Page:The Oak.djvu/56

 nearly horizontal radiating cylinders of soil are placed under contribution as before. Then the secondary rootlets emit tertiary rootlets in all directions—these and the rootlets of a higher order growing without any particular reference to the direction of gravitation, light, etc.—and so place successive cylinders of soil in all directions under contribution as before. By this time, however, the symmetry of the root-system is being disturbed because some of the rootlets meet with stones or other obstacles, others get dried up or frozen, or gnawed off or otherwise injured, and the varying directions in which new growths start and in which the resistances are least, influence the very various shapes of the tangled mass of roots now permeating the soil in all directions.

These roots supply the ever-increasing needs for water of the shoot-system, the leaf-surface of which is becoming larger and larger, and as the greater volume of water from the gathering rootlets has all to enter the stem via the upper part of the main root, we are not surprised to find that the latter thickens, as does the stem; and so with all the older roots—they no longer act as absorbing roots, but become merely larger and larger channels for water, and girder-like supporting organs.