112 



THE BIRCHES— BETULA, Tournefoirt (L.) 



This genus comprises about 35 known species of which numbei 

 25 are trees and the others shrubs. Of the known species about 1^ 

 are native to North America and 5 to Pennsylvania. 



The members of this genus are without exception called Birch 

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

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

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

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

 thin film-like papery layers. The wood is dense and hard, does 

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

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

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

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

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

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

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

 staminate flowers appear clustered in long tassel-like bodies hangi 

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

 filiate flowers appear below the staminate and are nearly erec 

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

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

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

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

 by the wind for considerable distances from the mother or seed tr 



The commercial products derived from some of the members of 

 genus are rather important and valuable. The species found in 

 eastern part of North America yield products of more value th 

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

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

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

 domestic use. Some of the species found in the eastern part of No 

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

 ber trees which yield not only excellent wood but also valuable oi 

 flavors, and bark. 



The subjoined key will aid in distinguishing the species of Bir 

 found in Pennsylvania. Separate summer and winter keys were 

 developed since the following key is based primarily upon bark ch 

 acteristics which are present at all seasons of the year. 



