LIFE-HISTORY OF A TREE 75 



lows, but a new one is taken on at the same time. Some- 

 times the dead layer falls off annually, as with the Syca- 

 more and Paper Birch, but generally it changes into a corky 

 condition and adheres as the tree expands. As this occurs 

 the dead parts arrange themselves in more or less vertical 

 valleys and ridges, although in some trees the dead bark 

 cracks irregularly and in patches, in which case it generally 

 falls off in scales in a few years after its death ; hence some 

 trees show a thinner bark than others. The inner layers 

 are called the live and the outer ones the dead bark. There 

 is quite a plain line of demarcation between them. 



In some species the annual layers are much thicker than 

 in others and are distinctly shown in the dead bark when 

 cut radially from the heart of the tree. The bark of some 

 trees contains chemical properties, such as tannic acid 

 an astringent much used in tanning leather and other use- 

 ful ingredients, and on that account may have a commercial 

 value. This is especially true of the Eastern and Western 

 Hemlock and some of the Oaks, notably the Chestnut Oak. 

 While the live bark is the most heavily charged with tannin 

 it does not rapidly disappear from the dead bark. Old Hem- 

 lock bark retains its tannin for a long time. 



An interesting experiment which discloses something of 

 the process in the deposition of the material constituting 

 the bark may be made by carefully removing some of the 

 bark of a tree at the time of the tree's most rapid growth in 

 early summer, when the bark most readily separates from 

 the cambium layer, then active in distributing both wood 

 and bark materials, which are then soft and easily placed. 

 If the exposed surface of the cambium is carefully shaded, 

 or the weather is moist and cloudy for a few days, a coat 

 of bark will be formed over the entire surface that has been 

 exposed, providing the cambium has not been bruised 

 or broken, and the wood and bark growth will go on 

 under the new bark the same as if the old had not been re- 

 moved. A young and thrifty tree can, at the time of its 

 most rapid growth, be completely deprived of its bark for 



