THEIR RUGGED TRUNKS 



outer surface which becomes active and forms bark 

 and continues to do so during the life of the individ- 

 ual. In some trees, like the Birch, as new layers are 

 formed the older ones are partially or completely 

 thrown off. In others, like the Beech, the growth is 

 such that the bark firmly coheres and remains smooth 

 on the outside. In the Oak, Elm, and Chestnut suc- 

 cessive formations are amassed and the bark, though 

 firmly coherent, becomes fissured and with age deeply 

 and ruggedly so. In some trees the bark-forming 

 cells, after a time, cease to function and fresh layers 

 arise successively deeper and deeper within the tis- 

 sues. When this happens, as in Sequoia, the bark is 

 made up of different tissues and is known as fibrous. 

 In most cases the bark is either thin and papery, 

 firm and smooth, or fissured, but in some— Cork 

 Oak, Cork Elm, and other trees — it is thick and 

 corky. 



Without entering further into the origin of bark, 

 our purpose is served if it be remembered that the 

 character of the bark depends largely upon its seat of 

 origin and the nature of the tissues of which it is 

 composed; that its appearance depends mainly upon 

 degrees of coherence and upon the stress and strain it 

 is submitted to as growth continues year after year. 

 It is the tree itself that fashions the bark in all its 

 varied forms and not external elements, though wind, 



29 



