CHAPTER XII. 



TEGUMENTARY SYSTEM continued. 

 CORK PERIDERM. 



Persistent epidermis Replacement by cork Origin of periderm 

 Superficial periderms Deep-seated periderms Lenticels 

 Bark Smooth and rough barks. 



ONE of the striking differences between trees, shrubs and 

 other woody plants, as contrasted with herbs, depends 

 on the formation of corky or other coverings on the older 

 twigs, branches and stems, to replace the delicate epidermis 

 which is always found on the young green shoots. In very 

 few cases do we find the true epidermis persist beyond a few 

 months on the twigs, though in these exceptional instances 

 e.g. Yew, Spindle Tree (Euonymus), Aristolochia Clema- 

 titis, Acer Negundo and the Mistletoe it goes on growing 

 for from two to several years as the twig thickens. 



In the vast majority of cases, on the contrary, the epi- 

 dermis ceases to grow towards the end of the first summer, 

 and is to be seen at the base of the twigs dying and 

 cracking and peeling off under the tension exerted by the 

 thickening tissues beneath, and it is this dead papery 

 flaking epidermis, exposing the layers beneath, which 

 initiates the changes in smoothness and colour from 

 green to grey, brown, &c. on the older twigs, of which 



