50 STRUCTURAL BOTANY 



from the stem into the leaf, are called common bundles, 

 because they are common to both organs. That part 

 of a common bundle which passes through the stem 

 is called a leaf-trace, because its position is dependent 

 on that of the leaf with which it is connected. We 

 see, then, that the whole bundle - system of the stem 

 is built up of these leaf-trace bundles, the upper ends 

 of which run out into the leaves themselves. As the 

 various bundles are united to one another at the 

 nodes, it is equally easy for the food-substances which 

 they conduct to pass out into one of the leaves or to 

 continue their course straight up the stem, while, 

 on the other hand, those food-substances which are 

 formed in the leaves have ready access through the 

 stem-bundles to all parts of the plant. In fact, the 

 arrangement of the bundles is adapted to facilitate 

 communication in every direction between leaf and 

 stem. The connection with the root will be described 

 later on. 



Where an axillary branch is given off, its leaf -trace 

 bundles are continuous with those of the main stem. 



Having described the longitudinal course of the 

 vascular bundles, we have next to examine their 

 internal structure. We will fix our attention on a 

 single leaf-trace bundle, such as that which is shown 

 in transverse section in Fig. 21. 



The bundle is made up of two quite different kinds 

 of tissue. The inner half of the bundle that, namely, 

 which is turned towards the centre of the stem- 

 consists chiefly of rather large cells, the walls of which 

 are somewhat thickened, and are hard and woody 



