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Mat-forming herb with a branched, horizontal rhizome system some centimetres down in the substrate with vertical branches to aerial shoots, forming small or large, loose mats. The uppermost 1–2 cm of the rhizome branches are black, dull and transversely papillose or rugose. Aerial shoots often in large, dense bundles from the ends of rhizome branches, perennial (only one type of shoots), 5–8(12) cm long, 0.5–1 mm broad, procumbent, ascending or erect, unbranched (above rhizome), terminating in a spike with sporangiophores (lost on older shoots). Stems dark green, articulate, angled with 6 sharp ribs (in three pairs of two), strongly rugose by silica tubercles, 3 leaves at the end of each joint.
Leaves reduced to a fused sheath with free teeth (as many as ribs) in whorls. Leaves initially green at base, with a broad, black band and a narrow, white hyaline margin. They are retained as long as the shoot lasts, then as a black sheath with white, withered teeth. The teeth are narrowly triangular, apiculate with a long extended, white hair point, especially in shoot apices, but the apical parts are lost with age.
INFLORESCENCE AND FLOWER
Spikes ovoid, 2–3 mm, initially enclosed within the uppermost whorls of leaves (teeth) and not much emerging from these, with sporangiophores (transformed leaves carrying sporangia) in ca. 3 dense whorls. Sporangiophores peltate with an angular end plate participating in a compact outer surface of the spike until the spores are mature and the spike opens with slits between the sporangiophores to let the spores loose. Each sporangiophore carries ca. 3 oblong sporangia in a whorl on the inner surface of the plate. Sporangia open by longitudinal slits. Spores globose, green, each with four long, club-shaped bands (hapters), stretching out in dry weather, making the spore mass fluffy and wind dispersed in such conditions. In moist weather the hapters curve inwards making the spore mass immobile.
Sexual reproduction by spores; moderately efficient local vegetative reproduction by rhizomes.
Efficient wind dispersal of the spores due to the hapters and the small size of the spores. Downstream dispersal of rhizome fragments is possible.
The genus Equisetum is immediately recognized by the articulate stems and branches and the characteristic spike with angled sporangiophores.
Equisetum arvense is distinguished from the two other species in Svalbard – E. scirpoides and E. variegatum – especially in that the former has annual dimorphic shoots (two different shoot generations), the two latter perennial monomorphic shoots. Even if this difference is very marked, it has often been overlooked and there have been numerous mistakes in identifications. Some additional characters should be consulted. Firstly, E. arvense has branched aerial shoots, mostly yellowish green, the two others simple shoots with dark green colour. Furthermore, the aerial shoots of E. arvense are soft, with intact whorls of teeth on the main stems, whereas those of E. scirpoides and E. variegatum are firm, with withered teeth (often as a bicolorous band in black and white).
Equisetum scirpoides and E. variegatum may be superficially similar but with some significant differences. The shoots of E. scirpoides are nearly always more slender (0.5–1 mm broad), more crowded in dense bunches from the rhizome branches, the shoots have 6 ribs or ridges in three pairs but only 3 teeth in each whorl, and (a most important character), the uppermost 1–2 cm of the black rhizome branches are dull and strongly transversely papillose or rugose. In E. variegatum the shoots are more stout (> 1 mm), have 4–8 broad ribs and 4–8 teeth, and the uppermost 1–2 cm of the black rhizome branches are shiny and smooth.
Most often found in moderately dry heaths and meadows with dense vegetation cover. The substrate is usually sand or gravel, rarely finer fractions (we suspect that the species do not tolerate water-logged ground). It seems to be largely indifferent as to soil reaction (pH) but seems to be absent from the areas with the most acidic substrates.
Found in all sections except the polar desert and in all sections. The species is not common anywhere but widespread on Spitsbergen from Sørkapp Land north to the north coast in Andrée Land and Ny-Friesland. It is also recorded from several localities on Bjørnøya.
Frequent throughout the arctic circumpolar and reaching south into mountains in the boreal zone, in Europe to S Norway.