Page:The Oak.djvu/46

 transverse section exactly at this place we should see no differentiation into axial cylinder and root-cortex, etc.; the small circular mass would consist of cells all alike, and with very thin walls and full of dense protoplasm. This undifferentiated formative tissue is called the embryonic tissue of the root (Fig. 6, m). A little behind this we see the axis-cylinder and root-cortex already formed; still farther away we see the vascular bundles appearing, first as very thin cords, and then getting stronger and stronger as we recede from the tip (Fig. 6, ph and x); and similarly we trace the gradual development of the other parts in acropetal succession—i.e., the nearer we go to the apex the younger the parts are.

Now, there is a conclusion of some importance to be drawn from the putting together of these facts—namely, that all the structures found between the embryonic tissue at the tip of the root and the place where the root joins the stem have been gradually formed from the embryonic tissue in acropetal succession. We may picture this by marking a given level on the root, some distance away from the tip, where the axis-cylinder is sharply marked and has well-developed vascular bundles, the root-cortex is distinct, and the piliferous layer bears root-hairs, and remembering that so many days or weeks ago this very spot was in the then growing-point, and consisted of embryonic tissue with the cells all alike. Or we may put it in a different way thus: the present growing-point consists of embryonic