THE ROOT 



109 



serves as a shield, under the cover of which the delicate 

 growing point of the root pushes its way into the soil. 

 If the growing point of the root ended with the unpro- 

 tected youngest cells, it would evidently not be able to 

 fulfil its function ; it is only by pushing forward the 

 cap that it is able to force its way without serious harm 

 through the hard, rough, and sharp 

 particles of the soil. 



At a short distance from the 

 very tip, protected by the cap, the 

 whole external surface of the root 

 is covered with long thin hairs 

 (fig. 33) .^ Every such hair is simply 

 a very much elongated cell of the 

 surface layer. Further from the 

 tip this belt of hairs comes to an 

 end ; there the surface of the root 

 is protected by a hardened outer 

 layer (fig. 33) which has lost its 

 hairs. Still further up this layer 

 becomes torn, and is replaced by 

 another protective tissue like that 

 which covers the stems of trees, 

 and which botanists generally call 

 cork ; like all cork it is impervious 

 to water. Thus the root is differ- 

 entiated into three zones : the cap at the very tip of 

 the root, then a belt of hairs, and, lastly, the oldest 

 part with a dried skin and a corky tissue. This 

 oldest portion cannot absorb water and nutrient 

 substances ; the very tip is also unable to absorb, 

 or at any rate absorbs inadequately, a fact which can 

 be proved experimentally. The absorbing surface of 



' Fig. 33 shows a young root (A) covered with hairs ; a similar root 

 with particles of soil sticking to the hairs (B) ; an older branched root 

 with old parts already without hairs (C) ; and part of the transverse section 

 of a root under the microscope, showing the structure of the hairs and 

 their adhesion to the particles of the soil (D). 



Fig. 32. 



