8 4 



ORIGIN OF LEAVES AND BRANCHES 



deeply located cells in the cortex co-operate in their formation. 

 This relationship of the leaves and branches to the stem is shown 

 in Fig. 62, which is a diagram of an elongating stem, showing the 

 relation of the apical region of the stem (a in Fig. 61), to the 

 lower and older portion. The cells in these young leaves and 

 branches by rapid division and growth soon form the character- 

 istic tissues already noted in the leaves and stems; while a cor- 



FIG. 63. Section through the trunk of basswood showing relation of 

 branches to main stem. In the upper portion of the figure are three 

 branches that were killed after a few years' growth by shading and that 

 have been overgrown by the annual rings of the wood. The branch shown 

 at a has remained alive and increased in size after the manner of the 

 main stem. 



responding growth in the cortex and adjacent regions results in 

 the formation of vascular bundles that connect the vascular 

 bundles of the stem with those of the leaves and branches. When 

 a leaf falls off, the ends of these vascular bundles can be easily 

 seen in the leaf scar (Fig. 28, A) but owing to their minuteness 

 it is not an easy matter to trace them through the stem to the 

 point where they join on to the bundles of the stem. In the 

 branch, however, owing to its size and woody character, the 

 union with the stem is very manifest. Fig. 63 shows several 

 very small branches that continued to keep pace for one or more 

 years with the growth of the stem, but eventually they were 

 killed by the overhanging branches and in time became over- 

 grown with the annual layers of xylem. If the branch is favor- 



