5. Leaves lanceolate to ovate-lanceolate. 2-4 inches long; 

 petioles short Oess than H length of blade); buds M-M inch 



long, resinous; Rocky Mountains 



P. angustifolia, Narrowleaf Cottonwood, p. 129. 



5. Leaves ovate to ovate-lanceolate; petioles at least H as long 

 as leaf blade. 



6. Leaves 3-6 inches long, margins finely toothed; buds ^i-1 

 inch long, very resinous and aromatic. 

 7. Capsule 3-valved, hairy; stigmas 3; 40-60 purple 



stamens; Pacific region 



P. trichocarpa, Black Cottonwood, p. 12 7. 



7. Capsule 2-valved, glabrous; stigmas 2; 20-30 pink 



stamens; northern North America 



P. balsamifera. Balsam Poplar, p. 127. 



6. Leaves 2-4 inches long, margins coarsely toothed; buds 

 H-M inch long, slightly resinous, not aromatic; Rocky 

 Mts. . P. X acuminata, Lanceleaf Cottonwood, p. 129. 

 Leaf petiole definitely flattened laterally. 



8. Leaves broadly ovate to suborbicular, clearly defined translucent 



border, 1-3 inches long, base roimded; buds almost nonresinous; 



capsules 2-valved, thin-walled; 6-12 stamens. 



9. Leaves finely toothed, glabrous; buds glabrous; Atlantic to 



Pacific P. tremuloides, Quaking Aspen, p. 123. 



9. Leaves with few coarse teeth; buds hairy; eastern 



P. grandidentata, Bigtooth Aspen, p. 123. 



8. Leaves deltoid to rhombic-ovate, with clearly defined trans- 

 lucent border, 2-7 inches long; buds resinous; capsules 3- or 

 4-valved (2-valved in Black and Lombardy Poplars) thick- 

 walled; 12-60 stamens. 



10. Leaves rhombic-ovate (rarely deltoid), long acuminate, base 

 wedge-shaped or rounded, hybrids or naturalized trees 

 widely planted through North America. 



11. Leaves longer than broad, 2-6 inches long, coarsely 

 toothed. . P. X canadensis, Carolina Poplar, p. 125. 

 11. Leaves about as long as broad, 2-4 inches long, finely 

 toothed. 



12. Tree with wide-spreading, stout branches 



P. nigra. Black Poplar, p. 123, 



12. Columnar tree with erect branches narrow, spinelike 

 crown. P. nigra italica, Lombardy Poplar, p. 123. 

 10. Leaves deltoid in shape, 2-4 inches long; native trees. 



13. Leaves with glands at point where petiole and blade 

 meet; margins cUiate, with numerous teeth. 

 14. Leaves thick, very broad and truncate at base; 

 t\dgs thick. 



15. Buds glabrous; leaves finely toothed, eastern. . 



. . .P. deltoides. Eastern Cottonwood, p. 125. 



15. Buds slightly hairy, leaves coarsely toothed; 



Great Plains to Rocky Mountains 



... P. sargentii. Plains Cottonwood, p. 125. 

 14. Leaves thin, often rounded or wedge-shaped at 



base; twigs slender; Texas 



P. palrneri. Palmer Cottonwood, p. 125. 



13. Leaves without glands at point where petiole and blade 

 meet; margins not ciliate, with usually less than 15 

 teeth on a side. 



16. Pedicels longer than capsules; leaves with no more 

 than 10 teeth on a side; Colorado, New Mexico, 



west Texas and Mexico 



P. fremontii wislizenii, Rio Grande Cottonwood, 

 16. Pedicels shorter than capsules; leaves with more 

 than 10 teeth on a side; New Mexico west to Cali- 

 fornia P. fremontii, Fremont Cottonwood, p. 125. 



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