Page:The Oak.djvu/48

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30
THE OAK.

sheath limiting what belongs to the axis-cylinder (Fig. 5, c, sh). Inside this endodermis are about two rows of thin-walled cells full of protoplasm, and forming a continuous layer beneath the endodermis. This layer is termed the pericycle (Fig, 5, c, per) and it is a very important structure, because its cells give rise, by repeated divisions, to the lateral rootlets, which then grow out and burst their way through the endodermis, cortex, and piliferous layer, and so reach the soil. It is, of course, necessary to bear in mind that the endodermis and pericycle are concentric cylinders superposed on the axis of the root, as it were, and only appear as rings on the transverse section.

Inside the pericycle are arranged the vascular bundles, and we shall have to devote a few words of explanation to these remarkable and somewhat complex structures.

The section shows that there are about ten alternating groups of tissue constituting these bundles, and again the reader must bear in mind that each group is the transverse section of a long cord running up and down the root. Of these groups five are much more conspicuous than the other five, because they consist chiefly of more or less polygonal openings with firm, dark contours. These are the xylem vessels of the vascular bundles (Fig. 5, c, x), and we must note the following facts about them: In the first place, they are smaller nearer the pericycle than they are nearer the center of the axial cylinder, and the comparison of