48 VEGETABLE PHYSIOLOGY 
physical properties render it particularly adapted to serve 
ag the material of which the tissues conducting the stream 
of water are composed. Its slight 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 
construction 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 
Fic. 45.—SEcTION THROUGH EPIDERMIS OF LEarF, : se 
SHOWING THE OUTER WaLLS MATERIALLY parts of it remaiming 
THICKENED. 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 
