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.—OvurEr Portion oF Cortex 
ap VouNG: Tate on Lain, cells of the 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. 
Fig. 24.—Srorion or Bark or Quercus sessiliflora, (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 
