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 (C5H8O4)n, or hexosans having the formula (C6H10O5)n, or combined pentosan-hexosans.
In general, these compounds make up the skeleton, or structural framework material, of the plant, in contrast with the protoplasmic 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 pentosans, the pectins and mucilages, and the celluloses. The segregation into these three groups is not sharply defined. The distinction 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 chemistry of plant growth.
THE NATURAL GUMS AND PENTOSANS
The natural gums, when hydrolyzed, yield large proportions of sugars, but most of them also contain a complex organic acid nucleus, by means of which they form salts with calcium, magnesium, etc. Some of them, such as cherry gum and those which are found in the woody stems of plants (wood gum, and those found in corn stalks, the straw of cereals, etc.) yield practically pure pentoses. These are known as pentosans. They bear the same relation to the pentose sugars as do the dextrosans to glucose, etc. The wound gums, for example, yield arabinose, and the wood gums yield xylose. But most of the natural gums yield a mixture of galactose, some pentose, and some complex organic acid.
The gums are translucent, amorphous substances, whose solutions in water are levorotatory. They are precipitated out of solution by alcohol and by lead subacetate solution.
Gums are extremely difficult to hydrolyze, the laboratory process of hydrolysis usually requiring from eighteen to twenty-four hours of continuous boiling with acids for its completion. Because of this difficulty of hydrolysis, gums are practically indigestible by animals and of little use as food.
The following common examples will serve to illustrate the general nature of these compounds.
Gum arabic, found in the exudate from the stems of various species of Acacia, is a mixture of the calcium, magnesium, and potassium salts of a diaraban-tetragalactan-arabic acid. Arabic acid has the formula C23H38O22, and one molecule of this acid serves as the nucleus for the union of eight galactose and four arabinose groups, linked together in some unknown way. The formula for the compound, exclusive of the metallic elements with which it is loosely united is C91H150O78. This gives some idea of its complexity.