80 FIRST STEPS IN GENERAL KNOWLEDGE. 



them nearly every day, and I do not see the old 

 bark fall off, and a new one come instead of it." 



" These new layers of wood are formed under 

 the bark, and out of our sight. You have seen 

 people stripping off the bark from trees after they 

 are cut down, and you must have noticed how 

 easily it comes off; it is, indeed, quite distinct from 

 the wood, and has a gummy substance between it 

 and the true wood." 



" Then as the wood goes on increasing, does the 

 bark stretch very much, or how does it manage 

 to make room ?" 



" The bark itself is also gradually renewed on 

 its inner side, while the outer portion cracks and 

 peels off, or perishes by slow degrees. Thus, 

 while the solid trunk of the tree is formed by zone 

 upon zone of new wood, which is added in suc- 

 cession, year after year, the bark is renewed by 

 zone within zone of fresh material ; but with this 

 difference ; in the bark, the outside perishes, and 

 a new layer adapted to the increasing size of the 

 tree gradually takes its place ; in the true wood 

 every layer is permanent, and forms solid tim- 

 ber." 



" Then," said Mary, " the old bark is obliged to 



