52 VEGETABLE PHYSIOLOGY 
possibly to the conversion of the pectose into pectic acid and 
the interaction of the latter with some salt of calcium derived 
from the cell-sap which infiltrates the wall. The calcium 
pectate becomes deposited in this way halfway between 
the contiguous cells which are separated by the particular 
membrane in which the change is taking place. 
Tf the cell-wall is not at first homogeneous, we must 
suppose that the original thin membrane is composed of 
three layers, a central one of calcium pectate, on each face 
of which is a layer composed of the mixture or compound 
of cellulose and pectose. We never find, even at the moment 
of cell division, that the membrane is formed of calcium 
pectate only. 
It is possible to explain the growth in thickness of the 
middle lamella on either hypothesis. It is clear that the 
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. 
It is formed in the substance of the cell-wall, and in parti- 
ally lignified membranes the lignin can be dissolved out 
by appropriate reagents, leaving a cellulose basis. In its 
