Polygonum sect. Polygonum
Herbs, annual (perennial and rhizomatous in P. striatulum), not compact, not cushionlike, homophyllous or heterophyllous, frequently heterocarpic, rarely subsucculent (in P. marinense and P. fowleri). Stems prostrate, decumbent, or ascending to erect, straight (zigzag in P. fowleri and P. humifusum), distinctly and ± regularly 8–16-ribbed, smooth (papillose-scabridulous in P. plebeium). Leaves: ocrea 4–12-veined (1-veined in P. plebeium), proximal part not pruinose (pruinose in P. glaucum and P. oxyspermum); petiole articulated to proximal part of ocrea, when absent, blade is directly articulated to ocrea; blade linear to elliptic or obovate, rarely coriaceous (in P. glaucum), smooth (papillose in P. plebeium, rugulose and glaucous in P. glaucum); venation pinnate, secondary veins conspicuous. Inflorescences usually axillary, less commonly axillary and terminal; cymes 1–7(–10)-flowered. Pedicels erect to spreading, 0.5–7 mm. Flowers usually closed, sometimes semi-open; tepals ± monomorphic, outer tepals equaling or somewhat larger than inner, apices of outer tepals rounded; anthers whitish yellow. Achenes ovate to ovate-lanceolate, 2–3-gonous, shiny or dull, smooth to roughened or tubercled.
Distribution
Nearly worldwide.
Discussion
Species ca. 45 (13 in the flora).
Plants of sect. Polygonum produce two types of fruits (heterocarpy). Summer achenes are brown, ovate, and tubercled to smooth; late-season (autumn) achenes are hypertrophied, olivaceous, two to five times as long as the summer achenes, lanceolate, and smooth. The two types differ in their germination biology. Late-season achenes possess a low, innate dormancy and can germinate immediately after being produced, at 20–25°C (O. V. Yurtseva et al. 1999). Lower temperatures will delay germination until the following spring. Summer achenes are dormant when produced and undergo a cyclical dormant/nondormant pattern in the soil (A. D. Courtney 1968; J. M. Baskin and C. C. Baskin 1990).
Mature, early-season plants bearing leaves, flowers, and achenes are necessary for accurate determinations. Late-autumn specimens with hypertrophied achenes often are difficult or impossible to identify. In the descriptions, measurements of leaves, ocreae, and petioles are from the middle of the main stem. Leaf length includes the petiole. Heterophyllous taxa have cauline leaves at least three times as long as the branch leaves; homophyllous plants have stem and branch leaves about equal in size. Heterophylly is easily detected in most cases. However, some taxa show considerable variability in leaf morphology; identification should not rely on heterophylly alone. The character states “flowers closed” and “flowers semi-open” should be observed on herbarium specimens. Perianth description refers to the fruiting perianth measured from the pedicel joint to the apex of the tepals. Tepal descriptions refer to the outer three tepals (except for P. heterosepalum). Trigonous achenes have one face broader than the other two. Descriptions refer only to the two narrower faces, which can be subequal or distinctly unequal. Surface of achenes is best observed at magnifications of (or higher than) 100×. The achene surface may be: smooth, when no ornamentation is visible; roughened, when the tubercles are not discernible but the surface appears roughened; striate-tubercled, when conspicuous tubercles are oriented in rows; uniformly tubercled, when the tubercles are dense and rows difficult to distinguish; and obscurely tubercled, when the tubercles are weakly marked, inconspicuous, or restricted to a few (three to 15) rows on the central parts of faces. Late-season achenes in all species are hypertrophied, olivaceous, lanceolate, exserted, and smooth. They have little taxonomic significance.
