Solitary herb with thin pale rhizomes branching below ground level, growing in dense or diffuse tussocks. Aerial shoots ascending to erect with leafy flowering stems up to 10–15 cm. Stems, leaves and calyces with long (more than 0.5 mm), articulate, often gland-tipped, translucent hairs. Short hairs (less than 0.3 mm) absent.

Leaves opposite, up to 25 × 8 mm, narrowly oblong, obovate or oblanceolate, subacute, concentrated at base of shoots with more distant and smaller leaves upwards on flowering stems. Hairs on lower leaves 0.7–3.4 mm long, predominantly 4–12-celled and predominantly eglandular; hairs on upper leaves shorter (0.5–3.0 mm), 3–10-celled and both glandular and eglandular.

Flowering stems with a single terminal flower or a few-flowered dichasial cyme with moderately to widely diverging branches (at most 80–100º). Bracts small, green, hyaline margin absent or inconspicuous. Pedicels densely covered on all sides with long, usually glandular hairs.

Flowers radially symmetric with 5 free sepals and petals. Calyx broadly obconical or urn-shaped in outline, base of calyx rounded to obtusely angled. Sepals (3)4–10(12) × (1.5)2–3(4) mm, varying very much in size and also in shape from narrowly triangular to lanceolate or ovate, obtuse, subacute or slightly lacerate; the central part is green with dense glandular hairs, the margins broadly hyaline, especially apically. Petals 6–10 × 3–5 mm, ca. 1.5 times as long as sepals, spathulate, distinctly cleft, white. Stamens 10 (number sometimes reduced). Gynoecium of five carpels with 5 styles.

Fruit a capsule, 7–15 × 3–5 mm, about 1.5 times as long as sepals when mature, urn-shaped, slightly skewed at apex, opening by 10 teeth. Seeds numerous, with high, acute tubercles.

Sexual reproduction by seeds; no vegetative reproduction. Flowers are adapted to insect pollination. Plants flower regularly, sometimes 2–3 times during a single season, and set an abundance of mature seed. Seeds germinated to 34 % in an experiment (Alsos et al. 2013).

There is no special adaptation to seed dispersal.

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 arcticum, C. alpinum, and C. regelii differ from C. cerastoides in several characters: by having five styles, a capsule with more or less skewed top opening by ten teeth, by the shoots not long and procumbent, with hairs on all sides and the leaves not upturned, and petals non-translucent white (“whole milk”). Cerastium cerastoides 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 proposal to recognize C. cerastoides in its own genus, Dichodon, has some merit.

Cerastium arcticum, C. alpinum, and C. regelii are closely related in the so-called C. alpinum group (Hultén 1956; Böcher 1977; Brysting & Hagen 1999; Brysting & Borgen 2000; Brysting & Elven 2000; Brysting et al. 2007a, 2007b). Cerastium regelii differs in its leaves being glabrous or nearly glabrous, almost succulent, and nearly orbicular. This plant usually grows in more or less compact tussocks or mats of shortened vegetative shoots, and flowering is normally restricted to only some plants in a population and mainly at the end of the season. The two other species have much more elongate leaves, are always densely hairy, and they flower profusely in most plants of the populations throughout the season.

Cerastium alpinum and C. arcticum may be very difficult to keep apart, and the Svalbard material has not been sufficiently revised. Reported differences are that C. arcticum only has the long hairs (more than 0.5 mm) on leaves and stems, whereas C. alpinum has an admixture of such long and much shorter hairs (less than 0.3 mm); that the bracts on the flowering stems are without distinct hyaline margins in C. arcticum, whereas distinct hyaline margins nearly always are present in those of C. alpinum; that the base of the calyx in C. arcticum is rounded to obscurely angled, whereas that of C. alpinum is more sharply angled; and that the capsule of C. arcticum is urn-shaped, whereas that of C. arcticum is more cylindrical and narrower.

Common in a very wide range of dry to moist vegetation types, both on open ground and in closed heaths, meadows and early snowbeds; common on raised river terraces and river bars with intermittent inundation; in bird cliff meadows and vegetated parts of scree; common in polar desert vegetation. Indifferent as to soil fraction size and reaction (pH).

Common in all zones and sections, reported from all larger islands, including Bjørnøya, and also present on a majority of the visited smaller islands. Cerastium arcticum is among the five most common and widely distributed of Svalbard vascular plants.

