86 NATURE OF CORK TISSUE 



increase materially in size, such as the majority of our annual 

 plants; but in long lived stems, as shrubs and trees, where growth 

 goes on from year to year the epidermis is not able to keep pace 

 with the growth. 



38. Cork Tissue. — ^To meet this condition a new tissue, the 

 cork, is developed from certain cells called the cork cambium, 

 usually situated near the epidermis (Fig. 47). The cells of the 

 cork cambium divide much after the manner noted in the cam- 

 bium of the vascular bundle, but there is this difference, the outer 

 cell of the two daughter cells becomes a cork cell while the inner 

 cell remains capable of further division. Only rarely does the 

 outer daughter cell act as the cambium cell while the inner cell 

 develops into one of the cells of the cortex. As soon as the 

 cork cells have reached their growth, a substance called suber 



Fig. 47. Cross-section of the outer part of the stem of geranium. The 

 •corlc cambium, c, originating in the cells immediately below the epidermis. 



^begins to be deposited in their walls. This completely changes 

 the properties of the cellulose walls and renders them impervious 

 to fluids and gases. So the cork cells have the same physical 

 properties as the cuticle of the epidermis and owing to the 

 continued activity of the cork cambium they are able to keep 

 pace with the growth of the stem. It is evident that the cork 

 cells must die as soon as they become impervious to fluids, and 

 it must also follow that all cells lying outside of these cork cells 

 will die since no fluids can reach them from the vascular bundles. 

 These dead cork cells give the characteristic aspect to the outer 

 bark of trees and we would naturally come to think of the coarse, 

 dark bark as composed of rather thick cells. As a rule, however, 

 the cork cells are quite delicate and compactly put together (Fig. 

 48) and the dark color is more usually due to a discoloration of 

 their walls or to the dried remains of the cell contents. The 



