48 VEGETABLE PHYSIOLOGY 



physical properties render it particularly adapted to serve 

 as the material of which the tissues conducting the stream 

 of water are composed. Its lack of flexibility or extensi- 

 bility makes it suitable for the securing of rigidity in tissues 

 or structures needing considerable power of resistance to 

 winds or storms. It is thus a valuable material in the con- 

 struction of sclerenchyma. 



The protective tissues show a different modification of 

 the original structure. In the simplest cases we have seen 

 that the degree of protection secured is slight, and evidently 

 only transitory. The epidermis is, in these cases, the seat 

 of the changes which may be observed. The cells show 

 their walls sometimes very materially thickened on the 



exposed sides (fig. 45), 

 though the thickness 



varies in different 



cases. Layer after 

 layer of substance is 

 deposited upon the 

 original wall in these 

 regions, the other 



FIG. 45. SECTION THROUGH EPIDERMIS OF LEAF, t . . 



SHOWING THE OUTER WALLS MATERIALLY pai'tS Of it 1'emaining 



THICKENED. ... m , ,-1 i 



thin. The thickness 



itself secures a certain amount of protection against cold, 

 but to prevent absorption or dissipation of water or of 

 gases by these membranes, a chemical change also is 

 brought about. The outer layers of the wall undergo a 

 process known as cuticularisation, which generally extends 

 about halfway through its thickness. This change in 

 the outer walls of numbers of contiguous cells renders 

 it possible to strip off from such a tissue a piece of 

 apparently structureless membrane which is technically 

 called the cuticle, and which consists of nothing more 

 than these altered layers of the outermost walls of the 

 contiguous cells. The alteration of the chemical character 

 of this membrane in forming the cuticle of the epidermis 

 is due to the transformation of its cellulose or pectose 



