957) Saxifragaceae. In: Robyns W. (ed.), Flore Générale de Belgique, vol. 2, fasc. 3. Jardin Botanique de l’Etat, Bruxelles: 445-475.
Webb D.A. (1963) Flora Notulae Systematicae No. 2. Saxifragaceae. Feddes Repert. Spec. Nov. Regni Veg. 68: 198-210.
Webb D.A. (1987) Taxonomic and nomenclatural notes on Saxifraga L. Bot. J. Linn. Soc. 95: 227-257.
Webb D.A. (1987) Typification of the Linnean species of Saxifraga. Bot. J. Linn. Soc. 95: 259-272.
Webb D.J. & Gornall R.J. (1989) Saxifrages of Europe p. viii + 307 + 60 coloured plates with 61 figures in the text