THE COVERING TISSUES 103 



cells are obscure. Upon sectional view the cells appear 

 rectangular or oblong in form and are usually arranged 

 in regular rows (Plate 39). Irregular thickening of 

 the walls of cork cells may be noted in some instances, 

 the walls toward exposed surfaces being usually so 

 thickened. In fully mature plants, especially old 

 trees, the periderm consists of cork together with 

 parenchyma, fibrous elements and, in some instances, 

 stone cells (Plate 39). These additional structures 

 result from the formation of corky tissues beneath 

 fissures or cracks in the original periderm. The corky 

 layers are produced by a secondary phellogen region 

 established well within the fissure. Increasing amounts 

 of cork are .produced and the tissues external to this 

 new periderm are thus forced outward, finally becoming 

 part of the outer bark or bork. 



The functions of cork cells are to afford protection 

 against mechanical injury, insect attacks and access 

 of water to the inner tissues, and to prevent loss of 

 water through excessive evaporation. 



