BETULACE^ 129 



2. Alnus, L. 



Stamens 4 ; scales of female catkin persistent, woody. 



A, glutinosa^ Gaertn., our common Alder; also grown 

 largely for making charcoal for gunpowder. A. viridis^ 

 DC; leaves ovate, acute, doubly serrate, green on both 

 sides, fruit broadly winged ; frequent at a high elevation. 

 A. incana^ Willd. ; leaves glaucous or downy beneath, 

 fruit not winged ; sides of streams ; Switzerland, Jura, 

 Lombardy. A. corylifolia, Kern. ; leaves roundish-ovate, 

 nearly uniformly serrulate, covered with short hairs be- 

 neath ; Tirol, rare (Pusterthal, Scheiblingstein). 



Order LXXXIIL— SALICACE^. 



Flowers unisexual, dioecious ; male and female flowers 

 in catkins; calyx o; stamens 2 or more; ovary i -celled; 

 styles 2 ; ovules numerous. Trees and shrubs, belonging 

 chiefly to the Northern Temperate and Arctic Zones. 



I. POPULUS, L. 



Catkins drooping, scales lobed or cut, crenate ; stamens 

 4-30. Lofty trees ; not alpine. 



P. alba^ L., White Poplar; leaves cottony beneath; 

 occasional. P. nigra, L., Black Poplar; more common, 

 especially by water. P, canescens, Sw. ; possibly a 

 hybrid ; occasional. P. tremula, L., Aspen ; common. 



2. Salix, L. 



Catkins usually erect ; scales entire ; stamens usually 



2-3. Small trees or shrubs. 



VOL. II. I 



