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. 



ABC 



Fig. 78. — Cross section of bean root, diagrammatic, a, Primary cortex, with 

 outer piliferous layer, and inner endodermis ; b, pericycle ; c, primary xylem ; 

 d, primary phloem ; e, a ring of cambium formed of ground tissue between 

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

 / secondary phloem ; g, secondary xylem ; h, 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 



I 



Dermatogen 



Periblem 



gives rise to 



J'. 

 Epidermis 



Cortical tissue 



Epidermal system 



{Hypodermis 

 General cortex 

 Endodermis 



Plerome 



Stele 



Ground JMeiulk' 

 tissue \ - - - - 



I Medullary rays 



1 



System 



ground 

 tissue. 



I" Phloem 

 Vascular \ Xylem I Vascular 



bundles [Vascular [system 

 cambiumj 



