Trees of New York State 353 



5. Filaments united; capsule sessile S. purpurea 125 



5. Filaments separate ; capsule pedicelled 6 



6. Mature leaves glabrous and glaucous beneath; pedicel of capsule shorter 



than the scale S. discolor 121 



6. Mature leaves pubescent and somewhat glaucous beneath; pedicel of capsule 



longer than the scale S. rostrata 123 



7. Leaves without petiolar glands; sterile aments elongated, slender 



cyhndrical 8 



7. Leaves with petiolar glands; sterile aments short cylindrical or elhpsoid- 

 ovoid 9 



5. Leaves pale or glaucous beneath; petioles generally ^ of an inch or more 



in length S. amygdaloides 109 



8. Leaves green beneath; petioles less than § of an inch in length. . . .S. nigra 107 



9. Leaves ovate-lanceolate or lanceolate, attenuate-acuminate at the apex 



S. lucida 113 



9. Leaves ovate or oblong-ovate, acuminate at the apex S. pentandra 111 



THE POPLARS AND COTTONWOODS. Genus POPULUS 



(Tourn.) L. 



Trees of rapid growth and large size, with scaly, generally resin- 

 ous buds, long-petioled leaves, fugacious stipules, and bark which 

 is at first smooth but eventually furrowed. Some twenty-five 

 species have been described, natives of the northern hemisphere 

 and ranging from the Arctic Circle to the tropics. Eleven species 

 occur within the boundaries of the United States. 



Leaves alternate, simple, ovate-lanceolate to orbicular or deltoid, long- 

 petioled ; stipules minute, fugacious. Flowers dioecious, borne in aments, each 

 flower subtended by a fimbriate, caducous scale and cup-shaped, oblique disk; 

 staminate flowers consisting of 4-60 stamens, with short free filaments and 

 purplish anthers; pistiDate flowers consisting of a sessile, 1-celled ovary sur- 

 mounted by a short style and 2-4 entire or 4-lobed stigmas; ovules numerous. 

 Fruit a subglobose or ovoid-oblong capsule subtended by the persistent disk, 

 maturing in the late spring or early summer and dehiscing by 2-4 recurved 

 valves; seeds minute, comose, exalbuminous. Winter buds covered wdth a 

 number of scales, generally resinous. 



KEY TO THE SPECIES page 



1. Petioles strongly flattened laterally 2 



1 . Petioles terete or channeled, not strongly flattened 4 



2. Leaves deltoid P. deltoides 123 



2. Leaves broadly ovate or orbicular 3 



3. Leaves round-ovate, coarsely dentate ; buds scurfy-pubescent 



P. grandidentata 115 



3. Leaves ovate to orbicular, finely crenate-serrate ; buds glabrous 



P. tremuloides 113 



4. Leaves densely white- to mentose beneath P. alba 111 



4. Leaves essentially glabrous when mature 5 



5. Leaves densely white-tomentose when young; capsules slender-pedicelled. . . . 



P. heterophylla 117 

 5. Leaves not densely white-tomentose when young; capsules stout-pedicelled. .6 



6. Leaves broadly ovate, cordate or truncate at the base ; petioles and lower 



surface of leaves pubescent P- candicans 121 



6. Leaves ovate-lanceolate to cordate-ovate; petioles and lower surface of 



leaves smooth P. balsamifera 119 



12 



