38 THE VEGETABLE CELL IN GENERAL. 



any marked advantage over that which gives nearly the saie 

 color, 11 am eh', phloroglucin. 



155. B}' the employment of these reagents many cell- walls 

 have been shown to be distinctly lignified when the older re- 

 agent- -iodine in solution- -foiled to detect the change. 



156. Cutiiiization. Ordinary and lignitied cell-walls, and those 

 which have undergone the mucilaginous modification, absorb 

 water freely. On the other hand, the walls of certain cells found 

 chiefly on the exterior of organs are repellent. The substance 

 which imparts the repellent character to the cell-wall is known 

 as cutin ; when restricted to cork it is called suberin. 



157. Cutin and suberin have been described as different sub- 

 stances ; but although the former is more generally associated with 

 waxy matters, its reactions are essentially the same as those of 

 suberin. The water-proofing of the cell-wall ma}' be superficial, 

 as in most young epidermal cells, or it may affect the whole 

 structure of the wall, as in the case of cork. If a distinction is 

 made between the two states, the first may be termed cutiniza- 

 tion, the second, suberification. 



158. Cutin can be removed -from the walls with which it is 

 associated, by the use of Schulze's macerating liquid, subsequent 

 treatment with potassa, and careful washing. It is sometimes 

 necessary to heat the section in potassa before the cellulose can 

 be completely freed from the other matters. 



159. Holmel 1 has shown that the wall of a cork-cell, with 

 the exception of the young cork-cells in Conifers, is composed of 

 five plates: (1) a middle plate, common to the two contiguous 

 cells; (2) two plates, one on each side of the latter, consisting 

 of cellulose which is both cutinized and lignified ; (3) two plates 

 of cellulose forming the inner lining of the respective cells. The 

 latter plates may be more or less lignified. Differences in the 

 relative proportions of these constituent plates give rise to dif- 

 ferences in the character of different kinds of cork. 



160. As in the case of lignin, the difficulty of extracting cutin 

 renders its chemical composition doubtful. It is usually given 

 as follows : 



Carbon 73-74 per cent. 



Hydrogen 10 " 



Oxygen 17-16 



But there is also a trace of nitrogenous matter demonstrable ; 

 this probably belongs to residual protein matters which are in 



1 Sitzungsber. d. k. Akad. Wien, Bd. Ixxvi. 1 Abth. 



