THE SKELETON OF THE PLANT 47 
wall is the seat of a considerable chemical change which 
affects its whole substance, though the degree, and possibly 
the character, of the change may vary in the different 
layers of which the wall is built up. 
Not infrequently it is noticeable that the intercellular 
spaces contain small concretions of various form, which 
consist of the same substance as the middle lamella. This 
is scarcely to be wondered at, as, when the intercellular 
spaces are formed by the splitting of the cell-wall, the 
region of the middle lamella, which is the central part of 
the membrane, must abut upon the space formed in the 
rupture. The calcium pectate which is formed or deposited 
in the central region, and which causes the thickening of 
the middle lamella, may well exude to a certain extent into 
the intercellular space that has been formed. 
In such parts of the framework of a well-differentiated 
plant-body as need considerable rigidity, a conversion of 
cellulose into lignin takes place. This material is found 
conspicuously in the walls of wood-cells and sclerenchyma. 
Tt is formed in the substance of the cell-wall, and in par- 
tially lignified membranes the lignin can be dissolved out 
by appropriate reagents, leaving a cellulose basis. In its 
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 absorb 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 successively formed from the 
cellulose. 
Walls containing it subserve a double purpose. Its 
