40 THE PLANT CELL WALL 



possesses both hydrophilic and hydrophobic groups, hence can 

 serve as a "'cement" holding together the hydrophilic polysaccharides 

 and hydrophobic waxes. In succession, we have, from the cuticle 

 inward: cutin only, cutin plus wax, cutin-wax-polysaccharide, and, 

 innermost, if present at all, typical polysaccharide layers. 



Lignin as an internal substance serves as a mechanical and 

 water-proofing agent. Cuticular substances retard water loss, 

 and possess ultraviolet absorbing constituents which may act as 

 a superficial radiation screen. 



