11. Polygonum aviculare Linnaeus, Sp. Pl. 1: 362. 1753.
Doorweed, knotgrass, renouée des oiseaux
Plants green or bluish green, green after drying, sometimes whitish from powdery mildew, homophyl-lous or heterophyllous. Stems prostrate to erect, branched, flex-uous, 5-200 cm. Leaves: ocrea 3-15 mm, proximal part cylindric or ± funnelform, distal part silvery, hyaline, soon disintegrating into persistent fibers or nearly completely deciduous; petiole 0.3-9 mm; blade green to gray-green, narrowly elliptic, lanceolate, elliptic, obovate, or spatulate, 6-50(-60) × 0.5-22 mm, margins flat, apex acute, obtuse, or rounded; stem leaves 1-4 times as long as adjacent branch leaves; distal leaves overtopping flowers. Inflorescences axillary; cymes uniformly distributed or aggregated at tips of stems and branches, 1-6(-8)-flowered. Pedicels enclosed in or exserted from ocreae, 1.5-5 mm. Flowers closed or semi-open; perianth 1.8-5.5 mm; tube 20-57% of perianth length; tepals overlapping or not, green or reddish brown with white, pink, or red margins, petaloid, not keeled, oblong to obovate, often cucullate in fruit; midveins branched or unbranched, thickened or not; stamens 5-8. Achenes enclosed in or exserted from perianth, light to dark brown, ovate, (2-)3-gonous, 1.2-4.2 mm, faces subequal or unequal, apex not beaked, edges slightly concave, dull, usually coarsely striate-tubercled, sometimes obscurely tubercled; late-season achenes common or not, 2-5 mm.
Subspecies 7+ (6 in the flora): nearly worldwide.
Polygonum aviculare is a taxonomically controversial polyploid complex of selfing annuals. Although members of the complex have been considered inbreeders, they possess some structures that make cross pollination possible. Cleistogamous and chasmogamous flowers, heterostyly, protandry, and the capacity to secrete nectar suggest an ancestral mixed-mating system. Isoenzyme studies showed that the complex has an allopolyploid origin (P. Meerts et al. 1998) and has evolved as a swarm of inbreeding lines (“Jordanons”) (J. Gasquez et al. 1978). The six subspecies included here have been treated variously (T. Karlsson 2000; M. Costea and F. J. Tardif 2003). Complex intergradation patterns among them make their recognition at the species level impractical. Multivariate analysis and isoenzyme studies show that populations with intermediate characteristics may occur (Meerts et al. 1990, 1998). Except for subsp. boreale, which occurs in Greenland and Labrador, all subspecies are partially sympatric and their distributions have been influenced greatly by humans.
Meerts, P., T. Baya, and C. Lefèbvre. 1998. Allozyme variation in the annual weed species complex Polygonum aviculare (Polygonaceae) in relation to ploidy level and colonizing ability. Pl. Syst. Evol. 211: 239-256. Meerts, P., J.-P. Briane, and C. Lefèbvre. 1990. A numerical taxonomic study of the Polygonum aviculare complex (Polygonaceae) in Belgium. Pl. Syst. Evol. 173: 71-90. Styles, B. T. 1962. The taxonomy of Polygonum aviculare and its allies in Britain. Watsonia 5: 177-214.