Solitary herb. A very short, subterranean caudex, simple or moderately branched, each branch ending in one or more rosettes at ground level. Flowering stems (scapes) up to 15(20) cm, usually one per rosette but sometimes several.
Leaves alternate, all in basal rosettes, 0.5–2 cm long, narrowly obovate or spathulate, evenly tapering towards base, margins in the proximal part entire, in the distal part sharply dentate with more or less triangular teeth, glabrous except for a few multicellular hairs in the margins, dark green.
Inflorescence in upper 1/3 or less of flowering stem, a panicle with short branches. Flowers either all replaced by purplish, small, leafy bulbils crowded at the end of branches or sometimes with one terminal flower (only observed a few times in Svalbard).
The bulbils are transformed flowers, each consisting of a bunch of minute globular leaves.
Flowers radially symmetric with 5 free sepals and petals. Sepals short, 1–2 × 1.0–1.2 mm, oblong, obtuse or subacute, red-dotted. Petals 4–5 × 1–2 mm, lanceolate or narrowly ovate, obtuse or acute, white. Stamens 10, with bright red anthers and pollen. Gynoecium of 2 fused carpels with free upper parts and styles, red or pink, scarcely developed in the few observed flowering plants. Stamens and gynoecia probably very rarely functioning.
In the material we have inspected, flowering plants have only been seen in three collections: Hotellneset by Longyearbyen (1924), Todalen in Adventdalen (1993), and Colesbukta (1997).
Not observed in Svalbard material; replaced by bulbils.
Vegetative reproduction by bulbils only. The species produces large amounts of bulbils which fall to the ground. Tests have shown that ca. 70 % of the bulbils sprout after one year cold storage (Alsos et al. 2013). Bulbils have also sprouted from seed bank (Cooper et al. 2004). Thus, although they are more fragile than seeds, they may provide an efficient way of spread and recruitment, and due to their construction they furnish the emerging new plant with more nutrients than a seed normally receives from its endosperm. Sexual reproduction by seeds is not proven but may perhaps happen now and then (see Comments).
Bulbils may be eaten by, e.g., reindeer or ptarmigan and thereby be spread over longer distances.
The two Saxifragas in Svalbard with bulbils – S. cernua and S. svalbardensis – both differ from Micranthes foliolosa by having leafy flowering stems and bulbils in the leaf axils (i.e., transformed lateral shoots). None of the other species of Micranthes have bulbils. The only generally similar species in N Europe is M. stellaris (Scandinavia, Iceland, and farther south), differing in having normal flowers and absence of bulbils, and also in a different leaf shape.
Of the four species of Micranthes in Svalbard, M. foliolosa and M. hieraciifolia are easily separable from the two others and from each other even by their leaves. Micranthes foliolosa has characteristic, obovate or obcuneate leaves with a few triangular teeth in the distal 1/3 only, and the leaves are thin and glabrous; M. hieraciifolia has ovate or lanceolate, subacute or acute leaves with sparse, shallow teeth along the sides, and the leaves are thick and with white, articulate hairs along the margins and on the lower surface. Micranthes nivalis and tenuis both have thick, rounded leaves with obtuse, forward pointing teeth and are less readily recognized (see these species).
Confined to shallow marshes, open patches in moist or wet tundras, moist snowbeds, and moist to wet patterned ground. Usually scattered occurrence, but sometimes frequent. Nearly always on fine-grained substrates. On substrates with a circumneutral to basic soil reaction (pH), rarely on very acidic soils.
Cryophilous. Distributed throughout all sections and zones. The species is found on all major Svalbard islands, except for Bjørnøya, and also on several smaller ones.
The general range is arctic circumpolar, in Europe reaching south to S Norway.
Micranthes foliolosa is a mainly octoploid (2n = 56) species (Elven et al. 2011) and belongs in a group together with the mainly tetraploid (2n = 28), amphi-Atlantic M. stellaris, the mainly tetraploid (2n = 20) NE Asian M. redofskyi, and the tetraploid and higher polyploid (2n = 20 and 38) NW North American M. ferruginea. An origin of the bulbil-reproducing M. foliolosa from the seed-reproducing M. stellaris has been suggested, but this is a provincial European viewpoint. Morphological features and molecular evidence both suggest a much closer relationship with M. redofskyi, possibly in combination with M. ferruginea (A. Tribsch pers. comm.).
Hultén (1968) described a Saxifraga (Micranthes) foliolosa var. multiflora from W Alaska, characterized vs. M. foliolosa s. str. by presence of multiple flowers besides bulbils. Such multi-flowered M. foliolosa is frequent on both sides of the Bering Strait as are plants with lower chromosome numbers (2n = 40, 48) than typical of M. foliolosa. These plants are intermediate between M. foliolosa and M. redofskyi. Recently a similar, multi-flowered plant (Micranthes foliolosa var. sisimiutii Wilken & Jürgensen) has been described from W Greenland (Wilken & Jürgensen 2013), just at the northern boundary of M. stellaris in Greenland (Sisimiut). Both these multi-flowered plants distinctly suggest that M. foliolosa has a hybrid origin, that the parentage may be complicated and involve several parental species, and that the transition to bulbil reproduction has been a success as M. foliolosa is much more widespread (circumpolar) than any of its suspected parental relatives.
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.
Cooper, E.J., Alsos, I.G., Hagen, D., Smith, F.M., Coulson, S.J. & Hodkinson, I.D. 2004. Recruitment in the Arctic: diversity and importance of the seed bank. – Journal of Vegetation Science 15: 115–124.
Elven, R., Murray, D.F., Razzhivin, V. & Yurtsev, B.A. (eds.) 2011. Annotated Checklist of the Panarctic Flora (PAF) Vascular plants. http://panarcticflora.org/
Hultén, E. 1968. Flora of Alaska and Yukon. – Arkiv för Botanik, ser. 2, 7(1). 147 pp.
Wilken, L.R. & Jürgensen, T. 2013. Micranthes foliolosa nov. var. sisimiutii from Sisimiut, Greenland. – Books on Demand GmbH, Copenhagen.