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 @). 
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. 
