WOOD AND BARK. 



181 



that the name of the whole plant often means only 

 the useful bast which the plant contains. 



The outer bark, or bark proper (primary cortex), 

 consists mainly of fundamental tissue (Figs. 157 pr, 

 Z 59 /) Generally, upon its outside layer, and 

 so just under the epidermis, are formed cells, with 

 walls of cork. This cork cell tissue, through 

 which water hardly penetrates, prevents nourish- 

 ment from reaching the epidermis of the young 



Fig. 1 60. Transverse section of a one year's shoot 



of Ailanthus (foreign), magnified 350 times. 

 r, primary cortex ; k, cork cells ; c, dead epidermis. 



stem, and so towards autumn the once green 

 and living skin becomes brown, and at length it 

 withers and dies away. Its place is taken by the cork 

 tissue, which has caused its death (Figs. 160, 157 k}. 

 Often the cork tissue grows to a considerable thick- 



