CARBOHYDRATES AND FATS 477 



Substances allied to mannite are the Dulcite found in Melampyrum, Euonymus, 

 and many other plants, and the rarely occurring Sorbite (cf. Tollens). 



Inosite, a sugar-like substance, which according to Maquenne 1 is a hexa- 

 hydroxyhexamethylene, disappears during the ripening of the fruit of Phaseolus as 

 if it were a plastic product. 



Mucilage. Gums and Pectin compounds form a large group of substances 

 having similar physical properties, and include bodies of widely different chemical 

 composition. They may take part in the formation of the cell-wall, or may 

 be dissolved in cells or secretory receptacles, or may be excreted externally. 

 They consist for the most part of different forms of carbohydrates, but according 

 to Ishii "' the mucilage from the roots of Dioscorea japonica is composed of 

 a proteid resembling animal mucin. As in animals, gluco-proteids, &c. may often 

 occur in plant-mucilage. 



The molecules of a number of carbohydrates appear to be large and complex, 

 and often combine with molecules of other substances 3 . Thus the typical gums are 

 mainly pentosans (compounds of pentose), while dextrins are glucosans, substances 

 whose gradual decomposition produces every variety of transition forms from 

 gelatinous or colloid to crystalline bodies. Similarly the molecules of the partially 

 gelatinous reserve-celluloses (hemi-cellulose) yield on hydrolysis either hexoses or 

 both pentoses and hexoses (Sect. 83). 



Recent researches (Tollens, 1. c., p. 242) render it almost certain that the 

 group of pectins are carbohydrates, which contain in their molecules hexoses 

 and pentoses as well as other substances. Neutral pectins are complex in 

 character, and when by hydrolysis compounds are produced which contain a 

 carboxyl group, pectic acids result which are either soluble in water or in alkalies, 

 but form insoluble calcium salts. Many gums show similar characteristics, and 

 seem to be in part allied to pectin substances. Like other carbohydrates, pectins 

 enter into the composition of the cell-wall either as dissolved substances or in 

 solid form. 



The different forms of mucilage may undoubtedly be produced in a variety 

 of ways, and either by synthesis or by decomposition, as when the cell-wall becomes 

 mucilaginous. Mucilage is, however, often formed in the interior of the cell 

 without the intervention of cellulose as an intermediate product, and gelatinous 

 coverings of comparatively firm consistency are often formed from excreted 

 mucilage. Direct observations show that pectin substances are not usually 

 formed by a metamorphosis of the cell-wall, an erroneous generalization formerly 

 made by certain authors (Wiesner, &c.) 4 . 



The retrogressive metamorphoses of the cell-wall are probably as a general 



1 Maquenne, Ann. d. chim. et d. phys., 1887, vi. se'r., T. xil ; Tollens, iSSS, p. 253; 1895, 



P- 2 93- 



'- Ishii, Versuchsst., 1895, Bd. XLV, p. 434. On animal mucins, &c., cf. Neumeister, Physiol. 

 Chemie, 1893, Bd. I, p. 35. 



3 Cf. Tollens, Handb. d. Kohlenhydrate, 1895, Bd. II, pp. 198, 240. 



4 Nageli u. Schwendener, Mikroskop, 1877, 2. Aufl., p. 507 ; Wiesner, Sitzungsb. d. Wien. Akad., 

 1865, Bd. L, Abth. ii, p. 442, and the literature here quoted. 



