7 6 



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 



within the stele and follows a 



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 



the older into those of the xylem, often 



^/ f ^S*«Ta1te b r e %VcoX" formin 8 a nearl 5' ^broken mass 



^Hs'of each (figs. 89, 90). The 



lative amount of the different 



tissues which make up these 



ably. Compare with fig. 89, which 

 about five times as highly magnified 



b, b, b, b, four primary phioem bundles; 1 eld 

 b', secondary phloem produced by 

 stelar cambium, as are the four wedges 



b^^^^-iSSSfLSft bundles goes far to determine 

 l!^riS-Af^S emwedges) th e 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 



