THE STEM 55 
As the oak-tree becomes older there is a more com- 
plex formation of cork, which leads to the rough scaly 
appearance of the bark characteristic of full-grown trees. 
31. Lenticels.—When cork has been formed round 
a stem the stomata of the stem are useless, for cork 
being impermeable to cell-sap all the cortex outside it 
dies. To replace these stomata new organs are formed 
Fic. 36.—Cork Layer of Beech, 
ce. Cuticle. ¢. Epidermis, y. Cork. ph. Phellogen, col. Collenchyma 
of cortex. cor. Cortex. (From Darwin’s ‘‘ Klements of Botany ”.) 
in older stems called LunricEts (fig. 37). Underneath 
the stomata a meristematic tissue arises called the cam- 
bium of the lenticel, which gives rise on the outside to 
a number of cells with numerous air spaces between 
them called the COMPLEMENTARY CELLS of the lenticel, 
and on the inside to secondary cortex. The activity of 
this meristematic tissue soon causes the epidermis to be 
ruptured, and thus air can pass freely in and out of the 
