20 



VEGETABLE PHYSIOLOGY 



complexity in the different groups. In the smallest forms, 

 which are only herbaceous in habit, we find the protective 

 mechanism takes the shape of a thickening and cuticulari- 

 sation of the outer walls of the cells of the epidermis 

 (fig. 22). The protection secured is twofold ; evaporation 



of water is prevented, and so 

 an economy of the supply is 

 secured, while the dangers 

 incident to cold or heat are 

 minimised. 



In plants of sturdier habit 

 the protection afforded by 

 after a while by a more 

 complicated tegumentary sheath. Certain cells become 

 specialised and form layers of cork (fig. 23), which arise 



FlG. 23. OUTEB PORTION OF CORTEX 

 OF YOUNG TWIG OF LIME. 



per, cork layer. 



the epidermis is replaced 



FIG. 24. SECTION OF BARK OF Quercus sessiliflora. (After Kny.) 

 IH-, cork layers arising at different depths in the cortex. 



successively at gradually increasing distances from the 

 exterior, and in the case of trees finally lead to the con- 

 struction of a bark (fig. 24). The corky formations are 



