4 THE PLANT CELL WALL 



remain are solubilized with hot ammonium oxalate and after 

 separation from the oxalate by precipitation, may be redissolved 

 in ammonia and reprecipitated as the calcium salt. 



As we have noted, the hemicelluloses are distinguished by 

 their solubility in cold dilute alkali, a property which provides 

 the basis for their determination. The hemicelluloses may be 

 recovered from alkaline extracts by precipitation with cold acid 

 and alcohol. Commonly, alkali extraction is preceeded by treatment 

 of the tissues with ammonium oxalate in order to remove the 

 pectic substances. 



Fractional precipitation has been used in the separation of 

 various polysaccharides, the hemicelluloses especially. Fractiona- 

 tions have been carried out with acids, alcohol, or by complexing 

 with copper. Cetyltrimethyl ammonium bromide will precipitate 

 acidic polysaccharides as insoluble quaternary ammonium salts, 

 leaving neutral polymers in solution. Even the latter may be 

 precipitated if first rendered acidic by formation of borate complexes. 



Among the major cell wall substances lignin is distinctive for 

 its aromatic character and the relative difficulty encountered in 

 separating it without alteration from other components. The 

 many separation techniques which have been developed range 

 from the delignification of wood for practical ends (in sulfite 

 pulping processes, for example) to mild treatment of the cell 

 wall with cellulose to render the residual lignin more soluble. 



Traditional procedures are based principally upon the removal 

 of other substances leaving the lignin as a residue. The resistance 

 of lignins to strong mineral acids permits separation of carbohydrate 

 derivatives which are solubilized by 72 per cent sulfuric acid or 

 fuming (40-42 per cent) HC1. Although such procedures modify 

 the lignins they are probably suitable for gross analyses. A milder 

 isolation procedure is based upon treatment of cell walls with 

 dilute alkali, hot dilute acid, and cuprammonium solution. There 

 are some losses of lignin in alkaline solutions, but a more native 

 product may be obtained. 



The reverse procedure, removal of lignin from other wall 

 constituents, has been accomplished using bases such as alcoholic 

 NaOH or ethanolamine; phenols; or alcoholic HC1. Again, such 



