THE PLANT-CELL. 21 



mixture ; the intermediate layer is affected to some extent by 

 both these reagents. The effect of lignification is to make 

 the cell-wall harder and more elastic, and to diminish its 

 capacity for absorbing water, without, however, rendering it 

 impermeable : on the contrary, it is a characteristic property 

 of a lignified cell-wall, as Sachs has shewn, that water readily 

 traverses it. The importance of this property will become 

 apparent hereafter when we are discussing the movement of 

 water in plants. 



Lignin cannot be regarded as a definite chemical compound : the 

 name includes probably a number of different substances which are 

 formed by the gradual alteration of cellulose in the process of lignifica- 

 tion. Since treatment with solution of potash or with nitric acid does 

 not disorganise the lignified cell- wall although it removes about one-tenth 

 of its substance, there is reason to believe that the lignin substances are 

 chemically combined with the cellulose. It has been suggested that 

 cellulides are present, that is, compounds of cellulose with aromatic 

 bodies : and since treatment of the cell-walls with hydrochloric acid or 

 with chlorine sets free a substance which reduces cupric oxide, it seems 

 probable that glucosides may be present also. Erdmann considers that 

 he has obtained from pine-wood a definite substance to which he gives 

 the name of Glycolignose. When boiled with dilute hydrochloric acid it 

 is decomposed according to the equation 



CaoH^O,! + 2H 2 O = 2C 6 H 12 O 6 + C 18 H 26 O n (Lignose) : 



and when lignose is decomposed by long-continued boiling with dilute 

 nitric acid the following change is effected : 



C 18 H 26 On + O = 2C 6 H 10 O 5 (Cellulose) + C 6 H 6 O 2 (Pyrocatechin) : 

 thus glycolignose appears to be a glucoside, and lignose a cellulide. 



The inorganic, or rather the mineral substances which are 

 found deposited in cell-walls are principally silica and salts of 

 calcium ; iron, manganese, aluminium have also been detected. 

 Cell-walls which contain these substances, leave, after com- 

 bustion, a considerable ash ; in some cases the ash forms a 

 complete skeleton of the tissue. 



Silica especially occurs in the cuticularised walls of epi- 

 dermal cells, but, as we have already seen, it is not confined 

 to them. The amount present is often very large; thus 

 Struve found that it constitutes 99 per cent, of the dry 

 epidermis of Calamus Rotang, 



