52 VEGETABLE PHYSIOLOGY 
physical, and mechanical properties, which fit the definite 
tissues formed for the functions which fall to them. 
Temporary structures generally possess a different chemical 
composition from permanent ones. The transitory cuticle 
gives place to the more permanent cork, and this becomes 
strengthened by the introduction of sclerenchymatous 
elements as the cork formation becomes continuously more 
deep-seated. The strengthening tissue varies similarly ; 
the walls of collenchyma, though thickened in a particular 
way, are not chemically changed in the same manner as those 
of sclerenchyma or woody tissue, for their cellulose under- 
goes no conversion into, or impregnation with, lignin. 
The fibres of the bast differ from those of the wood in His 
same particulars. 
