294 



A TEXT-BOOK OF BOTANY. 



(Fig. 158). When the phellogen develops in the deep-seated 

 tissues, the cells outside of the corky layer sooner or later die and 

 slough off. This is due to the fact that the cork cells are suberized 

 and do not permit the passage of the cell-sap containing food sub- 

 stances. In large shrubs and trees with thick stems and trunks 



A B 



Bf JK A A^sm^ ^"^ 

 p 



Fig. 159- Development of Bork: A, in bark of cherry (Prunus Cerasus), showing 

 a layer of periderm (k) with thin-walled eork cells; bast fibers (Bf); parenchyma (p) ; 

 stone cells (st) occasionally branching and lengthened into fibers. B, inner layer of periderm 

 of Quercus Robiir, showing compactly arranged, thick-walled cork cells (P) filled with a 

 reddish phlobaphene or altered tannin; starch-bearing parenchyma (p) ; stone cells (st) ; 

 sieve tubes (Bg) ; bast fibers (Bf) ; prism of calcium oxalate (kr) ; several rows of thick- 

 walled, porous cells (x). — After Dippel in "Das Mikroskop." 



a number of successive layers of cork or periderm are formed. 

 These layers with the dead cortical tissues between them persist 

 to some extent and constitute what is known as bork, i.e., bork 

 consists of a number of alternate layers of periderm and cortical 



