110 



PLANT LIFE. 



The outside tissues of the periderm 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 mechanical tissues by 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 layer of cells which 



v 



FIG. 127. 



FIG. 128. 



FIG. 127. Part of a transverse section through the cork cambium and the tissues it pro- 

 duces in the European elm. k, cork cells, the innermost layer still with some proto- 

 plasmic contents; ph, cork cambium; pd, secondary cortex. Highly magnified. 

 After Haberiandt. 



FIG. 128. Part of a transverse section of young stem of cherry, showing formation of 

 periderm. e, epidermis; k, cork; ///, cork cambium, with one row of secondary 

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



becomes the generating layer. It may be formed in one 

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

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



