56 SALICACEAE. 



POPULUS. Polar. Aspen. 



Deciduous trees often very rough-barked when old, with 

 rather soft white or browning wood with minute scattered ducts 

 and very fine medullary rays ; moderate rounded or acutely 5- 

 angled twigs ; 5-angled continuous brown pith ; somewhat raised 

 rather 3-lobed large leaf-scars with 3 large bundle-traces ; : nar- 1 

 row stipule-scars ; large buds, the terminal angularly ovoid, the 

 lateral often falcately spreading, with several scales of which 

 the lowermost is in front, immediately over the leaf-scar ; small 

 dioecious naked flowers in catkins ; and small ovoid capsules 

 with numerous cottony seeds. 



1. Buds plump, not resinous or gummy. 2. 



Buds elongated, more or less balsamiferous. 9. 



2. Leaves white- or gray-woolly beneath : petioles. 



little flattened. (White Poplars). 3. 

 Leaves not woolly: petioles much flattened. (Aspens). 6. 



3. Leaves gray beneath. P. canescens. 

 Leaves white beneath. 4. 



4. Tree rounded or oblong. 5. 



Tree pyramidal. P. alba Bolleana. 



5. Leaves coarsely toothed rather than lobed. P. alba. 

 Leaves deeply lobed. P. alba nivea. 



6. Leaves glabrate and nearly entire. P. tremuloides. 

 Leaves somewhat pubescent, coarsely toothed. 7. 



7. Buds gray-pubescent. P. grandidentata. 

 Buds glabrate. 8. 



8. Not weeping. P. Tremula. 

 With hanging branches. P. Tremula pendula. 



9. Leaves pale beneath : petioles little flattened : buds mostly 



very balsamiferous. (Balsams). 10. 

 Leaves green beneath : petioles much flattened : buds less 



balsamiferous. (Cotfonwoods). 14. 

 iq. Leaves deeply heart-shaped, broad. P. candicans. 



Leaves scarcely heart-shaped. II. 

 ii. Leaves elliptical or ovate. 12. 

 Leaves lanceolate. 13. 



