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. Hach of these secondary rootlets 
