Solitary herb with one erect, flowering stem or small tussocks with up to 15–20 stems. Stems simple or branched, up to 15 cm tall or more. Subterranean runners have been observed in some Svalbard populations but very rarely (see Comments). Pale, overwintering, bulbil-shaped buds at stem bases. Stems, petioles and leaf margins with glandular and eglandular hairs.
Leaves alternate. Basal leaves long-stalked with petioles (2)3–5(8) cm, bases sheating, blades (0.4)0.6–1.2(1.6) × (0.8)1.0–2.0(2.2) cm, palmately lobed, usually with 7 subacute lobes. Stem leaves smaller, with several dark red to blackish bulbils in each leaf axil.
Either one single, terminal flower or rarely several in a cyme.
Flowers radially symmetric with 5 free sepals and petals. Sepals 2–4 × 1.5–2.5 mm, ovate, red, with abundant glandular hairs. Petals 8–12 × 3–6 mm, nearly three times the length of sepals, obovate and slightly notched, white with yellowish green or rarely pink veins. Stamens 10. Ovary superior, of two carpels with two rooms, split apically.
Fruit a capsule, potentially with numerous seeds, but the fruit very rarely develops to maturity.
Vegetative reproduction by bulbils is the nearly exclusive way of reproduction in this species. Tests have shown that 90–100 % of the bulbils sprout after one year cold storage (Alsos et al. 2013.; Müller et al. 2011). Thus, although they are more fragile than seeds, they provide an efficient way of spread and recruitment. Flowers of this species are adapted to insect pollination and are mainly cross pollinated, although selfing may lead to a low proportion of seeds. Seeds have never been observed in Svalbard but plants collected in Svalbard and germinated in a greenhouse by C. Brochmann and A. Håpnes produced a low number of seeds. However, the level of genetic diversity in Svalbard is high, indicating that the species is able to maintain this level through a combination of occasional sexual recruitment and efficient spread of bulbils (Gabrielsen & Brochmann 1998).
Saxifraga cernua produces large amounts of bulbils which fall to the ground. They may also be eaten by e.g. reindeer or ptarmigan and thereby be spread over longer distance.
Saxifraga svalbardensis and S. cernua are similar. Plants of S. svalbardensis had been identified as S. cernua until the new species S. svalbardensis was described by Øvstedal (1975) The most evident differential character is that S. cernua is almost entirely without runners, whereas S. svalbardensis regularly has very thin runners deep in the moss layer where it grows. This difference results in different growth patterns: whereas the flowering shoots of S. svalbardensis are rather regularly spread, those of S. cernua grows much more clumped and irregular. In addition, the flowers of S. cernua are large and regular with white petals, whereas those of S. svalbardensis are smaller and often irregular with partly reduced petals with lilac stripes or lilac more or less throughout. Saxifraga cernua grows in a variety of site types, but very rarely in the wet, mossy mires where S. svalbardensis grows. Very small-grown plants of these two species have occasionally been mistaken for S. rivularis and S. hyperborea but the two latter species lack bulbils.
Saxifraga cernua occurs in a rather wide range of site types but is most frequent in stony snowbeds and other moist places, often following water courses and seepages, with sparse vegetation. It also occurs on moist substrates covered by mosses but rarely or never in deep wet moss mats (see S. svalbardensis). Often inhabits bird cliff meadows where it may become especially large-grown. It is best developed in places with snow protection but small plants may occur also in more exposed sites. Probably indifferent as to inclination, aspect and soil reaction (pH).
Common in all zones and sections. One of the most widely distributed of all Svalbard plants and even reaches the harshest of islands (Storøya and Kvitøya east of Nordaustlandet).
The general range is circumpolar and arctic–alpine, very widespread.
Saxifraga cernua is polyploid with varying chromosome numbers in the range 2n = 24–72 (Elven et al. 2011), not conforming to discrete ploidy levels. Several studies by a group of investigators (C. Brochmann, P.B. Eidesen, T.M. Gabrielsen, M. Kapralov, and others) have addressed this interesting plant (see Gabrielsen & Brochmann 1998; Kapralov et al. 2006). Saxifraga cernua is a complex result of hybridizations between at least three species at lower ploidy levels: S. rivularis (2n = 52), the E European and Siberian S. sibirica L. (2n = 26 and upwards), and the Beringian S. radiata Small (2n = 26 and upwards). Saxifraga sibirica and S. radiata have comparatively large and numerous flowers, the latter also subterranean runners, but both lack bulbils. All three assumed parents are sexual and reproduce by seeds (S. radiata also by runners). Saxifraga cernua has a much larger range than any of its assumed parents and reaches much farther into the Arctic and the high alpine mountains. Plants of S. cernua with S. radiata in their parentage seem to be more rich-flowered and have runners, whereas those with S. sibirica in their parentage are more few-flowered and lack runners. A few plants with runners and with genetic markers from S. radiata have been proved from Svalbard, in spite of the distance to the nearest S. radiata (2–3,000 km).
Alsos, I.G, Müller, E. & Eidesen, P.B. 2013. Germinating seeds or bulbils in 87 of 113 tested Arctic species indicate potential for ex situ seed bank storage. – Polar Biology 36: 819–830. Doi 10.1007/s00300-013-1307-7.
Elven, R., Murray, D.F., Razzhivin, V. & Yurtsev, B.A. (eds.) 2011. Annotated Checklist of the Panarctic Flora (PAF) Vascular plants. http://panarcticflora.org/
Gabrielsen, T.M. & Brochmann, C. 1998. Sex after all: high levels of diversity detected in the arctic clonal plant Saxifraga cernua using RAPD markers. – Molecular Ecology 7: 1701–1708.
Kapralov, M.V., Gabrielsen, T.M., Sarapultsev, I.E. & Brochmann, C. 2006. Genetic enrichment of the arctic clonal plant Saxifraga cernua at its southern periphery via the alpine sexual Saxifraga sibirica. – Molecular Ecology 15: 3401–3411.
Müller, E., Cooper, E.J. & Alsos, I.G. 2011. Germinability of arctic plants is high in perceived optimal conditions but low in the field. – Botany: 337–348.