112 



THE BIRCHES BETULA, Tournefort (L.) 



This genus comprises about 35 known species of which number 

 25 are trees and the others shrubs.. Of the known species about 15 

 are native to North America and 5 to Pennsylvania. 



The members of this genus are without exception called Birches. 

 In most of them the bark of the trees when young is ' smooth and 

 peels off into film-like papery layers which vary in color according 

 to the species from chalky white to reddish-brown. A few species 

 have, however, a close and smooth bark which does not peel off into 

 thin film-like papery layers. The wood is dense and hard, does not 

 show the annual rings very clearly, is of high fuel value and usually 

 reddish-brown in color, sometimes possessing a highly prized curly 

 or wavy figure. The twigs of the season produce only one leaf at a 

 point, while the twigs of the previous season produce two leaves from 

 the lateral buds situate on the short spur-like branches. The leaves 

 are simple, always alternate, occur singly or in pairs but never op- 

 posite. The flowers appear before or with the developing leaves. The 

 staminate flowers appear clustered in long tassel-like bodies hanging 

 down from the end of the twigs and are known as aments. The pis- 

 tillate flowers appear below the staminate and are nearly erect, 

 rather small and slender. The fruit is a cone-like structure known 

 as a strobile consisting of a central axis to which numerous scales 

 are attached. The scales are thin, 3-lobed, and bear the small flat 

 nuts with their wings. The nuts are very light and easily scattered 

 by the wind for considerable distances from the mother or seed trees. 



The commercial products derived from some of the members of this 

 genus are rather important and valuable. The species found in the 

 eastern part of North America yield products of more value than 

 those found in the western part. Most of the species found in the 

 western part of North America are too small, or infrequent in the 

 form of stands, to be commercially important for general or even 

 domestic use. Some of the species found in the eastern part of North 

 America are also small shrubs but others reach the size of large tim- 

 ber trees which yield not only excellent wood but also valuable oils, 

 flavors, and bark. 



The subjoined key will aid in distinguishing the species of Birch 

 found in Pennsylvania. Separate summer and winter keys were not 

 developed since the following key is based primarily upon bark char- 

 acteristics which are present at all seasons of the year. 



