44 THE CARBOHYDRATE ECONOMY OF CACTI. 



established as yet, nor is there any satisfactory scheme of classification or 

 identification, for different properties have been emphasized and varying 

 results obtained from considering the morphological, functional, or chemical 

 aspects. But from none of these aspects has a satisfactory or comprehensive 

 view been gained of the nature or behavior of the mucilages. Hemicellulose, 

 pectin, slime, and gum are names which have no definite or universal conno- 

 tation either as to origin, structure, or chemical composition, although some 

 efforts have been made for a system of classification. 1 All of these sub- 

 stances are carbohydrates in which a number of simpler sugars are con- 

 densed to form highly complex compounds. It is very questionable whether 

 any of these have ever been obtained in a pure state, and it seems very 

 probable that in most cases mixtures of carbohydrates as well as of other 

 substances have been dealt with. It has been known for some time that 

 complex organic acids, such as gluconic acid and a number of gum acids, are 

 also components, though their exact nature and the manner in which these 

 are placed in the compounds are not definitely known. 3 Furthermore, a 

 small amount of proteins and other nitrogenous substances as well as 

 inorganic salts usually are present as admixtures. All of these substances, 

 hemicellulose, pectins, slimes, and gums, on hydroylsis with dilute acids, 

 yield pentoses as well as hexoses, and they are therefore, in general, all 

 pentosans. 



It is indeed not surprising that such difficulty has been encountered in 

 working out specific reactions for the detection of these substances, as they 

 are so very much alike in general composition and undergo few character- 

 istic reactions that could be utilized, for instance, for microchemical detec- 

 tion. Most of the work in this field has been carried out through micro- 

 chemical means, and many of the tests employed for characterizing the 

 various mucilages are based rather upon the presence of admixtures or of 

 differences in the physical state than upon any definite chemical property. 

 As a matter of tradition plant slimes and gums are differentiated in that 

 under the former come all those polysaccharides which swell greatly in 

 water, but show limited dispersion, and can not be drawn out in threads, 

 while the gums are completely soluble in water. 3 A much more rational 

 system would be one based upon the nature of the products of hydrolysis and 

 perhaps especially of the gum acids. 



A very extensive literature has accumulated on the subject of these 

 colloidal polysaccharides, dealing mostly, however, with the cytological 

 aspect of their formation. The opinions run widely divergent, there is no 

 uniformity of terms, and the subject in general is a most uncoordinated one. 

 It is certain, however, that the various forms of mucilage are produced in a 



* TUNMANN, O. Pflanzen microchemie. Pages 560-592, 1913. Berlin. 

 ' CZAPEK, P. Biochem. d. Pflanzen. 2d edition, p. 676 for literature, 1913. 

 RUHLAND, W. Ueber Arabinbildung durch Bakterien und deren Beziehung zum 



gummi der Amygdaleen. Ber. d. deutsch. Bot. Ges., 24, 393-401, 1906. 

 FRANK, A. B. Ueber die anatomische Bedeutung der vegetablischen Schleime. 



Jahr. f. wiss. Bot., 5, 198, 1867. 



