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. 



If 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 



