EXPANSION OF TEE BARK 71 



75. The lower part of the limb shows that the 

 onter layers of bark (which are long since dead, and 

 act only as protective tissue) have reached the limit 

 of their expanding capacity and have begun to split. 

 The pupil will now be interested in the bark upon 

 the body of an old elm tree (Fig. 69) ; and he 

 should be able to suggest one reason why stems 

 remain terete, and why the old bark becomes 

 marked with furrows, scales and plates. 



76. If, for any reason, the bark should becomo- 

 so dense and strong that the trunk cannot expand, 

 the tree is said to be "bark- bound." Such condi- 

 tion is not rare in orchard trees which have been 

 neglected. When good tillage is given to such 

 trees, they may not be able to overcome the rigid- 

 ity of the old bark, and, therefore, do not respond 

 to the treatment. Sometimes the thinner - barked 

 limbs may outgrow in diameter the trunk or the 

 old branches below them. The remedy is to release 

 the tension. This may be done either by softening 

 the bark (by washes of soap or lye), or by separa- 

 ting it. The latter is done by slitting the bark- 

 bound portion (in spring), thrusting the point of a 

 knife through the bark to the wood and then draw- 

 ing the blade down the entire length of the bark- 

 bound portion. The slit is scarcely discernible at 

 first, but it opens with the growth of the tree, 

 filling up with new tissue beneath. Let the pupil 



