30 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



cuprammonium (Schweitzer's reagent) (though again this fails in some 

 algae) and by its transparency to ultra-violet radiation. Undoubtedly 

 the surest indications of the presence of cellulose are, however, those 

 associated with its crystalline nature. These will be discussed at some 

 length in the next chapter. 



Pectin can be recognized by the deep blue coloration in methylene 

 blue, and the red coloration on prolonged immersion in aqueous 

 ruthenium red (1/10,000), These reactions are, however, given by other 

 substances present in some cases (oxidized cellulose, cytoplasmic debris, 

 etc.), and it is imperative therefore in all cases of doubt to check on the 

 results by solubiUty tests. Pectin itself is soluble in 0-5% ammonium 

 oxalate and is reprecipitated in alcohol; if other pectic derivatives are 

 involved it may be necessary to apply a preliminary hydrolysis in, for 

 instance, alcoholic hydrochloric acid. The presence of xylan or mannan 

 can be demonstrated only by hydrolysis followed by a test for the 

 corresponding monosaccharide. 



There are several characteristic colour tests for lignin, but again these 

 suffer from lack of absolute specificity. The red coloration in phloro- 

 glucin followed by concentrated hydrochloric acid is a test for pentosans 

 rather than for the particular groupings present in lignin, and the same 

 probably applies to the yellow reaction in aniline chloride. On the other 

 hand, the magenta coloration when the material is treated with chlor- 

 ine water (or its equivalent) and subsequently heated in 3% sodium 

 sulphite can be regarded as typical, though the colour developed depends 

 upon the material used, being quite different in, for instance, the 

 Angiosperms and the Gymnosperms; and the silver staining method 

 recently introduced by Coppick and Fowler (11) promises to be of 

 considerable value. Lignin can be extracted from walls by the former 

 of these two treatments but during the process very considerable degrada- 

 tion occurs. In fact, there cannot be said to be any true solvent for 

 lignin, and the substance is therefore usually isolated by removal of the 

 other accompanying substances. 



With this brief description the way is now open for the study of the 

 complex of these various substances by the physical methods to be 

 described. Before proceeding to the study proper, a chapter will be 

 devoted to a simple account of these physical methods in order to confer 

 upon the reader some ability critically to assess the value of the work 

 to be described. 



