116 American Midland Naturalist Monograph No. 2 



eels jointed at base; leaves linear-IanceoIate, narrowed at base, undu- 

 late or crisped; sandy shores, local. McHenry Co., Vasey; Cahokia, 



Eggert. July-Sept. [/?. per s tear io ides sensu auth., non L.} 



R. fneginus Phil. 



2. Polygonum L. 



(Persicaria Mill.) 

 1 . Stems not twining. 



2. Stems not retrorsely bristly. 

 3. Flowers axillary. 



4. Stems leafy throughout; achenes included in or only shortly exserted 

 from the calyx. 



5. Sepals with yellowish green margins; stems erect or ascending. 

 6. Leaves oval, obtusish; achenes dull; waste ground. Aug.-Oct. 

 Erect Knotweed P. erectum L. 



6. Leaves lanceolate or narrowly oval, acute; achenes glossy; sandy 



soil. July-Sept P. ramosissimtim Michx. 



5. Sepals with pink or white margins; stems prostrate or spreading. 



7. Leaves thick, prominently veined, oval, pale green; stipules very 



conspicuous; achenes granular; waste ground, not common. 



Aug.-Oct P. buxijorme Small 



7. Leaves thin, not prominently veined, bright green, narrowly 

 elliptical to linear; stipules not conspicuous; achenes striate; 

 waste ground, very common; variable in appearance. July-Oct. 



Common Knotweed P. aviculare L. 



4. Stems with the upper leaves reduced in size. 



8. Achenes conspicuously exserted from the calyx; branches terete, 

 striate; stamens 5 or 6; sandbars or rocky ground, not common. 

 Aug.-Oct. Long-fruited Knotweed P. exsertuin Small 



8. Achenes included in the calyx; stem and branches angular, slen- 

 der; leaves plicate; stamens 8; sandy soil. July-Sept. Slender 



Knotweed P. tenue Michx. 



3. Flowers in terminal or axillary panicles. 



9. Styles short; calyx not curved, the sepals usually 5. 



10. Panicle terminal, usually solitary; perennial marsh or aquatic or 

 more or less amphibious herbs with long rhizomes. 

 11. Panicle ovoid or ellipsoid, 1-3 cm. long. 



12. P'eduncle glabrous or nearly so; leaves elliptical, glabrous, 

 glossy above, obtuse or acute; shallow water. June-Aug. 

 [P. fluitans Eaton; P. amphibiuiii sensu Am. auth., 



non L.] P. natans (Michx.) Eaton 



12. Peduncle pubescent; leaves elliptical-lanceolate, pubescent, 

 not glossy, acute or attenuate; swampy ground. June- 

 Aug. [P. hartwrightii Gray; P. amphibium var. hart- 



wrightii Bissell] 



P. natans f. hartwrightii (Gray) Stanford 



