Mat-forming dwarf shrub with extensive below and above ground branching resulting in large, often very dense stands up to 10–20(30) cm tall. Branching potentially at opposite leaves but often only one branch developed. Aerial branches densely covered by leaves.
Leaves opposite, 4–5 × 1.3–1.7 mm, evergreen, lasting for 4–5 years or more, appressed and densely imbricate on branches in four rows, thick, boat-shaped, with a very short petiole (ca. 1 mm) hidden inside the leaf cover, oblong to triangular, obtuse, with the mid vein impressed in a groove along the back, dark green, densely pubescent with very short hairs, young leaves sticky.
Flowers 2–4 together at the end of previous year's shoot, overtopped by the emerging current year's shoot.
Flowers nodding on a 7–14(16) mm, slender, red or yellowish pedicel curved at the end. Flowers radially symmetric with 5 free sepals and 5 fused petals. Sepals 1.8–2.2 × 0.8–1.2 mm, oblong, obtuse or slightly dentate at apex, whitish or pink. Corolla 6–8 × 6–8 mm, bell-shaped, fused for 2/3–3/4 of its length with 5 triangular petal lobes revolute at their ends, cream coloured. Stamens 5, ending ca. 5 mm inside the corolla; filaments 2–2.5 mm; anthers almost orbicular, ca. 1 mm, with two long spurs opposite to the two pores opening for pollen dispersal. Gynoecium of 5 carpels, with a 2 mm long, straight, slender style and stigma exposed near the mouth of the corolla.
Fruit a nearly globular capsule, 2.5–4 × 2.5–4 mm, becoming erect in fruit stage, opening by 5 apical slits into 5 valves, with numerous, very small seeds. The style remains on the capsule until seeds are dispersed.
Sexual reproduction by seeds; very restricted and local vegetative reproduction by layering. Flowers are strictly adapted to insect pollination. The spurs on the anthers function as levers; the anthers only empty pollen through the pores when an insect moves the spurs. Seed set is regular in Svalbard, vouching for efficient insect pollination. Seeds germinate to ca. 8 % in an experiment (Alsos et al. 2013); they probably depend on some interaction with fungi for efficient germination (mycotrophic).
Seeds are very small and light and dispersed by wind.
Cassiope tetragona is not similar to any other plant in Svalbard. Its nearest relative there is Harrimanella hypnoides, having similar but smaller bell-shaped, mainly white flowers. However, C. tetragona has much coarser and ascending to erect shoots with much coarser leaves organized in 4 rows, and the flowers are lateral on the shoots (not apical as in Harrimanella) and with revolute, cream coloured petal lobes (straight and often pink in Harrimanella).
A dominant species in heaths where frost movements of the ground is limited. Often growing on slopes with good drainage and some snow protection during winter. The range suggests that it is slightly basiphilous, not present in the areas in the south and west with the most acidic substrates.
Moderately thermophilous. In the middle arctic tundra zone and the clearly and weakly continental sections, slightly transgressing into the transitional section. Cassiope tetragona is present only on Spitsbergen and only north of Van Mijenfjorden. It is rather common in the areas around Isfjorden, Kongsfjorden, Liefdefjorden, Woodfjorden, and Wijdefjorden, with a few sites at Sorgfjorden and Lomfjorden (NE Spitsbergen, Ny-Friesland).
Globally, this is one of the more widespread of arctic plants, present and usually common on all northern continents, in Europe reaching south to N Fennoscandia. See also Comments.
Cassiope tetragona in Svalbard and throughout the Arctic belongs to ssp. tetragona with extended flower pedicels. Subspecies saximontana (Small) A.E.Porsild with very short-stalked flowers is distributed in the Rocky Mountains and reaches north to the non-arctic parts of Alaska and Yukon. A study based on AFLP markers (Eidesen et al. 2007) suggested that ssp. saximontana is 'sister' to the circumpolar ssp. tetragona. The study also suggested some interesting phylogeographic patterns concerning the Asian or Beringian origin of this circumpolar plant. All other species of the genus are confined to the North Pacific regions and E Asia (mainly China).
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.
Eidesen, P.B., Carlsen, T., Molau, U. & Brochmann, C. 2007. Repeatedly out of Beringia: Cassiope tetragona embraces the Arctic. – Journal of Biogeography 34: 1559–1574.