68 GROUPS OF TISSUES, OR TISSUE SYSTEMS 
manent but more often a layer of cells starting at the 
phellogen and dipping inward into the cortex and finally 
back to the phellogen also becomes converted into 
phellogen and begins to produce cork. The more or 
less lens-shaped mass of tissue cut off by this process 
promptly dies from lack of water and eventually scales 
off. Thus is formed the flaky type of bark. This proc- 
ess is repeated time and again so that the bark remains 
only about the same thickness, no matter what the age 
of the tree. 
100. Lenticels. As cork is about to form, a phellogen 
of special type arises under many of the stomata on the 
ao young stems and twigs. This forms 
a loose mass of cork with large inter- 
cellular spaces connecting through 
the intercellular spaces in the phel- 
logen (these being lacking in ordinary 
Fic. 37.—Lenticels, Phellogen and cork) with those of the 
cortex. This mass of cork cells rup- 
tures the epidermis and forms a minute lens-shaped 
fleck. These lenticels function then as openings for the 
exchange of gases while at the same time the mass of 
loose cork cells greatly reduces the water loss. 
101. In addition to the foregoing cases cork is also 
formed in many plants as a result of wounds. The 
injured cells die but those immediately or but a short 
distance below become converted into phellogen which 
produces a cork layer that forms an effective barrier 
against further water loss and probably also prevents in a 
large measure the entry of foreign organisms. Fre- 
quently this cork thus formed serves as an abscission 
layer, i.e. it splits, and permits the dead tissues to slough 
off. The layers normally found at the base of the leaf 
petiole in the autumn are of similar nature, serving to 
a 
