Mat-forming herb with an extensively branched shoot system, below and above ground, rooting, with branches 0.5–1 mm wide. Aerial stems procumbent, often with dwarf shoots (very short shoots with numerous, small leaves) in the leaf axils. Flowering shoots short, 0–1.5 cm tall, with several pairs of leaves. Glabrous.
Leaves opposite, dense on shoots (short internodes), 0.3–1.0 cm long, without sheaths or petioles, broadly elliptic or ovate, acute with a small blunt and often colourless tip, 1-veined, flat, fleshy, green. Dead (marcescent) leaves persist over winter, often well into the next season.
Flowering shoots with one flower, rarely 2–3 in a reduced dichasial cyme.
Pedicels very short, 3–10 mm. Flowers radially symmetric, with 5 free sepals and petals. Sepals 4–6 × 1.2–1.4 mm, lanceolate and subacute, green or tinged purplish with a broad, white hyaline margin. Petals 5–7 × 3–6 mm, longer than sepals, obovate in outline, deeply cleft (almost to the base), white. Stamens 10. Gynoecium of 3 carpels with 3 styles, with one room.
Fruit a capsule that opens apically with 6 teeth, with numerous seeds.
Sexual reproduction by seeds; vegetative reproduction by rooting procumbent shoots and detached parts of shoots, probably also dwarf shoots in leaf axils that may function as bulbils. The plant flowers profusely in Svalbard. The large mats of white flowers are probably attractive to insects, when present, and at least some insect pollination is assumed. It sets fruit regularly and seeds germinate to 94 % in an experiment (Alsos et al. 2013).
We suspect that shoot fragments and dwarf shoots is a main way of propagation in Svalbard and that these diaspores are dispersed with sea currents. Seeds are probably also dispersed mainly by sea currents.
The deeply cleft petals distinguish Stellaria from the other genera of Caryophyllaceae in Svalbard (Cerastium has shallowly cleft petals, the other genera with free sepals and petals have entire petals).
Stellaria humifusa can never be confused with the other native species (or species group) of Stellaria in Svalbard, the S. longipes group. Stellaria longipes s. lat. has long, stiff, very narrow and acute leaves and mostly long-pedicellate flowers. In addition, it grows in heath and meadow with subterranean branching only, whereas S. humifusa is restricted to seashores and with the main branching on the surface (i.e., above-ground).
Restricted to salt marshes and other seashore sites with fine-grained, regularly inundated substrates. The species is one of the most regular inhabitants of such site types in Svalbard.
Present in all zones and in the transitional to clearly continental sections. Frequent in seashore sites on Spitsbergen north of Van Keulenfjorden and known from Bjørnøya (fairly common along the coasts, see Engelskjøn & Schweitzer 1970), the northern parts of Prins Karls Forland, a few places on Nordaustlandet, and recorded from one locality on the southwestern part of Edgeøya.
Stellaria humifusa is a common seashore plant in the circumpolar Arctic, in Europe only reaching south to N Norway (perhaps extinct in NE Troms, common in Finnmark), N Russia, and previously to the innermost parts of the Bothnian Bay in the Baltic in Finland. It is more common along the North Pacific and there reaches much farther south than it does along the Atlantic coasts, for unknown reasons (see Elven et al. 2011).
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/
Engelskjøn, T. & Schweitzer, H.J. 1970. Studies on the flora of Bear Island (Bjørnøya). I. Vascular plants. – Astarte 3: 1–36.