CONSTITUTION AND ARCHITECTURE IN THE CELL WALL 7 



and "lignified wall" reagents, respectively. The great dependency 

 of staining reactions upon conditions and the frequent reversal of 

 staining affinities cast doubts upon the general value of these 

 dyes. Such staining reactions have undoubted utility in cytological 

 investigations, but may be better indicators of the organization 

 and polyelectrolyte character of macromolecules and their aggre- 

 gates, than of specific chemical groups. This view is supported 

 by dyes such as azure B , which exhibit the phenomenon of meta- 

 chromasy, or differential coloration in different structures. One 

 dye, ruthenium red, seems to be of comparatively great utility 

 as a differential stain for pectic substances. Although it cannot 

 be claimed that this dye is specific for pectins, it seems to have 

 particular affinity for highly polymerized acidic carbohydrate 

 derivatives of this type. 



Specific cellulose staining can be effected by virtue of its molecular 

 organization. The two most useful reagents are chlorozinc iodide 

 and KI 3 -72 per cent sulfuric acid. These reagents swell cellulose 

 and exhibit dichroism, appearing blue or violet when the electric 

 vector of incident light is parallel to the axis of the fiber and color- 

 less when the electric vector is normal to the fiber axis. 



Such effects are based upon the oriented deposition of iodine 

 within the ordered fiber structure. The staining of cuticular 

 materials also depends upon a physical property—solubility— in 

 lipophilic dyes such as sudan III. 



Lignin is unique among major wall substances in its ability to 

 form chromogenic condensation products when treated with 

 acidic solutions of many phenols and aromatic amines. When 

 suitable precautions are taken to remove soluble, low molecular 

 weight interfering substances, reagents such as phloroglucinol- 

 HC1 can be used to impart a brilliant red stain to lignins and 

 lignified walls. Other lignin-specific chromogens may be formed 

 by treatment of tissues with chlorine followed by alkali or sodium 

 sulfite. 



Physical Methods 



High polymers such as cellulose are organized into reticular 

 gel systems. The reticulum consists of variously ordered molecular 



