290 THE ROOT 



root cap is composed of living cells derived from the 

 primary meristem (p.m.) from which they are continually 

 renewed by active cell division, while those on the surface 

 of the cap break down into mucilage and are rubbed 

 off as the root tip is pushed through the soil. Behind 

 the root tip (Fig. 47, a, A) there is a bare stretch of 

 varying length, often about 5 to 10 mm. long, where 

 the root is actively growing in length : this is the 

 elongating region (E). Behind this again is the root- 

 hair region (R), thickly covered under favourable 

 conditions of soil moisture and aeration, with root 

 hairs, each of which is a tubular outgrowth from a 

 single cell of the surface layer (piliferous layer) (Fig. 48). 

 The root hairs come into intimate contact with moist 

 soil particles and absorb water owing to the greater 

 osmotic strength of the cell sap than of the dilute 

 solution outside, and also salts in solution. It should 

 be understood that the entrance of the salts into the 

 root hair is independent of the entrance of water, and 

 depends primarily on the difference of concentration of 

 the various solutes inside and outside the root-hair cell. 

 The ectoplasm of the root hairs, which is impermeable 

 to the " osmotic substances " in the cell, must of 

 course be permeable to these salts. New root hairs are 

 formed in front of the root hair region by the growing 

 out of the cells of the piliferous layer into papillae which 

 elongate to form the hairs (Fig. 49, A). The hairs 

 at the back usually die, but in some cases they are 

 persistent and clothe the root for a long distance even 

 to the point of origin of a root several inches long. 

 But ordinarily the life of a root hair is short, and behind 

 this region the root is again bare. 



Growth of the Boot Tip. — As new cells are formed by 

 division of the meristem the ones already formed pass 



