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-Strircture 

 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 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 



FIG. 70. Two Ash Trees naturally 

 grafted together. 



