20 BULLETIN 179, U. S. DEPAETMEJSTT OF AGEICULTTJEE. 



10. Young branches glabrous; pubescence of the leaves usually 

 more abundant along the midrib and more or less tufted in the ' 

 axils of the lateral veins. Flowers 9 or 10 mm. broad, the 

 calyx lobes glandular 11. 



10. Young branches more or less pubescent; pubescence of the 



leaves usually neither more abundant along the midrib than 

 elsewhere, nor tufted in the axils of the lateral veins. Flowers 

 6 to 10 mm. broad, the calyx lobes eglandular 17. 



11. Leaves lanceolate to ovate-lanceolate, rather strongly condupli- 



cate or trough shaped. Fruit ripening in July and August. . P. reverchonii. 



11. Leaves ovate to oblong-ovate or sometimes slightly obovate, 



usually not trough shaped. Fruit ripening in June P. rivularis. 



12 . Leaves usually 6 to 10 cm. long. Calyx lobes glandular P. munsoniana. 



12. Leavesmostly 2 to6 cm. long. Calyx lobes eglandular 13. 



13. Leaves oblong-lanceolate to obovate-lanceolate, mostly 4 to 6 



cm. long. Flowers 10 to 14 mm. broad. Fruit ripening at the 



Arnold Arboretum in September P. orthosepala. 



13. Leaves lanceolate, oval-lanceolate, or oblong-oval, 2 to 6 cm. 



long. Flowers 8 to 12 mm. broad. Fruit ripening from the 

 last of May, in the South, to the fii-st of September (subsp. 

 tvatsoni) P. angustifolia. 



14. Leaves lanceolate to oval-lanceolate or sometimes ovate or oval, 



not strongly reticulate veiny below, and glabrous or pubescent. 

 Flowering pedicels glabrous or only sparingly pubescent 15. 



14. Leaves ovate, oval, or nearly orbicular, rarely somewhat obo- 



vate, rather strongly reticulate veiny below, and pubescent. 

 Flowering pedicels strongly pubescent 16. 



15. Petioles mostly 7 to 12 mm. long. Calyx lobes only slightly 



shorter than the tube. Tree of the northern Alleghenies and 



southern New England P. alleghaniensis. 



15. Petioles mostly 5 to 9 mm. long. Calyx lobes mostly shorter than 



the tube. Tree of the Southern States P. umbellata. 



16. Leaves orbicular to oblong-orbicular, obtuse at the apex P. gravesii. 



16. Leaves ovate or oval, rarely slightly obovate, usually acute at 



the apex P. maritimxi. 



17. Leaves oval or lance-oval, rarely ovate or obovate-lanceolate, 



narrowed or rounded toward the base, petioles 5 to 8 mm. 

 long. Flmvers 9 to 10 mm. broad, pedicels and calyx finely 

 pubescent P. gracilis. 



17. Leaves ovate to oblong-ovate, usually rounded at the base, 



petioles 10 to 12 mm. long. Flowers 6 to 7 mm. broad, pedicels 



and calyx glabrous or sparingly pubescent P. venulosa. 



18. Leaves lanceolate, oblong-lanceolate or oblong-oval, narrowed or 



rounded at the base, the apex acute or acuminate. Floivering 

 pedicels without subtending bracts, or when present the bracts 

 minute, not foliaceous. Eastern and Rocky Mountain species . P. pennsylvanica. 



18. Leaves oblong-oval, obovate or rarely lanceolate, narrowed or 



even cuneate toward the base, obtuse or rarely acute at the 

 apex. Flowering pedicels subtended by small foliaceotis dentate 

 or ladniate bracts. Southwestern and Pacific coast species P. emarginata. 



19. Leaves usually narrowly oblanceolate-spatulate and acute P. pumila. 



19. Leaves oblong-spatulate to oval or somewhat obovate, acute or 



obtuse 20. 



