THE SEEDLING AND YOUNG PLANT 37 



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 thin-walled cells full of rela- 

 tively 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 milli- 

 metres 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, &c., 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 downwards. 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 contain- 

 ing dissolved salts, &c. 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 



