52 VEGETABLE PHYSIOLOGY 



Temporary structures possess a very 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 

 deeply 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 the 

 same particulars. 



