MECHANICAL PROBLEMS 101 



sap. They are thus easily ruptured, and 

 every country boy knows that in late spring 

 the bark of willow or ash twigs can easily be 

 slipped off the wood as an unbroken cylinder. 

 This is because the debris of the torn young 

 cambial cells serve as a sort of lubricant 

 which facilitates the process. Later on the 

 bark will not slip off readily if at all, and 

 this is due to the fact that the inner and outer 

 cell layers of the previously undifferentiated 

 zone have gradually become changed into 

 wood and bast. The thin layer of residual 

 cambium is now not thick enough, nor can 

 it provide sufficient lubricant to enable the 

 ring of bark to slip off. 



The new wood thus produced consists of 

 young water-conducting tracheids and vessels, 

 as well as of other sorts of cells which have 

 various functions to discharge. Some of 

 these cells, as they change from the embryonic 

 to the permanent or adult state not only 

 thicken their walls, but grow considerably 

 in length, inserting their tips between other 

 similar cells above and below them. They 

 are largely, though not exclusively, of mechan- 

 ical significance. From a commercial point 

 of view it is mainly to the mechanical tissue 

 that woods of various sorts owe their technical 

 value as timber. 



A relatively considerable proportion of the 

 new wood is thus more or less definitely 

 differentiated to serve mechanical purposes. 

 This applies to most of the cells which have 



