422 MANUAL OF BOTANY 



all well understood. Certain solvents will extract them from 

 ordinary cell-walls, leaving cellulose as the basis of the latter. 



The reactions of cellulose have been already described in 

 an earlier section in dealing with the composition of the cell- 

 wall. The most characteristic one is the assumption of a blue 

 colour when treated with iodine and some hydrating agent, such 

 as strong sulphuric acid or chloride of zinc. 



Cellulose itself is capable of existing in more than one con- 

 dition. We find some kinds of it which will stain blue on 

 treatment with iodine alone. Examples are found in the cell- 

 walls of the bast of the stele of Lyoopodium, the endosperm of 

 the Pseony, the cotyledons of some of the Leguminoste, &c. 

 The walls of the hyphse of the Fungi differ again, in that they 

 will not give the blue colour with iodine even after treatment 

 with hydrating agents. Eecent observations suggest that this 

 variety of cell- wall approaches in composition the chitin of the 

 animal kingdom. 



It is probable that cellulose is chemically combined with a 

 certain amount of water, and that the degree of hydration differs 

 in the different varieties described. 



"Where considerable rigidity of structure is needed it is 

 obtained by the conversion of the cellulose into lignin, the 

 material fomid conspicuously in the walls of wood cells. This is 

 formed in the substance of the cell-walls, and in partially lignified 

 membranes the lignin can be dissolved out, leaving a cellulose 

 basis. In its chemical characters lignin differs materially from 

 cellulose ; it has no extensibility, nor can it absorb water and 

 swell as can the latter ; on the other hand it allows water to pass 

 through it with great rapidity and ease. It can be recognised 

 micro-chemically by staining red when treated with phloroglucin 

 and a mineral acid, or yellow with aniline chloride under the 

 same conditions. 



Lignin is probably not a definite chemical compound, but a 

 mixture of substances successively formed from the cellulose. 

 Its physical properties render it particularly adapted to serve 

 as the material of which the tissues conducting the stream of 

 water are composed. Its lack of flexibility or extensibility makes 

 it suitable for the securing of rigidity in tissues or structures 

 needing strong power of resistance to winds or storms. 



The modification of the tegumentary tissue of the higher 

 plants takes the form primarily of the transformation of the 

 cell-waU into cutin or suherin, the former being characteristic 

 of the outer layers of the epidermal cell-walls, and leading 



