Page:The Oak.djvu/52

 the xylem consists of vessels and cells, the former developed centripetally, while the phloëm consists of sieve-tubes and cells. Any cell-tissue which may lie in the center of the axial cylinder, and surrounded by the vascular bundles, corresponds, in popular language, to pith; any that runs between the bundles corresponds to medullary rays.

We now turn to the root as a whole, and examine its behavior in the soil as the young seedling develops further, and in the light of the above anatomical facts.

Although the root-system of the young plant is regularly constituted of a series of lateral rootlets springing from the primary root, the orderly arrangement is soon disturbed when the tertiary and other rootlets begin to develop from the secondary rootlets; moreover, as the age of the tree increases, the tendency to irregularity is increased owing to the production of rootlets of the higher orders at different places, thus interfering with the acropetal succession of the younger rootlets.

At first the root-system is especially engaged in boring into the soil, and, provided the latter is sufficiently deep and otherwise suitable, the tap-root will go down a foot or more in the first year. As the roots thicken they exhibit considerable plasticity, as is especially evinced on rocky ground, where the older roots may often be found in cracks in the rocks, so compressed that they form mere flattened sheets many times broader than they are thick (Fig. 8).