24 



VEGETABLE PHYSIOLOGY 



great specialisation for protective purposes, a particular tegu- 

 mentary system being developed, which varies in com- 

 plexity in the different groups. In the smallest forms, 



which are only herbaceous in 

 habit, we find the protective 

 mechanism taking the shape of 

 a thickening and cuticularisa- 

 tion of the outer walls of the 



FIG. 23. OUTER PORTION OF CORTEX ,, ., , , 



OF YOUNG TWIG OF LIME. G ^ f th e outermost layer 



per, cork layer. ph, meristem layer. (fig- 22). The protection 



secured is twofold : evapora- 

 tion of water is prevented, and so an economy of the supply 

 is secured, while the dangers incident to cold or heat are 

 minimised. 



~~pe 



FIG. 24. SECTION OF BARK OF Quercus sessili flora. (After Kny.) 

 pe, cork layers arising at different depths in the cortex. 



In plants of sturdier habit the protection afforded by 

 this outermost layer or epidermis is supplemented after a 

 while by the development of a more complicated tegu- 

 mentary sheath which replaces the epidermis when the 



