26 THE OAK 



of the section would be much as before, excepting that 

 the distinction between the axial cylinder and the root- 

 cortex would be less marked. 



Now contrast a section cut a couple of inches or so 

 away from the tip, in the region where the root-hairs 

 are well developed. Here we find the axial cylinder 

 much more strongly marked than before, and the pili- 

 ferous layer is very clearly distinguished by the fact that 

 it gives off the root-hairs, each hair arising from one of 

 its cells. 



A little investigation shows that the axial cylinder 

 is thus strongly marked because certain dark-looking 

 structures have now been formed just inside its boundary 

 i.e. just inside the line which delimits it from the 

 root-cortex. These dark structures are the sections of 

 several fine cords or bundles, called vascular bundles, 

 which can here be traced up and down in the root. As 

 the section shows, these bundles are arranged at approxi- 

 mately equal distances in a cylinder ; they form the 

 vascular system of the root, and they always run along 

 the region just inside the outer boundary of the axis- 

 cylinder (fig. 5, B, p and x). 



If we compare our successive transverse sections, 

 and cut others at various levels along the young root, it 

 will be clear that, as we pass from the tip of the root to 

 parts further behind, certain changes must be going on, 

 which result first in the definite marking out of the axial 

 cylinder, and then in the development of these vascular 

 bundles and of other parts we will not describe in detail. 



