76 PLANT LIFE. 



and are ordinarily cork-like, i.e., thin-walled and impervious 



to water. Those cells which lie outside a layer of cork are 



therefore cut off from a supply of food and soon perish. 



The inner growing layer, or stelar cambium, is developed 



y within the stele and follows a 



, ^-igrt^j^^^SSfejL tortuous course, lying outside the 



xylem and inside the phloem 

 bundles (fig. 88). As a result 

 of tangential divisions in this 

 region, tissues similar to those 

 already existing in the stele are 

 produced. On the outer side 

 the cells differentiate mainly 

 into the tissues of the phloem, 

 and on the inner side mainly 



Fig. go.-Transverse section of the older int O those of the Xylem, often 



^V^it7ZlT^rsLZS-y Arming a nearly unbroken mass 

 ffi^^^5SSRT&.S^ of each ( fi S s - 8 9> 9o). The 

 ^.MoS^LS^LSffi: relative amount of the different 

 &^L, p if«^^ tissues which make up these 

 bunX s n not y shown"" TrfevTi^LxfthS bundles goes far to determine 

 t SSS^-SS^^ 1 ! mwedge8 - ) the character of the mature root. 

 87. (a) Woody roots. — If mechanical tissues predominate, 

 particularly in the xylem, the root will become strong and 

 rigid, as in the case of trees and shrubs. When the root is 

 long-lived, the activity of this stelar cambium is usually 

 resumed with each season, a layer of tissue being thereby 

 added to the outside of the xylem region, and a thinner layer 

 to the inside of the phloem. The woody part, especially, 

 shows in cross-section concentric rings indicating the yearly 

 additions. Since the material produced by the stelar cam- 

 bium usually greatly increases the diameter of the root, the 

 outside parts become fissured lengthwise. Thus, in an old 

 and much-thickened root of the woody type, the periderm 



