CHAPTER V 

 GUMS, PECTINS, AND CELLULOSES 



THESE substances constitute a group of compounds which are 

 very similar to the polysaccharide carbohydrates in composition 

 and constitution, but which serve entirely different purposes in 

 the plant. As a class, they are condensation products of pentoses, 

 known as pentosans and having the formula (CsHsO^n, or hex- 

 osans having the formula (CeHioOs)^ or combined pentosan- 

 hexosans. 



In general, these compounds make up the skeleton, or struc- 

 tural framework material, of the plant, in contrast with the proto- 

 plasmic materials or food substances for which most of the other 

 types of organic compounds (discussed in other chapters of this 

 book) serve. They are the principal constituents of " woody 

 fiber," of cell-walls, and of the " middle lamella " which fills up 

 the spaces between the plant cells. They are, therefore, found 

 in largest proportions in the stems of woody plants; but they are 

 also present in every other organ of plants, as the cell-wall or 

 other structural material. 



For purposes of study, these compounds may conveniently be 

 divided into three groups; namely, the natural gums and pen- 

 tosans, the pectins and mucilages, and the celluloses. The segre- 

 gation into these three groups is not sharply defined. The dis- 

 tinction between the groups is based upon the solubility of the 

 compounds in water. The gums and pentosans readily dissolve 

 in water; the pectins form colloidal solutions which are easily 

 converted into " jellies"; the mucilages do not dissolve but form 

 slimy masses; while the celluloses are insoluble in and unaltered 

 by water. Some authors add a fourth group, known as " humins" ; 

 but as these are the products of decay (usually in the soil) of these 

 structural compounds, rather than of growth and development, 

 they need not be taken into consideration in a study of the chem- 

 istry of plant growth. 



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