STRUCTURE OF THE STEM 99 
about naturally by the branches of a tree chafing against one 
another until the bark is worn away and the cambium layer 
of each is in contact with that of the other, or two separate 
trees may be joined by 
natural grafting, as is 
shown in Fig. 70. 
110. Stem-Structure 
of Climbing Shrubs. — 
Some of the most remark- 
able kinds of dicotyle- 
donous stems are found 
in climbing shrubs. The 
structure of many of 
these is too complicated 
to be discussed in a 
botany for beginners, but 
one point in regard to 
them is of much inter- 
est. The bundles (as 
seen in the clematis and 
shown in Fig. 56) are 
much more distinct than 
in most other woody 
stems. Even after sev- 
eral years of growth the 
wood is often found to be 
arranged in a number of 
flattish twisted strands. 
It is evident that this 
Fic. 70. — Two Ash Trees naturally 
grafted together. 
is for the sake of leaving the 
stem flexible for twining purposes, just as a wire cable is 
adapted to be wound about posts or other supports, while 
