Mat-forming herb with an extensive, branched rhizome system; rhizome branches very slender (0.5–1 mm thick) with pairs of scaly, white hyaline leaves. Shoot system extensively branched at ground level into numerous (often hundreds) erect shoots, vegetative or flowering, at base with dense, protective covers of dead, straw-coloured to whitish marcescent leaves from previous years. Stems to 10–15 cm or occasionally more in sheltered or manured sites, with (3)5–10 pairs of leaves and with dwarf shoots (with densely crowded leaves) in their axils; stems angular with 4 pale ribs, otherwise green, reddish or covered with bluish bloom (wax), glabrous or occasionally pubescent on the youngest parts.

Leaves opposite, crowded on the proximal parts of a shoot, more distant on the distal parts, 5–10 × 2–4 mm, usually shorter than the internodes on the distal parts, without sheaths or petioles, ovate or lanceolate, acute, fleshy, uncommonly stiff (almost pungent) with strengthened and pale mid vein and margins, green or bluish green, very shiny or with a bloom.

Inflorescence a single flower or a dichasial cyme with 2–5 flowers. Bracts either like stem leaves and shiny green without hyaline margin, or smaller, more dull, often tinged purplish and with a white hyaline margin, or almost entirely hyaline, glabrous or more rarely ciliate.

Pedicels long, 3–5(8) cm, often much longer than the upper internodes (note the scientific name 'longipes' meaning on a long 'foot'). Flowers radially symmetric with 5 free sepals and petals. Sepals 2.5–4.0 × 0.8–1.2 mm, narrowly triangular or lanceolate, subacute, green or tinged purplish with a white hyaline margin, glabrous or rarely ciliate. Petals 5–7 mm long, twice as long as sepals, deeply cleft (almost to base) with two lobes 1–2 mm broad, translucent white. Stamens 10; filaments ca. 3 mm; anthers ca. 1 mm, red. Gynoecium of 3 carpels with 3 styles, with one room.

Fruit a brown or black capsule that opens apically with 6 teeth, with numerous seeds.

Sexual reproduction by seeds; local vegetative reproduction by rhizomes, possibly also by detached short shoots from leaf axils (bulbils). Morton (2005) reports the species to be self-compatible but with pollination usually performed by small flies. Flowering and seed set is irregular in Svalbard; very many stands are non-flowering, and even when flowering fully developed capsules are not seen regularly. The reasons are probably not climatic but genetic; the species is extremely polymorphic and also extremely varied as to ploidy levels, from diploid (2n = 26, × = 13) to ca. octoploid (2n = ca. 104). Svalbard plants are counted as octoploid but there are very few counts. We assume that there is much genetic imbalance and sterility, compensated for by very efficient ways of vegetative persistence and possibly dispersal.

The rhizomes are very brittle and parts may easily be dispersed some distance by land-slides or water. We also wonder about the abundant dwarf shoots produced in the leaf axils. These break off very easily (so easily that they often are lost in herbarium plants), and we assume that they can root and function as vegetative diaspores, even if we find no mentioning of this in authoritative surveys of the species (e.g., Morton 2005). Whether seed dispersal is efficient in Svalbard is unknown.

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 longipes s. lat. can never be confused with the other native species of Stellaria in Svalbard, S. humifusa. Stellaria longipes s. lat. has long, stiff, very narrow and acute leaves and mostly long-pedunculate flowers. In addition, it grows in heaths and meadows with subterranean branching only, whereas S. humifusa is restricted to seashores and with the main branching on the surface (i.e., above-ground).

Growing in a very wide range of vegetation types, from heaths and meadows to patterned ground (common both on soil polygons and in the mossy rims between polygons), screes and landslides, and in manured sites (bird cliff meadows, by cabins and in settlements). The substrate may be fine-grained or coarse-grained, dry or moist. It is one of the most wide-spanning of all Svalbard plants, only missing from wet vegetation types. It seems to be indifferent as to soil reaction (pH).

