CELL-CONTENTS AND FORMS OF CELLS. 291 



Cork not only occurs as a secondary protective layer, but may 

 also arise in other parts of the plant as a result of injury, as in 

 leaves, fruits, stems, and tubers. It also arises as a result of the 

 disarticulation of the leaf in autumn. 



PERIDERM. The epidermis is not adapted for the protection 

 of the perennial plant organs on account of its thin, frequently 

 delicate structure and its inability to continue with the increase in 

 thickness of stems and roots. Hence it becomes replaced by the 

 periderm, which consists of a lasting tissue, the CORK, and of a 

 meristematic tissue, the PHELLOGEN, which reproduces the cork 

 when it becomes torn or destroyed, by the continued growth in 



fihil 



FIG. 156. Section through a secondary lenticel in the bark of Sassafras; e, epidermis', 

 st, stone cells; phel, phelloderm derived from secondary phellogen and having thick lignj. 

 fied wall; p, parenchyma; c, cork; com, complementary cells. After Weiss. 



thickness of stems or roots. Cork is not only of sub-epidermal 

 origin, but may occur deeper in the cortex (Fig. 158), or even in- 

 side the endodermis. In the latter case, as in roots, it owes its 

 existence to the activity of the pericambium. Superficial, i.e., 

 hypodermal cork, is extremely rare in roots. Not infrequently a 

 layer of cells is formed inside of the phellogen, being termed the 

 phelloderm. They usually contain plastids ; the walls are moder- 

 ately thick and free from intercellular spaces (Fig. 156). 



Lenticels may be described as biconvex fissures in the periderm 

 which permit of the easy access of air to the intercellular spaces 

 of the rather loosely arranged cells lying beneath them (Fig. 



