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

Mertens, T. R. and P. H. Raven. 1965. Taxonomy of Polygonum, section Polygonum (Avicularia) in North America. Madroño 18: 85–92.

Wolf, S. J. and J. McNeill. 1986. Synopsis and achene morphology of Polygonum section Polygonum (Polygonaceae) in Canada. Rhodora 88: 457–479.

Lower Taxa

Polygonum achoreum, Polygonum argyrocoleon, Polygonum aviculare, Polygonum erectum, Polygonum fowleri, Polygonum glaucum, Polygonum marinense, Polygonum oxyspermum, Polygonum patulum, Polygonum plebeium, Polygonum ramosissimum, Polygonum striatulum

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	Polygonum humifusum
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