THE STRUCTURE OF THE ROOT 41 



its more external layers and in time ruptures them- 

 The pressure of wet soil against its epiblema is not 

 unlikely to set up decay. The cortical tissues and the 

 epiblema are therefore inadequate to protect the gradu- 

 ally thickening central cylinder. But these difficulties 

 become obviated as the growth proceeds. By the time 

 the central cylinder has become only slightly thickened 

 the zone of the root hairs has been removed to some 

 distance in advance, by the continuous elongation of the 

 root. The cortex of the thickened part is consequently 

 not supplied with water as before, and ceases to play its 

 original part in transporting the water upwards. The 

 hairs having disappeared from that region too, the 

 epiblema has not its first importance there. The pres- 

 sure of the gradually increasing girth stimulates the 

 cells of the pericycle and they again show the power of 

 increasing by tangential divisions. The pericycle be- 

 comes uniformly several cells thick, one layer of which 

 remains meristematic. It cuts off repeatedly bands or 

 shells of cells which remain very regular in shape,, 

 appearing in transverse sections like rows of bricks. 

 The outermost ones lose their contents and their walls 

 are transformed into suberin, a substance closely re- 

 sembling the cuticularised material of the endodermis. 

 This band of cells forms what is known as a cork layer. 

 It extends completely round the root and forms a 

 strongly protecting sheath. It is perforated here and 

 there by little rounded masses of cells loosely arranged 1 

 so that air can pass between them. These are known as 

 lenticels ; they serve to admit air to the interior of the 

 root. It is quite impervious to water except at these 

 spots, and hence preserves the root from loss of water by 

 outward leakage. The cells of the cortex and epiblema 

 may now rot away without causing any damage to the 

 root. The latter acquires, in fact, a fresh exterior of 

 a more resistant and permanent character than the 



