Page:The Oak.djvu/47

 cells all alike; in a few days some of these cells will have changed into constituents of the axis-cylinder and cortex, and subsequently some of them will give rise to vascular bundles, etc. Not all, however; and it is necessary to understand that as the embryonic tissue moves onward and leaves the structures referred to in its wake, it does so by producing new embryonic cells in front—i.e., between the present ones and the root-cap.

We must now look a little more closely into the structure of the axial cylinder, at a level a little behind the region where the root-hairs are produced on the piliferous layer.

A thin transverse section in this region shows that the root-hairs have all died away, and the walls of the cells of the piliferous layer are becoming discolored, being, in fact, converted into a brown, cork-like substance impervious to moisture, or nearly so; consequently the piliferous layer is no longer absorptive, and it will soon be thrown off, as we shall see.

The cortex offers little to notice, except that its cells are being passively stretched or compressed by the growth and processes going on in the axial cylinder; and it is this cylinder that attracts our special attention, and several points not noticed before must now be examined in some detail.

In the first place, the cylinder is demarkated off from the cortex by a single layer of cells shaped like bricks, and with a sort of black dot on the radial walls; this is called the endodermis, and may be regarded as a