20 



VEGETABLE PHYSIOLOGY 



complexity 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 cuticulari- 

 sation of the outer walls of the cells of the outermost layer 

 (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 



FIG. 23. OUTEB POBTION OF CosTEx minimised. 



OF YOUNG TWIG OF LIME. i n p i an t s of sturdier habit 



the protection afforded by 



this outermost layer or epidermis is replaced after a while 

 by a more complicated tegumentary sheath. Certain cells 

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



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

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



arise 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 



