More or less solitary herb with thin pale branched rhizomes below ground level, growing in diffuse tussocks. Aerial shoots ascending to erect with leafy flowering stems up to 20 cm. Stems, leaves, and calyces with a characteristic combination of both short (less than 0.3 mm) and long (more than 0.5 mm), articulate, translucent, often gland-tipped hairs.

Leaves opposite, up to 20 × 8 mm, narrowly oblong, elliptic or oblanceolate, subacute, concentrated at base of shoots with more distant and smaller leaves upwards on flowering stems. Lower leaves with the long hairs 0.8–3.2 mm, predominantly 6–17-celled, and eglandular; upper leaves with the long hairs slightly shorter (0.5–2.8 mm), 5–12-celled, and both glandular and eglandular. On upper leaves and stems mixed with the short hairs (less than 0.3 mm), sometimes also on lower leaves.

Flowering stems with a single terminal flower or a few-flowered dichasial cyme with moderately to widely diverging branches (rarely as much as 80–100º). Bracts small, green with a distinct white, hyaline margin broadening apically. Pedicels densely covered on all sides with both long, usually glandular hairs, and short hairs.

Flowers radially symmetric with 5 free sepals and petals. Calyx narrowly obconical in outline; base of calyx truncate, often quite sharply angled (more rounded in C. arcticum). Sepals (3)4–9 × 1.5–3 mm, narrowly triangular or lanceolate, subacute to acute or slightly lacerate, green in the central part but with a broad hyaline margin, especially apically, the green part with sparse or moderately dense glandular hairs. Petals 7–12 × 3–6 mm, ca. 1.5 times as long as sepals, spathulate, distinctly cleft, white. Stamens 10 (number sometimes reduced). Gynoecium of five carpels with 5 stigmas.

Fruit a single-room capsule, 7–15 × 2.6–4.0 mm, 1.5–2 times as long as sepals when mature, nearly cylindrical but slightly skewed at apex, opening by 10 teeth. Seeds numerous, with high tubercles.

Sexual reproduction by seeds; no efficient vegetative reproduction. Flowers are adapted to insect pollination. The plant flowers and probably sets seed regularly.

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.

The few plants in Svalbard confirmed as Cerastium alpinum are found in meadow and heath types of vegetation on climatically favourable slopes. From other places we know that this species prefers dry sites with well-drained substrates (coarse sand to stones) but that it is largely indifferent as to soil reaction (pH).

Probably thermophilous. Perhaps found in a few sites along the west coast of Spitsbergen from Hornsund (Sørkapp and Wedel Jarlsberg Lands) north to Isfjorden (Colesbukta, Nordenskiöld Land) and perhaps Kongsfjorden (Oscar II and Haakon VII Lands), in the middle and perhaps northern arctic tundra zones and in the weakly oceanic and transitional sections.

The global range is amphi-Atlantic in at least three races (see Comments). Cerastium alpinum ssp. alpinum may be European only, mainly Fennoscandian but reaching Iceland and perhaps Svalbard; ssp. glabratum is Fennoscandian and does not reach north of the European mainland; ssp. lanatum is the most widespread race in Europe from the S European mountains north to northernmost Fennoscandia and arctic Russia, across the Atlantic to Iceland, S and W Greenland, and NE Canada.

For more comments to the C. alpinum aggregate, see C. arcticum.

Cerastium alpinum is tetraploid (2n = 72; see Elven et al. 2011), as is the morphologically rather different C. regelii, whereas the more similar C. arcticum is at least predominantly hexaploid (2n = 108). Cerastium alpinum is polymorphic and has been described with three races (subspecies or varieties) or species: ssp. alpinum, ssp. glabratum (Hartm.) Á.Löve & D.Löve (var. glabrum Wahlenb.; C. glabratum Hartm.), and ssp. lanatum (Lam.) Asch. & Graebn. (var. lanatum (Lam.) Hegetschw.; C. lanatum Lam.). These three races were accepted by, e.g., Hultén (1956) and Jonsell et al. (2001). A morphological study did not show very consistent differences (Grundt et al. 2000), and neither did subsequent molecular works (Brysting & Borgen 2000; Brysting et al. 2007a). The extremes in morphology are rather distinctly different but the number of transitional forms is high, especially between ssp. alpinum and ssp. glabratum.

The arctic plants have, since Hultén (1956), been assigned to ssp. lanatum. The original C. lanatum of Lamarck was described from the Alps and is a rather southern plant, common in warm cliffs in the Scandinavian mountains and also in similar sites in Iceland, Greenland, and parts of Canada (see Distribution), but it is significantly different from the plants in more northern parts of Greenland, on Jan Mayen, and in Svalbard. The northernmost plants assigned to C. alpinum rather resemble ssp. alpinum but may belong to a yet undescribed arctic race.

The presence of C. alpinum in Svalbard is supported by a chromosome count of 2n = 72 from the Hornsund area (Sørkapp Land: Sergeijevfjellet; Dubiel 1990) and by a few herbarium specimens suggestive of this species.

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

Brysting, A.K., Aiken, S.G. & Scott, P.J. 2007. 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 Videnskaps-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.

Dubiel, E. 1990. Vascular plants of the NW Sørkapp Land (Spitsbergen). Distribution and habitats. – Zeszyty Naukowe Uniwersitetu Jagiellónskiego, Prace Botaniczne (Cracow) 21: 7–33.

Elven, R., Murray, D.F., Razzhivin, V. & Yurtsev, B.A. (eds.) 2011. Annotated Checklist of the Panarctic Flora (PAF) Vascular plants. http://panarcticflora.org/

Grundt, H.H., Borgen, L. & Elven, R. 2000 [2001]. Infraspecific variation in Cerastium alpinum L. s. l. (Caryophyllaceae). – Nordic Journal of Botany 20: 641–653.

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

Jonsell, B., Brysting, A. & Karlsson, T. 2001. Cerastium L. p.p. – In: Jonsell, B. (ed.), Flora Nordica. 2. Chenopodiaceae to Fumariaceae: 135–159.