60 BOTANY. 



some cases (e.g., the cork-oak) are thin and weak, while in 

 others (e.g., the beech) they are much thickened, and in 

 all cases they are nearly impermeable to water. True cork 

 is destitute of intercellular spaces, its cells being of regu- 

 lar shape (generally cuboidal) and fitted closely to each 

 other (Fig. 38). 



105. Cork-substance is formed by the repeated subdivi- 

 sion of the cells of a meristem layer of the fundamental 



Fig. 39.— Cross-section through a lentieel of Birch, e, epiaermis: s, a 

 breathing-pore. Magnified 280 times. 



tissue (Pig. 38) ; these continue to grow and divide by par- 

 titions parallel to the epidermis, forming layers of cork 

 with its cells disposed in radial rows (Pig. 38, k). Shortly 

 after their formation the cork-cells lose their protoplasmic 

 contents, while beneath them new cells are constantly be- 

 ing cut ofE from the cells of the generating layer; in this 

 way the mass of dead cork-tissue is formed and pushed out 

 from its living base. 



106. The generating tissue is called the Cork-cambium, 

 or Phellogen; it occurs not only in the hypoderma, but in 

 any other part of the fundamental system, and in the sec- 

 ondary fibro- vascular bundles. When a living portion of 



