THE SEEDLING AND YOUNG PLANT. 37 



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, be- 

 tween 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 circumnuta- 

 tion, and other irritabilities of the apical portions of 

 the root, and it bores its way several centimetres down- 

 ward. As it lengthens by the addition of cells pro- 

 duced by the division of those of the embryonic tissue, 

 and by their successive elongation the older parts be- 

 hind 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 devel- 

 oping 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 



