Ill 



THE BIRCH FAMILY— BETULACEAE. 



The Birch family comprises 6 genera with about 75 species of 

 trees and shrubs which are confined to the colder part of the northern 

 hemisphere. Of this number 5 genera with about 30 species are 

 native to North America and 5 genera with 11 species to Pennsylva- 

 nia. 



All the members of this family, even though they may belong to 

 different genera, have many morphological features in common. The 

 leaves are simple, alternate, borne singly or in pairs on the branches 

 but never opposite each other. The staminate and pistillate flow- 

 ers are separate, but are borne on different parts of the same tree 

 and usually on different parts of the same branCh. The staminate 

 flowers are long, usually in drooping aments, or in spike-like or knob- 

 like araents and may be with or without a perianth. The fruits 

 are small, one-celled, usually subtended by a large bract which in 

 the most important genera develops into a cone-like structure called 

 a strobile. 



Various products of high commercial importance are produced by 

 this family. The wood of the Birches is used extensively for furni- 

 ture, flooring, interior finishing and has a very high fuel value. The 

 fruit of the Hazelnuts is prized as food. The wood of some of the 

 Alders is especially adapted to the manufacture of gunpowaer and 

 charcoal. The bark of the Black Bircji yields a volatile oil of consid- 

 erable importance. The technical value of the products from the 

 members of this family are becoming more important every year. 

 The wood of some of the species which was despised formerly, is now 

 considered of high value in some particular industries, on account of 

 the new u.ses to which it is being put. The science of Xylology, 

 which is merely in its formative period, will do much in advancing 

 the position of the wood of species at present despised or at least not 

 fully known. The subjoined key will aid in distinguishing the genera 

 of this family. 



KEY TO THE GENERA. 



Page. 

 1. Staminate flowers solitary on each bract; pistillate flowers with a iicriantli; fruit 

 not a strobile ; 2 



1. Staminate flowers 2 to sereral on each bract; pistillate flowers wlthont a perianth; 



fruit :i St '.-obile 4 



2, Shrubs: twigs covered with stiff red hairs standlDg out at right angles; nuts large 



covered by leaf-like involucre CoryluB 122 



2. Trees: Twigs not covered with stift red hairs; nuts small and subtended by a large 



bract 3 



8. Bark close, smooth and fluted; nut subtended by a flat 3-lohed bract, terminal 



lobe serrate on one side Carpinus 120 



8. Bark thin covered by loose ribbon-like narrow brown scales; nut subtended by a 



closed bract arranged in hop-like clusters Ostrya 119 



4. Shrub with close, somewhat fluted bark; wood yellowish upon exposure; buds stalked, 



obtuse at apex, covered with l,wo exposed valvate scales; fruit woody and 



persistent Alnus 121 



4, Small to large trees with loose bark u'^ually peeling off into thin film-like layers; 



buds not stalked, acute at apex, covered with 3 or more overlapping scales; fruit 



membranous and deciduous Betula 112 