The general range is broadly amphi-Atlantic and includes N Canada, Greenland, Jan Mayen, Svalbard, Franz Joseph Land, and Novaya Zemlya.

Cerastium arcticum is extremely polymorphic and has been recognized as such for a long time. Several attempts have been made to subdivide it on two or more species, subspecies or varieties (e.g., Tolmachev 1930; Hultén 1956; Böcher 1977). It is a hexaploid (2n = ca. 108), whereas the related C. alpinum and C. regelii both are tetraploids (2n = 72). Hultén (1956) considered this C. alpinum aggregate a case of worldwide (ongoing) hybridization and introgression, whereas Böcher (1977) regarded it to be a mature polyploid complex.

Until quite recently, the plants in Scandinavia, Iceland, Svalbard, and Greenland were assigned to one species as C. arcticum. Molecular and morphological studies (Brysting & Hagen 1999; Brysting & Borgen 2000; Brysting & Elven 2000; Brysting et al. 2007a, 2007b) have produced another model for the group. Their data support a separation on two species: one in the Atlantic regions (Scandinavia, the British Isles, Iceland) and another in the arctic regions (Svalbard, Greenland, and also Canada). The name C. arcticum is based on Greenland plants and is the priority name for the arctic species. The Atlantic species is C. nigrescens (H.C.Watson) Edmondston ex H.C.Watson. These studies also suggest that C. arcticum and C. nigrescens in their parentage have C. alpinum as a common progenitor, whereas the other progenitors differ. Cerastium nigrescens has in its parentage some plants from C European mountains, missing in the parentage of C. arcticum which rather may have the broadly Beringian C. beeringianum Cham. & Schltdl. in its parentage.

The molecular studies included a high number of plants from Svalbard, covering all the ‘races’ proposed by Hultén and Böcher. These studies did not support any meaningful subdivision of C. arcticum in Svalbard or elsewhere. The variation is real, and also to a certain degree retained during cultivation under uniform conditions, but it may be impossible to handle taxonomically. As for the four varieties proposed by Hultén, the three varieties included in the synonymy above – arcticum, procerum, and vestitum – grow intermixed on the Svalbard islands (and elsewhere), often within populations, whereas the fourth variety – var. sordidum Hultén as identified by Hultén in the herbaria – also occurs with the same lack of pattern. The type of this last-mentioned name is, however, based on a Svalbard plant which rather belongs to the hybrid C. arcticum × regelii.

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.

Böcher, T.W. 1977. Cerastium alpinum and C. arcticum, a mature polyploid complex. – Botaniska Notiser 130: 303–309.

Brysting, A.K. 2000. Chromosome number variation in the polyploid Cerastium alpinum – C. arcticum complex (Caryophyllaceae). – Nordic Journal of Botany 20: 149–156.

Brysting, A.K., Aiken, S.G. & Scott, P.J. 2007a. Caryophyllaceae. – In: Aiken, S.G. (ed.) et al., Flora of the Canadian Arctic Archipelago: Descriptions, illustrations, identification, and information retrieval. – [CD-ROM version] National Research Council of Canada, Ottawa.

Brysting, A.K. & Borgen, L. 2000. Isozyme analysis of the Cerastium alpinum – C. arcticum complex (Caryophyllaceae) supports a splitting of C. arcticum Lange. – Plant Systematics & Evolution 220: 199–221.

Brysting, A.K. & Elven, R. 2000. The Cerastium alpinum – C. arcticum complex (Caryophyllaceae): numerical analysis of morphological variation and a taxonomic revision of C. arcticum Lange s. lat. – Taxon 49: 189–216.

Brysting, A.K. & Hagen, A. 1999. Species in polyploid complexes? The Cerastium alpinum – C. arcticum complex. – Skrifter Norske Videnskabs-Akademi Oslo, I. Matematisk–Naturvidenskapelig Klasse, n. s. 38: 183–190.

Brysting, A.K., Oxelman, B., Huber, K.T., Moulton, V. & Brochmann, C. 2007b. Untangling complex histories of genome mergings in high polyploids. – Systematic Biology 56: 467–476.

Hultén, E. 1956. The Cerastium alpinum complex. A case of worldwide introgressive hybridization. – Svensk Botanisk Tidskrift 50: 411–495.