5° 



THE STRUCTURE OF ECONOMIC PLANTS 



xy par 



in circumference, so that it persists as a continuous zone outside of 

 the secondary phloem. Commonly, the pericyclic region also 

 increases in width, owing to successive tangential cell divisions; 



and this results in the for- 

 mation of a few to many- 

 layered zone of active 

 pericyclic tissue. The epi- 

 dermis, cortical paren- 

 chyma, and endodermis 

 become fractured and dis- 

 integrate. As this occurs, 

 the outermost layers of 

 derivatives of the pericycle 

 form a phellogen (cork 

 cambium) which develops 

 a protective periderm. 



In many fleshy roots the 

 pericycle may form a zone 

 of storage parenchyma; 

 and, in some cases, the 

 term secondary cortex has 

 been applied to this region. 

 Since it is clearly of stelar 

 origin, this terminology is 

 apt to be misleading; and 

 it is more accurate and 



Fig. 19. Transection of a sector of the stelar Convenient tO USe the term 



portion of an old root of Pastinaca showing the perky elk Xpne. In Very old, 



primary xylem strand and the development of -^ , ■' 



secondary xylem. The enlargement and division WOody rOOtS, especially of 



of the parenchymatous cells have resulted in a dis- t5erennial t)lantS the tDCri- 

 persion of the secondary xylem vessels : >nx, metaxy- . 1 ' u 1 



lem; par, interstitial parenchyma which has become CyCle may alSO De lOSt, 



radially elongated; px, protoxylem; ra, pericyclic and, when this OCCUrS, a 

 ray; xy par, xylem parenchyma; xy 1, secondary ■ • 1 



xylem. ' ' ^ ' ■" •' new protective periderm is 



formed from a phellogen 

 arising in the tissues of the secondary phloem. While the 

 primary phloem and other tissues outside the cambium may be 

 lost, the primary xylem remains in place at the center of the axis. 

 Frequently, there is a proliferation of the cells of fundamental 

 parenchyma adjacent to the primary xylem strand, or separating 

 the metaxylem from the protoxylem, and this results, in a spreading 



mx 



— par 



