66 



OUTLINES OF PLANT LIFE. 



In a large number of roots, the secondary changes result 

 in increasing the diameter, sometimes very greatly, by the 

 formation of concentric layers of new tissue in two or more 

 regions, called the cambium regions. 



The outer growing layer, or cork cambium, usually formed in the 

 cortex, produces tissues which are of such a nature as to protect the 

 parts within. They constitute ^& periderm, and are ordinarily cork-like, 

 i.e., thin-walled and impervious to water. Those cells which lie outside 



Fig. 54 — A , diagram of primary structure. B, C, diagrams showing the results of 

 secondary thickening from the stelar cambium in the two extreme forms, c, cortex ; 

 en, its innermost layer ; _?*, outermost layer of stele ; ^A', primary bast ; fi/t", sec- 

 ondary bast ; jr', primary wood ; x" x" , secondary wood ; cb, stelar cambium ; r, sec- 

 ondary pith-rays ; m, pith. — After Van Tieghem 



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 wood strands and 

 inside the bast strands (fig. 54). As a result of tangential divisions in 

 this region, tissues similar to those already existing in the stele are pro- 

 duced. 



The relative amount of the new tissues goes far to deter- 

 mine the character of the mature root. 



79. (a) Woody roots. — If mechanical tissues predomi- 

 nate, the root will become strong and rigid, as in the case of 

 trees and shrubs. When the root is long-lived, the forma- 

 tion of new tissues is usually resumed with each season, and 

 the central part, especially, shows in cross-section concentric 

 rings indicating the yearly additions. As the root thickens 



