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 débris 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 
