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Mat-forming herb with thin rhizomes, long, procumbent leafy shoots with short, possibly articulate, eglandular hairs, mostly on one side, and flowering stems ascending from procumbent shoots, up to 2–7 cm with 1–3 pairs of leaves.
Leaves opposite, 4–7 × 1–2 mm, narrowly oblong, obtuse, evenly spaced along shoots, upturned on procumbent shoots, glabrous or more often with short hairs at base.
Inflorescences terminal, of a single flower or 2–3 in a dichasial cyme. Pedicels with dense pubescence of very short, glandular hairs on one side.
Flowers radially symmetric with 5 free sepals and petals. Sepals 4–6 × 1.2–1.4 mm, narrowly ovate or lanceolate, obtuse or subacute, with glandular hairs at base, green or purplish green in the middle part with distinct hyaline margins on one or both sides. Petals 6–8 × 2–3.5 mm, ca. 1.5 times as long as sepals, narrowly spathulate, distinctly cleft, translucent white ("skimmed milk"). Stamens 10. Gynoecium of 3 carpels with 3 styles.
Fruit a capsule, 6–10 × 2.5–4 mm, fusiform, straight, ca. 2 times as long as sepals when mature, opening by 6 teeth. Seeds numerous, with very low tubercles or ridges.
Sexual reproduction by seeds; local vegetative reproduction by rhizomes and rooting prostrate shoots. Flowers are adapted to insect pollination. Ripe capsules and seeds have been observed in a few collections from Bjørnøya.
There is no special adaptation to seed dispersal except that the capsule with its teeth inhibits spread of seeds unless it is moved, by wind or animals.
The species of Cerastium can be mistaken for those of the related genus Stellaria. Cerastium has petals cleft at most to 25 %, whereas Stellaria has petals split nearly to the base (or, in species not occurring in Svalbard, without petals). When in fruit, Cerastium has a fusiform to nearly cylindrical capsule opening by 10 or rarely 6 teeth (Cerastium cerastoides), whereas Stellaria has a subglobular to short cylindrical capsule opening by 6 teeth.
Cerastium cerastoides differs from C. arcticum, C. alpinum and C. regelii in several characters: It has three styles, a more or less straight capsule opening by 6 teeth, shoots are procumbent with hairs only on one side and leaves upturned, and the petals are translucent white (“skimmed milk”). The others have five styles, a capsule with more or less skewed top opening by ten teeth, the shoots are not long and procumbent and they have hairs on all sides and the leaves not upturned, and petals are non-translucent white (“whole milk”). The proposal to recognize C. cerastoides in its own genus, Dichodon, has some merit.
Moist snowbeds and depressions with seepage, often associated with moss mats, and occurring together with Oxyria digyna, Ranunculus pygmaeus and Saxifraga cernua. Grows in substrates such as coarse sand to stones. Indifferent as to soil reaction (pH).
Thermophilous. In Svalbard only known from Bjørnøya and thereby confined to the middle arctic tundra zone and the weakly oceanic section. Relatively frequent on Bjørnøya and found both in the northern and southern parts.
The general range is amphi-Atlantic, European and C Asian, reaching the Arctic in NE Canada, Greenland, the North Atlantic islands, and the European mainland in Norway and Russia.
Cerastium cerastoides has sometimes been assigned to another genus, Dichodon, distinguished from Cerastium by, e.g., the three styles (and carpels) and a different basic chromosome number (x = 19 vs. x = 18). The results of a phylogenetic study of the genus (Scheen et al. 2003) gives some support to this suggestion as the group of C. cerastoides and its few relatives is ‘sister’ to all other Cerastium. It also means that it is taxonomically isolated in Svalbard, whereas the other species of Cerastium in the islands are closely related and assumed or proven to hybridize, sometimes to a degree that obscures the limits between species. However, hybrids or at least assumed hybrids between C. cerastoides and species of the C. alpinum group are reported from Scandinavia.
Scheen, A.-C., Brochmann, C., Brysting, A.K., Elven, R., Morris, A., Soltis, D.E., Soltis, P.S. & Albert, V.A. 2003. Northern Hemisphere biogeography of Cerastium L. (Caryophyllaceae): Insights from phylogenetic analysis of non-coding plastid nucleotide sequences. – American Journal of Botany 91: 943–952.