86 STUDIES ON APPLES. 



COMMENTS ON THE PROBLEMS INVOLVED. 



From the foregoing- review pectin bodies would be defined as sab- 

 stances of undetermined function, very widely distributed in plant tis- 

 sues. Pectin bodies occur both in the j uice and marc, i. e. , in soluble and 

 insoluble forms. The latter form, according to the work of Scheibler, 

 Wohl, Van Niessen, and others, and from the work done in the Bureau 

 of Chemistry, seems to be resolved into soluble forms by boiling with 

 water. The solutions possess considerable viscosity, the property of 

 forming jellies with precipitants such as alcohol, sugars, solutions of 

 salts, and pectase and usualby rotate polarized light to the right. 

 The rotating power of the different pectin bodies appears to vary con- 

 siderably. The distinctions laid down by Fremy as existing between 

 .the pectin bodies seem to be based on uncertain physical properties a 

 and on the amount of lead with which they will combine. Criteria 

 adopted by later workers are based mainly on chemical behavior and 

 action on polarized light. 



Chemically, pectin bodies are characterized by yielding reducing 

 sugars, furfurol, and mucic acid in widely vaiying amounts, according 

 to the source of the pectins and the method employed in isolating 

 them. These variations may be largely due to varying degrees of 

 hydration and to impurities in the pectin bodies examined. Dif- 

 ferences in the pectin bodies themselves, however, are indicated 

 by the results of Ullik and Ilerzfeld. Pectins are profoundly 

 changed by alkalis with the formation of salts of the so-called 

 pectic acid, the free acid being insoluble in water. Acid groups 

 appear to be formed even by treatment with very dilute alkali for a 

 very short time, so that extraction with alkali of pectins from plants 

 does not recommend itself to the writers when a study of the dissolved 

 material is desired. 



The most important problem appears to be the quantitative deter- 

 mination of the pectin bodies occurring in a given tissue, because 

 such a method could be used jbo determine the function of the material 

 in plants whether, for example, it is a reserve material, a by-product, 

 is used for structural purposes, or has all three functions or two of 

 them; whether the nature of the pectin body changes with the 

 growth or age of the tissue, or possesses a practically constant com- 

 position; whether the pectin bodies obtained from different sources 

 are identical, are mixtures of the same substances (such as araban and 

 galactan) in varying proportions, or are inherently different. 



It is of interest to note that all pectin bodies thus far studied have 

 been derived from the softer tissues. The harder woody material has 

 not been considered. 



<* J. prak. Chem., 1884, SO: 370. Neue Ztg. Zucker-Ind., 1885, 14: 151. 



