BETULACEAE. 63 



fiiVe medullary rays ; mostly very slender terete twigs ; small 

 3-sided or flattened homogeneous greenish pith ; alternate, often 

 2-ranked low rather crescent-shaped leaf-scars with 3 bundle- 

 traces, short narrow evanescent stipule-scars ; ovoid or oblong 

 sessile solitary buds, the terminal sometimes absent and the lat- 

 eral rather appressed with about 6 alternate exposed scales ; 

 simple usually toothed petioled leaves ; small imperfect incon- 

 spicuous apetalous flowers in catkins ; and small 2-winged 

 fruits in cone-like catkins with deciduous 3-lobed scales. 



1. Shrubs. 2. 

 Trees. 4. 



2. Leaves glabrous. 3. 



Leaves pubescent, dentate. B. pumila. 



3. Twigs glandular-warty. B. glandulosa. 

 Twigs not glandular-roughened. B. nana. 



4. Bark cherry-like, wintergreen-flavored. 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. B. nigra. 



7. Leaves triangular. B. populifolia. 

 Leaves ovate. 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. 



ic. Leaves not lobed. n. 

 Leaves lobed or cut. 12. 



11. Scarcely weeping or pyramidal. B. pendula. 

 Weeping. B. pendula tristis. 

 Narrowly conical. B. pendula fastigiata. 



12. Scarcely weeping. B. pendula dalecarlica. 

 Weeping. B. pendula gracilis. 



ALNUS. Alder. 



Rather oVoid much branched deciduous tree, or more often 

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