Usually common in all zones and sections. Common on Spitsbergen and the other larger islands, on many smaller islands, but not recorded from Hopen. Obviously absent from Bjørnøya.

The global range of S. longipes s. lat. is broadly circumpolar, reaching well south in the forest lands of North America and reaching the C Asian mountains. In Europe, its southern limit is found in some small and very isolated populations in the Dovrefjell mountains in S Norway.

Stellaria longipes s. lat. is one of the most intricate plants in the Arctic. Previous authors, and also many modern ones, have considered it a large group of species, especially those authors with much field experience from the Arctic. Several North American authors more concerned with the genetic variation found in laboratories have rather merged the entire group into a single, polymorphic species, for which the priority name will be S. longipes. The solutions of these two schools have not yet been critically compared. Field botanists still tend to see interesting variation where laboratory botanists see none.

There are several important contributors to the several-species model. An early one is Hultén (1943), recognizing eight species with strongly overlapping geographical ranges (i.e., ranges that are parapatric or sympatric), and assigning the Svalbard plants to S. crassipes Hultén. His approach was followed by Porsild (1963) and Porsild & Cody (1980) detailing this treatment for North America, Böcher (1951), Philipp (1972), and Böcher et al. (1978) for Greenland, and Matzenko (1971), Antonova & Petrovsky (1986), and Vlasova (1993) for the Russian areas. The diagnostic differences reported by these authors are mainly found in the absence or presence of pubescence on stems and sometimes leaves, whether the plants are bluish green (with bloom) or not, leaf shape, whether the bracts in the inflorescence are hyaline throughout, hyaline-margined, or not hyaline at all, numbers of flowers in the inflorescence, pedicel length, sepal shape, degree and location of pubescence on sepals, capsule colour, and seed surface features. Such characters are considered diagnostic among species elsewhere in Stellaria. This species aggregate approach was until the 1990s applied in all major floristic surveys for all regions.

For Europe, Chater & Heywood (1993) accepted three species: S. longipes, S. crassipes and S. ciliatosepala (probably the same as S. edwardsii), all three as possibly present in Svalbard. These authors, however, indirectly referred to the other approach in the following comment: "The three species ... are morphologically quite distinct in Europe, but not in North America, and a satisfactory interpretation of the group cannot be made without further investigation throughout the range". We have surveyed the Svalbard material and find that it would be possible to recognize two, possibly three species, following the key characters emphasized by, e.g., Hultén (1943), Porsild (1963), Porsild & Cody (1980), and Böcher et al. (1978). If we follow the key of Porsild & Cody (1980), the Svalbard plants fit into S. edwardsii (ciliate sepals and bracts, bracts with hyaline margins), S. longipes (glabrous sepals and bracts, narrow leaves, many-flowered inflorescences, black capsules), and S. crassipes (glabrous sepals and bracts, short and broad leaves, one-flowered inflorescences, and brown capsules). The same would be the result if we applied the key of Böcher et al. (1978). As to the comment that the variation is less clear-cut in North America than in Europe, we (Elven & Solstad) have observed similar distinctive variation in morphology among North American (and Greenland) plants as we have among Svalbard and Russian plants.

From the 1970s onwards, Chinnappa, Morton, and collaborators have studied morphology, cytology, reproduction, and isoenzyme variation in the complex, especially in North America (e.g., Chinnappa & Morton 1974, 1976, 1984, 1991; Chinnappa 1985; MacDonald & Chinnappa 1988). These authors have found (according to the summary by Borgen & Often 2001): "great phenotypic plasticity, low correlation between cytotype and morphology, and high fertility [in crosses?] among cytotypes ... It seems therefore appropriate to treat the complex as one polymorphic species". They assumed the complex to be a polyploid reticulum based on several parental diploids (diploid S. longipes s. str. one of them). This approach has been followed in later floristic surveys (e.g., Borgen & Often 2001 for NW Europe; Morton 2005 for North America; Brysting et al. 2007 for the Canadian Arctic Archipelago). We also follow this approach here, with some doubts, and prefer to wait until the controversy between morphology and genetics is better resolved.

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