Page:The Oak.djvu/111

Rh of that of the primary xylem; there are also no spiral vessels formed now. In fact, the structure of the vascular bundles of the root has now changed its character, and from this point forward the root increases in thickness exactly as the stem does, whence I refer the reader to the following chapter for further details.

The development of the layers of cork which now surround the thickening axis-cylinder go on forming year after year, as the cambium forms more xylem and phloëm and so thickens the root; were this not the case, the layer of cork would soon be ruptured as the root increases in diameter. Such rupture, in fact, does occur, but the cork-forming tissue in the pericycle goes on growing and acts as a cork-cambium, and repeatedly develops more cork to make good the layers which are being split and worn off in the soil.

From what has been said it will be understood that a transverse section of an old root differs entirely in structure from that of a young one, although all the changes in the former can be correlated with the primary structures of the latter. In the first place, such a section shows no piliferous layer or cortex, both having been sloughed off long ago; the protective function of these layers is now assumed by the cork jacket (often called periderm) developed by the cork-cambium cylinder in the pericycle, and even this will not show all the cork that the cambium has developed, because many outer layers will have flaked away, just as the present outer layers are doing.