VII] 



BARK OF BIRCH 



93 



by the peculiar constitution of the periderm, which is 

 composed of alternating layers of tijiu-walled cuboidal 

 cork-cells empty of recognisable contents, and of tougher 

 flattened cells often tilled with brown debris : although 



"b. « 



Fig. 44. Birch (Iletula alba) : louj^-h bark and base of stem (Irv). 



the whole mass is only a few millimetres thick it may 

 flake off in thin broad ring-like papery lamellae, often 

 to the number of 20 or more in a year. As tiie stem 

 oi' the Birch approaches 20 years old and upwards, 



