134 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



6. Salix babylonica L. WeepinCx willow. 

 Along the upper Potomac; infrequent. Apr. -May. Native of Asia; cultivated 



and naturalized in eastern N. Amer. 



7. Salix cordata Mulil. 



Along the Potomac, Eastern Branch, and Hunting Creek. Apr. -May. Widely 

 distributed in N. Amer. 



A hybrid between this and 5. sericea is found occasionally. 



8. Salix sericea Marsh. Silky willow. 

 Along streams; common. Apr.-May. Eastern N. Amer. 



9. Salix humilis Marsh. Prairie willow. 

 Dry fields, thickets, and open woods; frequent. March-Apr. Eastern N. Amer. 



10. Salix tristis Ait. Dwarf prairie willow. 

 Dfy fields and woods, chiefly eastward. Apr.-May. Eastern N. Amer. 



2. POPTJLUS L. Poplar. 



Leaves deltoid or cordate-deltoid, glabrous on the surfaces, ciliate; buds glabrous, resi- 

 nous; floral bracts falling early 1. P. virginiana. 



Leaves rounded, oval, or cordate-oval; buds neither glabrous nor resinous; floral 

 bracts persistent. 

 Leaves on the young branches cordate-ovate, 15-20 cm. long or more. 

 Leaves crenate, long-petioled, auriculate at base, the amides overlapping. 



5. P. lieterophylla. 

 Leaves dentate or denticulate, the normal leaves broadly oval, acute, 5-10 cm. 



long, sericeous at fijst, glabrous in age 2. P. grandidentata. 



Leaves on the young branches broadly cordate-ovate, rarely over 10 cm. long, irreg- 

 ularly toothed or lobed. 

 Leaves on the young branches deeply 5-lobed, dark-green above, white-woolly 

 beneath; normal leaves with more or less persistent wool; petioles shorter than 



the blades 3. P. alba. 



Leaves on the young branches irregularly toothed, not deeply lobed, silvery- 

 canescent or woolly; normal leaves commonly ovate, acute, canescent, 

 becoming glabrous; petioles equaling or exceeding the blades. 



4. P. canescens. 



1. Populus virginiana Fouger. Cottonwood. 

 River bottoms and low places. Apr. Middle Atlantic states. (P. monilifera Ait.) 

 Populus italica Moench (P. dilatata Ait.; P. nigra italica DuRoi), the Lombardy 



poplar, is extensively planted ; readily recognized by its very broad, deltoid, abruptly 

 acuminate root-shoot leaves and erect fastigiate branches. Native of Eur. 



Populus nigra L., the black poplar, is occasionally planted. Native of Eur. The 

 normal leaves are rhombic-acuminate, 5-8 cm. long, the branches spreading and in 

 age horizontal. 



Populus eugenei Simon-Louis (P. monilifera of authors, not Ait.), the Carolina 

 poplar, is extensively planted in our streets and parks. It has an excurrent trunk and 

 spreading branches; the leaves are triangular-ovate, broadest below the middle, long- 

 acuminate, those of the root shoots more or less deltoid, the teeth in both cases being 

 incurved. 



2. Populus grandidentata Michx. Large-toothed aspen. 

 Deciduous woods. Apr. Northern states, south to N. C. 



Populus tremuloides Michx., the American aspen, has escaped from cultivation near 

 Chevy Chase Lake {Maxon & Standley). Can. to Pa. and Nebr. Readily recognized 

 by its broadly ovate or orbicular, crenulate, abruptly acuminate leaves, 3-6 cm. long. 



