no 



PLANT LIFE. 



The outside tissues of the jjeridevm rarely remain living^ 

 No intercellular spaces arise between the flat cells, which 

 early lose their contents, while the walls become waterproof. 

 Such a tissue is known as cork (fig, 128). Other cells may 

 be altered into mecliaiiical tissues bv the thickening of their 

 walls and the death of the protoplasm. Zones of cork often 

 alternate in the periderm with zones of mechanical tissues 

 Since no water solution can pass through a cork zone, it is 

 evident that all parts lying outside of one are cut off from a 

 supply of nourishment, and must therefore perish sooner or 

 later. 



137. Location of cork cambium. — How much will thus be 

 killed depends upon the position of the la\ er of cells which 



Fi 



'^^^^ 



Fig. 12.S. 



Fir.. 127. — Part of a trnns\X'rsij section tlir<nig]i the cork canihiuni and the tis.^iies it {iro- 

 diices in tile European elm. /.-, cork cells, the iiniennost la\er stiil with some [noto- 



plasinic contents; /'It, curk cambium; /</, secondaiy cortex. Highly iii.iynilied 



After Haberiandt. ' ^ . ^ ■ 



Fit;. J2.S. — Part of a trans^-erse section of young stem of c hcriT. sliow-ing foimatioii of 

 periderm, c, epidermis; /■. cork; /// , cork camliium, with one row of sectuidary 

 cortex below; c, cortex. Highly magnified.— After Haberiandt. 



becomes the generating layer. It may be formed in one 

 of three places: (</) It is sometimes in the epidermis itself 

 (fig. 125), in which case only the outer half of the epidermal 



