34 THE VEGETABLE CELL IN GENERAL. 



144. The principal modifications of the cell- wall are the fol- 

 iowing : — 



(1) Partial or complete conversion into mucilage (Gelatina- 

 tion) ; (2) Lignification ; (3) Cutinizatiou (or Suberification) ; 

 (4) Mineralization. 



145. All of tliese, except tlie first, change the chemical char- 

 acter of the cell-wall onl^- by what maj' be regarded as infiltra- 

 tion ; upon removal of the infilti-ated matter bj^ means of proper 

 agents, the cellulose basis of the wall is left behind with very 

 little if anj- change. 



146. It sometimes happens that one part of the membrane of 

 a cell, or even one of its layers, may be modified in one way, and 

 another in another ; it is also possible for the same membrane to 

 undergo two of the changes above mentioned ; namely, Lignifi- 

 cation and Mineralization. 



147. The mncilaginoTis modillcation. Commonly the cell-wall is 

 not much changed by immersion in water. It may become more 

 nearly transparent, but its size and densit3' are not essentially 



(3) Soluble in sulpluiric acid, insoluble in cuprammonia (unless jireviously 

 acted on by acids or alkalies) ; c. g., the pitJi, ami medullary rays of woods. 



(4) Soluble in concentrated sulpburic acid ; insoluble in cuprammonia, but 

 becoirjng soluble in tbis upon previous treatment with Schulze's macerating 

 liquid ; e. g., wood-cells of pine, oak, yew, etc. 



'5) Insoluble in concentrated sulphuric acid and cuprammonia, but soluble 

 in boiling concentrated potassic hydrate ; e. </ ., cuticle, and the outer layer of 

 Qie "".embrane of older ducts. 



II. Iodine Eeactions. 



(1) With iodine and water, a blue color ; lichen-filaments, etc. 



(2) With iodine and water, no color ; but giving a blue tint with iodine and 

 a metallic iodide ; or when iodine is followed by sulphuric acid : — 



A. Thin-walled Parenchyma (which will often turn blue when a pure iodine 

 solution acts with repeated diying), older Parenchyma, the inner part of thick- 

 ened wood-cells of Pinus and Abies, and the basi-fibres of hemp. 



B. Only when the reagents have been preceded by the ajiplication of nitric 

 acid : all membranes in the interior of the plant, e. g., the outer part of wood- 

 cells and ducts, the brown cells which surround the vascular bundles in 

 ferns, etc. 



C. Only when the reagents have been preceded by the use of boiling potassic 

 hydrate : cork, etc. 



According to Fremy and Urbain, the substances which foim the skeleton of 

 plants are principally pectose and derivatives fi-oni it, cellulose and its isomers, 

 Ys-sculose, and cutose. These four groups are thus distinguished from one 

 another. 



Pectose acted on by alkaline carbonates is changed into pectio acid, and 



