82 



Plants and their Ways in South Africa 



activity is first evident. Once the ring is formed it lays down 



wood to the inside, phloem to the outside, with medullary rays 



at intervals, much as secondary thickening takes place in a stem. 



When cork is formed, it also takes its rise in the pericycle, 



FlG. 78. Cross section of bean root, dia 

 outer piliferous layer, and inner endodermi: 

 d, primary phloem ; e, a ring of cambium 



matic. a, Primary cortex, with 



, pericycle ; c, primary xylem ; 



formed of ground tissue between 



the phloem and xylem, and of pericycle outside the points of protoxylem ; 

 f t secondary phloem ; g t secondary xylem ; //, primary medullary rays ; b' peri- 

 cycle which has formed an outer layer of cork, and secondary cortex within ; 

 a' remains of primary cortex cut off by the cork. 



and in time cuts off the entire cortex ; so that a root in which 

 considerable secondary growth has taken place, may show very 

 little actual increase in diameter, as in C, Fig. 78. 



The tissues of roots and stems will be understood better 

 by tabulating them : 



The primary meristem 

 Dermatogen 



Periblem 



Plerome 



gives rise to 



\ 



Epidermis 



Cortical tissue 



Stele 



Epidermal system 



[ Hypodermis 

 | General cortex 

 Endodermis 



( Pericycle 



j Medulla 



I Medullary rays 



System 



of 



ground 

 tissue. 



[Phloem 



Vascular -j Xylem 

 bundles ( Vascular j 

 iumj 



cambi 



Vascular 



system 



