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. 



Fig. 78. — Cross section of bean root, diagrammatic, a. Primary corte.x', with 

 outer piliferous layer, and inner endodermis ; /', pericycle ; <-, primary xylem ; 

 t/, primary phloem ; e, a ruig of cambium formed of ground tissue between 

 the phloem and xylem. and of pericycle outside the points of protoxyleni ; 

 j\ secondary phloem ; o-, secondary xylem ; li , primary medullary rays : /'' 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 



J' 

 Dcrmatogen 



Periblcm 



Plerome 



chives rise to 



Epidermic 



Epidermal system 



CHypodermis 

 Cortical tissue General cortex 



Endodermis 



Stele 



Ground 

 tissue 



rPericvcle 

 ■ Medulla 

 I Medullary 



System 



of 

 ground 

 tissue. 



i Phloem \ 

 Vascular -. Xylem '^Vascular 



bundles \ Vascular I system 

 cambium J 



