54 BOTANY. 



cells, whose cavities are filled with air only. The walls in 

 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 regular 

 shape (generally cuboidal) and fitted closely to each other 

 (Fig. 33). 



103. Cork-substance is formed by the repeated subdi- 

 vision of the cells of a meristem layer of the fundamental 



Fig. 34. — Cross-section through a lenticel of Birch, e, epidermis; s, a breath- 

 ing-pore. Magnified 2M times. 



tissue (Fig. 33); these continue to grow and divide by par- 

 titions parallel to the epidermis, forming layers of cork 

 with its cells disposed in radial rows (Fig. 33, k). Shortly 

 after their formation the cork-cells lose their protoplasmic 

 contents, while beneath them new cells are constantly being 

 cut off 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. 



104. 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- 



