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 
Fic. 23.—OuTER Portion oF CornTEX minimised. 
or Youve Twice or Lime. In plants of sturdier habit 
per selene 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. 28) which 
Fic. 24.—SEctTIon of Bark or Quercus sessiliflora. (After Kny.) 
pe, 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 
