THE SKELETON OF THE PLANT 53 



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 little extensibility, nor can it imbibe 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 formed successively from the 

 cellulose. 



Walls containing it subserve a double purpose. Its 

 physical properties render it particularly adapted to serve 

 as the material of which the tissues conducting the stream 

 of water are composed. Its deficiency in flexibility or 

 extensibility makes it suitable for the securing of rigidity 

 in tissues or structures needing considerable power of re- 

 sistance to winds or storms. It is thus a valuable material 

 in the construction of sclerenchyma. 



The protective tissues show a different modification of 

 the original structure. In the simplest cases we have seen 

 that the degree of protection secured is slight, and evidently 

 only transitory. The epidermis is, in these cases, the seat 

 of the changes which may be observed. The cells show 

 their walls sometimes very materially thickened on the 

 exposed sides (fig. 45), though the thickness varies in 

 different cases. Layer after layer of substance is deposited 

 upon the original wall in these regions, the other parts of 

 it remaining thin. The thickness itself secures a cer- 

 tain amount of protection against cold, but to prevent 

 absorption or dissipation of water or of gases by these 

 membranes, a chemical change also is brought about. 

 The outer layers of the wall undergo a process known as 

 cuticularisation, which generally extends about halfway 



