Page:The Oak.djvu/44

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rest 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 piliferous 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 approximately 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 axial 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 farther 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.