64 



FOUNDATIONS OF BOTANY 



and truncate base, unevenly twice serrate, with rather long, slender 

 petioles, which allow the leaves to quiver like those of the aspen. 

 Bark scaling off in white strips and layers, but not in nearly as 

 large sheets as that of the rarer canoe birch (B. papyrifero^. The 

 commonest birch of New England. 



4. B. alba, L. European White Birch, Cdt-leavkd Birch. 

 A tree 50 to 60 ft. high, often with drooping branches. Leaves 

 triangular-ovate, truncate, rounded or somewhat heart-shaped at 

 the base, not strongly taper-pointed except in the cut-leaved form. 

 Commonly cultivated from Europe. Resembles No. 3, but has 

 whiter bark and (the weeping form) much more slender branches. 



V. ALNUS, Toum. 



Shrubs or small trees. Leaves petioled, serrate. Elower- 

 buds stalked, appearing the previous season ; staminate cat- 

 kins racemed, drooping, flowers 3-6 in the axil of each bract, 

 subtended by 1-2 bractlets, perianth 4-parted, stamens 4, fila- 



FiG. 8. — Alnus glutinosa. 



A, a flowering twig ; ;(, staminate catkins ; p^ pistillate catkins ; £, a group ol 

 staminate flowers, enlarged; C, two pistillate flowers, enlarged. 



