THE SKELETON OF THE PLANT 53 
chemical characters lignin differs remarkably from cellulose. 
It does not stain blue with iodine and sulphuric acid, but 
can be recognised by its property of becoming red when 
treated with phloroglucin and a mineral acid, or yellow 
with anilin chloride under the same conditions. Its physical 
properties are also different, and bear a definite relation to 
the function of the tissue as a medium for the transport of 
water. It has little extensibility, nor can it imbibe water 
and swell as can unaltered cell-wall ; on the other hand, it 
allows water to pass through it with great rapidity and 
ease. 
Lignin is probably not a definite chemical compound, 
but a mixture of substances formed successively from the 
cellulose. 
Walls containing it subserve a double purpose. Its 
physical properties render it particularly adapted to serve 
as the material of which the tissues conducting the stream 
of water are composed. Its deficiency in flexibility or 
extensibility makes it suitable for the securing of rigidity 
in tissues or structures needing considerable power of re- 
sistance 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 parts of 
it remaining thin. The thickness itself secures a cer- 
tain 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 
