258 MISC. PUBLICATION 42 3, U. S. DEPT. OF AGRICULTURE 



Many of the species are weeds, but are not troublesome in Arizona. 

 The knotweeds (section Avicularia) tend to become abundant on 

 overgrazed land. Some of the smartweeds (section Persicaria) have 

 acrid juice which is irritating to the skin, eyes, and nostrils. 



Key to the species 



1. Stems twining; leaf blades (at least the lower ones) ovate, cordate-sagittate; 

 flowers in loose axillary and terminal racemes or racemiform panicles; 

 outer sepals winged or strongly keeled; flowers greenish white: Section 



Bilderdykia 1. P. convolvulus. 



1. Stems not twining; outer sepals not winged (2). 



2. Leaves with a hingelike joint at the point of attachment of blade and 



sheath, the blades oblong, elliptic, lanceolate, or linear; flowers in axillary 



clusters; bracts of the inflorescence with well-developed blades; flowers 



greenish, whitish, or tinged with pink: Section Avicularia (3). 



3. Flowers crowded toward the ends of the branches, the inflorescences 



appearing like terminal spikes; stems mostly erect (4). 



4. Stems slender, usually less than 15 cm. long; floral leaves little reduced; 



achenes granular-striate 2. P. watsoni. 



4. Stems stout, 30 to 100 cm. long; floral leaves greatly reduced except 



near the base of the spike; achenes smooth, shiny. 



3. P. ARGYROCOLEON. 



3. Flowers scattered along the stems in small axillary clusters (5). 



5. Stems decumbent or prostrate; achenes dark brown, the surface dull 



and minutely roughened 4. P. a viculare. 



5. Stems usually erect or ascending (6). 



6. Pedicels mostly reflexed or deflexed; leaf sheaths 10 mm. long or 

 longer; perianth 3 to 4 mm. long; achenes black, the surface 

 (sometimes only the angles) shiny and smooth. 



5. P. DOUGLASII. 



6. Pedicels erect or ascending (7). 



7. Stems usually slender; upper leaf blades reduced to subulate bracts; 

 sheaths commonly less than 10 mm. long; perianth not more 

 and usually less than 3 mm. long; achene black, very acutely 

 angled, shiny and smooth 6. P. sawatchense. 



7. Stems stout; upper leaf blades often reduced but not subulate; 

 achenes minutely roughened, not very shiny. 



7. P. RAMOSTSSIMUM. 



2. Leaves without a distinct joint at the point of attachment of blade and 



sheath; flowers in terminal (sometimes also axillary) spikelike racemes 



or panicles; bracts of the inflorescence reduced to sheaths (8). 



8. Rootstock usually much thickened and bulblike; radical leaves well 



developed; inflorescence solitary; plants alpine or subalpine; flowers 



white or tinged with pink: Section Bistorta (9). 



9. Inflorescence narrowly cylindric, 5 to 8 mm. wide, usually viviparous 



(bearing bulblets)^ below 8. P. vi viparum. 



9. Inflorescence broadly cylindric or somewhat ovoid, 10 to 20 mm. wide, 



not viviparous 9. P. bistortoides. 



8. Rootstock, if any, not bulblike; radical leaves none: Section Persicaria (10) . 



10. Inflorescences often solitar}^, all terminal or nearly so; plants aquatic 



or semiaquatic; flowers bright pink (11). 

 11. Leaf blades obtuse or acute, commonly widest near the middle; 

 inflorescence seldom more than 3 cm. long, more than 10 mm. wide. 



10. P. AMPHIBIUM. 



11. Leaf blades acuminate, commonly widest near the base; inflorescence 



3 to 10 cm. long, seldom more and often less than 10 mm. wide. 



11. P. COCCINEUM. 

 10. Inflorescences usually several, axillary as well as terminal; plants not 



aquatic, but often growing in wet soil (12). 



12. Sheaths without marginal bristles (occasionally short-ciliate when 



young); flowers pink or pinkish (13). 

 13. Inflorescences nodding or drooping, usually numerous, slender and 

 elongate 12. P. lapathifolium. 



