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 
sh 
KIOY B 
Fia. 34.—Cross-section through a lenticel of Birch. e, epidermis; s, a breath- 
ing-pore. Magnified 280 times. 
i, 
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, &). 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- 
