SUBERISATION OF THE CELL-WALL. 



89 



require to be protected against evaporation, is surrounded by suberised cell-tissue ; 

 as the stems and older branches of woody plants, in the wood of which the current 

 of water moves. 



In like manner the outside of leaves and of young shoot-axes is clothed with 

 a pellicle, generally exceedingly thin, the so-called cuticle, the substance of which 

 possesses all the essential properties of suberised cell-walls. While lignificalion 



Fig. 87, — Cells from a leaf of //oya carnosa 

 (X Sod), showing the striation ; though far less 

 strongly marked in nature, the strise are quite 

 as evident, a optical longitudinal section of the 

 crossed annular striation ; * external view of the 

 side where the annular strire cross ; c, d external 

 view of the side where they do not cross; e a 

 portion of cell-wall where only isolated annular 

 striae are evident. 



Fig. ^.— Cells with brown walls in the stem 

 of Pteris aguUina. A half a cell, isolated and 

 decolorised' by Schulze's maceration; B s. piece 

 more highly magnified {i< 550)— the fissure-like 

 pits are crossed, i, e. the fissure becomes twisted 

 with increase of thickening; p lateral view of a 

 fissure, which here appears as a simple canal, 

 since it exhibits the narrow section. 



makes the cell-walls predominantly hard and rigid, they gain by means of suberisation 

 in extensibility and elasticity. 



The third common form of metamorphosis of the cell-wall, the conversion 

 into mucilage, consists in a chemical alteration of the cellulose, in consequence of 

 which it obtains the property of absorbing large quantities of water, and of swelling up 

 in a corresponding degree, in many cases so strongly that the volume increases a 

 hundred-fold or more. According as this property is more or less developed, the 

 mucilaginous change of the cell-wall makes itself evident in a more or less gelatinous 

 consistence ; and it may even be converted into a liquid slime with water. As a further 



