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 



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 

 FIG. 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 