Selected References
Lower Taxa
Key
| 1 | Plants perennial, rhizomatous | Polygonum striatulum |
| 1 | Plants annual, not rhizomatous | > 2 |
| 2 | Leaves in distal part of inflorescence reduced, not overtopping flowers (shorter than or equaling flowers); inflorescences axillary and terminal, spikelike | > 3 |
| 2 | Leaves in distal part of inflorescence overtopping flowers; inflorescences entirely axillary | > 5 |
| 3 | Achenes striate-tubercled | Polygonum patulum |
| 3 | Achenes smooth or roughened | > 4 |
| 4 | Margins of tepals pink, rarely red or white; achenes 1.3-2.3 mm | Polygonum argyrocoleon |
| 4 | Margins of tepals greenish yellow or yellow, rarely pink or white; achenes (2.3-)2.5-3.5 mm | Polygonum ramosissimum |
| 5 | Proximal parts of ocreae pruinose | > 6 |
| 5 | Proximal parts of ocreae not pruinose | > 7 |
| 6 | Distal parts of distal ocreae silvery, persistent; leaf blades glaucous, rugulose; perianth 2-3(-4) mm | Polygonum glaucum |
| 6 | Distal parts of ocreae hyaline, soon disintegrating into fibers or deciduous;leaf blades not glaucous, not rugulose; perianth 3.5-5.5 mm | Polygonum oxyspermum |
| 7 | Achenes striate-tubercled, uniformly tubercled, or obscurely tubercled | > 8 |
| 7 | Achenes smooth to roughened | > 13 |
| 8 | Plants green to bluish green; margins of tepals white, pink, or red | > 9 |
| 8 | Plants light green or yellowish; margins of tepals green to yellow | > 11 |
| 9 | Achenes coarsely striate-tubercled | Polygonum aviculare |
| 9 | Achenes obscurely tubercled | > 10 |
| 10 | Plants dark brown to black after drying; distal part of ocreae dis-integrating into persistent fibers, brown | Polygonum ramosissimum |
| 10 | Plants green after drying (sometimes whitish from powdery mildew); distal part of ocreae persistent, silvery | Polygonum aviculare |
| 11 | Perianth tube 40-55% of perianth length; tepals ± keeled; pedicels 1.3-1.8(-2) mm, enclosed in ocreae | Polygonum achoreum |
| 11 | Perianth tube 20-38% of perianth length; tepals not keeled; pedicels 2-7 mm, exserted from ocreae | > 12 |
| 12 | Leaf blades elliptic to obovate; distal parts of ocreae ± persistent, silvery; achenes striate-tubercled | Polygonum erectum |
| 12 | Leaf blades narrowly elliptic to lanceolate, rarely ovate; distal parts of ocreae soon disintegrating into persistent brown fibers; achenes uniformly tubercled | Polygonum ramosissimum |
| 13 | Stems and leaf blades papillose-scabridulous; ocreae 1-veined | Polygonum plebeium |
| 13 | Stems and leaf blades glabrous; ocreae 4-12-veined. [14. Shifted to left margin.—Ed.] | > 14 |
| 14 | Leaves often opposite at proximal nodes; achenes 1.4-1.6(-2.2) mm; Yukon, N.W.T., Nunavut, Alaska 9 Polygonum humifusum | > 14 |
| 14 | Leaves all alternate; achenes 1.6-4.5 mm; broad distribution (including Yukon, N.W.T., Nunavut, Alaska) | > 15 |
| 15 | Perianth tube 40-57% of perianth length | Polygonum aviculare |
| 15 | Perianth tube 18-38% of perianth length | > 16 |
| 16 | Achenes beaked at apex, edges strongly concave; stems sometimes zigzagged | Polygonum fowleri |
| 16 | Achenes not beaked at apex, edges straight; stems not zigzagged | > 17 |
| 17 | Plants homophyllous; pedicels 1-2 mm, enclosed in ocreae | Polygonum ramosissimum |
| 17 | Plants heterophyllous; pedicels 2-6 mm, mostly exserted from ocreae | > 18 |
| 18 | Plants subsucculent; leaf blade apices rounded; proximal parts of ocreae funnelform; flowers semi-open; cymes in axils of most leaves; California | Polygonum marinense |
| 18 | Plants not subsucculent; leaf blade apices acute to acuminate; proximal parts of ocreae cylindric; flowers closed; cymes crowded toward tips of branches; broad distribution in United States and Canada (including California) | Polygonum ramosissimum |