THE CORK 69 



chapter on storage of reserve materials, but they also have a 

 place here because they protect the chemically active cells be- 

 neath by filtering out some of the sun's heat and by supplying 

 them with water when evaporation is reducing their turgidity 

 too low for the normal performance of their functions. In 

 some cases, however, the accessory layers have thick walls and 

 relatively small cavities and assist the outer layer chiefly in 

 protecting against mechanical injuries (Fig. 34). 



The Cork 



It is the rule in the perennial parts of trees and shrubs that 

 the epidermis is sooner or later replaced by the cork tissue (Fig. 

 24); in many instances the change beginning in the first year, 

 and in others not until the lapse of many years. The cork then 

 assumes the protecting and waterproofing functions of the dis- 

 placed tissue. 



By additions from the phellogen or cork cambium the cork 

 tissue becomes several to many cell-layers thick. After the 

 cork cells have attained their growth they die and their cell- 

 sap is replaced by air, a fact which accounts for the lightness 

 of cork. The walls of the cork, as a rule, are thin and in some 

 instances suberized through and through, while in others a 

 middle lamella of cellulose is present. The suberization or 

 waterproofing of the wall is accomplished by chemical changes 

 in the original cellulose wall and by additions of suberin layers 

 to this, or by the latter process alone. The suberin is quite 

 similar to cutin in its chemical constitution and physical prop- 

 erties, and both are of the nature of wax. 



Where the cork becomes only a few cell layers in thickness 

 the cells are apt to be flattened so that the tangential diameters 

 are broader than the radial, but where annual additions are 

 made by the phellogen, the cells first formed are not so flat- 

 tened and may be even larger in the radial diameter, and these 

 are succeeded at the close of the season's growth by radially 

 narrower cells, so that rings of growth appear in the cork as 



