6 THE PLANT CELL WALL 



Enzymes have found application in the determination of oc- 

 and ^-linkages. The a-glucosidase, maltase, and the (3-glucosidase, 

 emulsin, may be cited as analytically useful enzymes specific for 

 their respective isomers. The most useful analytical technique 

 which has been applied to these relational aspects of polysaccharide 

 structure is methylation prior to hydrolysis. Commonly, dimethyl 

 sulfate in alkali has found use in this connection. A linear glucose 

 homopolymer containing 1,4-linkages will yield principally 2,3, 

 6-trimethyl glucose. Short polymer chains will yield appreciable 

 amounts of 2,3,4,6-tetramethyl glucose which is derived from 

 frequent end groups, whereas only trace amounts of the tetra- 

 methyl derivatives can be isolated if the polymer is large and pos- 

 sesses a small proportion of end residues. The presence of branching 

 is shown by the occurrence of 2,3-dimethyl glucose. Additional 

 evidence about linkages may be obtained by methylation of acid 

 derivatives or bromine oxidation. 



Sugars other than glucose released by hydrolysis are often 

 identified by specific reactions. Mannose may be estimated by 

 precipitation of its phenylhydrazone. Galactose is readily identified 

 by oxidation (nitric acid) to water insoluble mucic acid. In general, 

 pentoses (xylose, arabinose) are identified and estimated by de- 

 hydration (12 per cent HC1) to furfural and formation of insoluble 

 colored products with reagents such as phloroglucinol. 



The degradation of lignin, like its isolation, presents distinctive 

 problems. Among the many analytical procedures, alkali fusion, 

 hydrogenolysis, various forms of pyrolysis, and oxidation with 

 permanganate or nitrobenzene are of considerable importance. 

 These chemical methods coupled with ultraviolet and infrared 

 spectrophotometry have contributed considerable knowledge about 

 lignin structure. Methylation procedures have also been applied 

 so degradation products for the identification of phenolic groups, 

 and hydroiodic acid treatment is useful in the determination of 

 methoxyl groups. 



Most histological staining methods which have been applied 

 in studies of various plant cells and tissues lack the specificity 

 of reliable microchemical reagents. For example, standard dyes 

 such as hematoxylin, and safranine, have been claimed as "cellulose" 



