Page:The Oak.djvu/54

 The root of the oak, speaking generally, is a typical root in the following respects: It consists, as we have seen, of a primary or tap root which develops secondary or lateral roots in acropetal succession, and these in their turn produce rootlets of a higher order. These secondary, tertiary, etc., rootlets arise endogenously, taking origin from the pericycle at the periphery of the strand of vascular bundles which traverse the central axis, and then bursting through the cortex to the exterior. The primary root, as well as the rootlets of all orders, are provided with a root-cap at the tips, and they all agree in being devoid of chlorophyll or stomata. From the outer layer of cells—the piliferous layer, corresponding to an epidermis—root-hairs are developed at some little distance behind the root-cap, and these superficial cellular outgrowths also rise in acropetal succession, the older ones behind dying oS as the younger ones arise farther forward. If we bear in mind all that has been shortly stated above, it will be very easy to figure the behavior of the root-system as it penetrates the ground, and the following short description of the biology of the root may render the matter clear. When the radicle commences to bore down into the soil it puts forth a large number of root-hairs from the parts a few millimetres behind the tip, and these attach themselves to the particles of soil and supply points of resistance; the tip of the radicle is protected by the slippery root-cap, and it must be borne in mind that the embryonic tissue of the growing-point consists of