Page:The Oak.djvu/55

 thin-walled cells full of relatively stiff protoplasm with very little water. Hence the growing-point is a firm body. The most active growth of the root takes place at a region several millimetres behind the root-cap, between it and the fixed point above referred to; hence the apex of the root is really driven into the ground between the particles of rock, etc., of which the latter is composed. This driving in is aided by the negative heliotropism, the positive geotropism, the circumnutation, and other irritabilities of the apical portions of the root, and it bores its way several centimetres downward. As it lengthens—by the addition of cells produced by the division of those of the embryonic tissue, and by their successive elongation—the older parts behind go on producing root-hairs, and thus a vertical cylinder of soil around the primary root is gradually laid under contribution for water containing dissolved salts, etc. In those parts of the root which are behind the growing region no further elongation occurs; hence the tips of the lateral rootlets (which have been developing in the pericycle at the circumference of the axial cylinder of vascular bundles) can now safely break through the cortex and extend themselves in the same manner from the parent root as a fixed base, without danger of being broken off by the elongation of the growing parts. Each of these secondary rootlets grows out at an obtuse angle from the primary root, and not vertically downward, and as it does so a similar wave of root-hairs is developed along it; thus a series of 4