It is the lack of any such ring of cambium as is found in 

 dicotyledonous plants, or even of permanent cambium in the 

 separate bundles, that makes it impossible for the trunks of 

 most palm trees to grow indefinitely in thickness, like that of 

 an oak or an elm. 



81. Stem structure of climbing shrubs. Some of the most 

 remarkable kinds of dicotyledonous stems are found in climbing 

 shrubs. The bundles, as shown in Fig. 57, are much more dis- 

 tinct than in most other woody stems. It is evident that this 



FIG. 68. Diagram to illustrate secondary growth in a dicotyledonous stem 



R, the first-formed bark ; p, mass of sieve cells ; ifp, mass of sieve cells between 

 the original wedges of wood ; fc, cambium of wedges of wood ; ic, cambium 

 between wedges ; 6, groups of bast cells ; fh, wood of the original wedges ; ifh, 

 wood formed between wedges ; x, earliest wood formed ; M , pith. After Sachs 



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 a solid steel or iron rod of the same size would 

 be too stiff for this use. 



82. Interruption of annual rings by branches ; knots. When 

 a leaf bud is formed on the trunk or branch of a dicotyledonous 

 tree it is connected with the wood by fibre-vascular bundles. 



