26 BETULACEAE 



3. Twige glandular-warty. B. glandulosa. 

 Twigs not glandular-roughened. B. nana. 



4. Bark cherry-like, wintergreen-fiavored. ( Sweet b.). B. lenta. 

 Bark flaking, or white or yellowish. 5. 



5. Bark gray or yellow or orange. 6. 

 Bark white. 7. 



6. Leaves ovate: bark gray or yellowish. B. lutea, 

 Leaves rhombic: bark orange (Red birch). B. nigra. 



7. Leaves triangular. B. populifolia. 

 Leaves ovate or else lobed. 8. 



8. Bark mostly flaking: leaves large (4-12 cm.). B. papyrifera. 

 Bark not flaking: leaves small (2-6 cm. long). 9. 



9. Leaves green. 10. 



Leaves purple. B. pendula purpurea. 



10. Leaves not lobed. 11. 



Leaves lobed or cut. (Cut-leaved birch). 13. 



11. Scarcely weeping or pyramidal. 12. 



Weeping. B. pendula tristis. 



Narrowly conical. B. pendula fastigiata. 



12. Twigs pubescent. B. pubescens. 

 Twigs glabrous*. B. pendula. 



13. Scarcely weeping. B. pendula dalecarlica. 

 Weeping. B. pendula gracilis. 



ALNUS. Alder. 



Rather ovoid much branched deciduous trees, or more often 

 seen as shrubs, with rather smooth bark; brownish rather hard 

 wood with minute diffused ducts and occasional thick medul- 

 lary rays accompanying the prevalent very fine ones; rather 

 slender often 3-sided twigs with 3-sided or flattened homogen- 

 eous pith; alternate crescent-shaped or half round somewhat 

 raised leaf scars with 3 bundle-traces 1 (or the lowermost broken 

 into a secondary group) in a single series; 3-ranked stalked 

 plump buds with about 3 exposed scales; rounded or ovate or 

 siomewhat lanceolate denticulate or once or twice serrate peti- 

 oled simple leaves; small monoecious apetalous flowers in cat- 



