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. 

 It 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 no 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 



