THE STEM. 



91 



formation of one or two layers of actively dividing cells, 

 roughly parallel to the surface. When there are two such 

 layers they are concentric. 

 They are formed from existing 

 cells which retain or resume 

 their power of active growth 

 and division. The develop- 

 ment of thp tissues from the 

 external growing layer, called 

 the cork cambium, results in the 

 formation of secondary cortex, 

 called periderm, while the 

 tissues arising from the in- 

 ternal growing layer, or sielar 

 cambium, form the secondary 

 wood and secondary bast (fig. 

 81). 



112. Cork. — ^The outer tis- 

 sues of the periderm rarely 

 remain living. The close-set 

 flat cells early lose their con- 

 tents, and the walls become 

 waterproof, forming cork (fig. 

 82). Other cells may be al- 

 tered into mechanical tissues by 

 the thickening of their walls 

 and the death of the proto- 

 plasm. Zones of cork often Fig. Si.— Part of a transverse section of 

 . , . J . 1 a young stem of cinchona in process of 



alternate in the periderm with secondary thickening, /z, hairs; c, epi- 



zones of mechanical tissues. 

 Since almost no water 



no water can 

 pass through a cork zone, it is 

 evident that all parts lying out- 

 side of one are cut off from a supply of nourishment, and must 



dermis ; k^ corlc-cambium ; m.r, cortex ; 

 J, gum-resin tubes in cortex ; sb, primary 

 bast strand ; c, stelar cambium ; g, h, 

 secondary wood \ ?nk, pith rays ; ?«, 

 pith The tissue between sb and c is 

 secondary bast. Highly magnified.— 

 After Tschirch. 



