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 (C_{5}H_{8}O_{4})_{n}, or hexosans having the formula (C_{6}H_{10}O_{5})_{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 C_{23}H_{38}O_{22}, 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 C_{91}H_{150}O_{78}. This gives some idea of its complexity.

When boiled with nitric acid, it is oxidized to mucic, saccharic, and oxalic acids. It gives characteristic reactions with alum, basic lead acetate, and other common reagents.

Gum arabic comes on the market as a brittle, glassy mass, which is used in the preparation of mucilages, and as a carrier for essential oils, etc., in certain toilet preparations.

Recent investigations have shown that the so-called "meta-pectic acid," which is often found in sugar beets and interferes with the process of sugar manufacture, is identical with gum arabic in composition and properties.

=Gum tragacanth= is the soluble portion of the natural gum which is found in several species of _Astragalus_. It constitutes only 8 to 10 per cent of the total gum-like material which is present, the remainder being composed of insoluble gummy substances of unknown composition. The soluble gum consists of calcium, potassium, and magnesium salts of an acid which, when hydrolyzed, yields several molecules of arabinose, six of galactose, and one of geddic acid (an isomer of arabic acid). It is said to be produced by the metamorphosis of the medullary rays under unfavorable conditions of growth. It comes on the market in globular masses of amorphous material, and is used in the manufacture of cosmetics, etc.

=Wound gum= is frequently found in the tracheæ of plants, and near surface wounds, which it stanches. It is secreted by the cells surrounding the injured part. It responds to the reactions of other gums and to some of those of woody fiber. Its exact composition is not known, but probably lies between that of the true gums and that of cellulose.

These gums are generally considered to be decomposition products of celluloses, resulting from the action of some hydrolytic ferment, usually stimulated by some unfavorable condition of growth, some injury, or some morbid condition.

The =pentosans=, araban and xylan, occur normally in the stems and outer seed coats of many common plants. They constitute a considerable proportion of these tissues, as indicated by the following results of typical analyses: Wheat bran, 22 to 25 per cent; clover hay, 8 to 10 per cent; oat straw, 16 to 20 per cent; wheat straw, 26 to 27 per cent; corn bran, 38 to 43 per cent; jute fiber, 13 to 15 per cent; various wood gums, 60 to 92 per cent.

They are white, fluffy solids, which are difficultly soluble in cold water, more readily in hot water. They are very difficult to hydrolyze, and indigestible by animals. When finally hydrolyzed, they yield arabinose and xylose, respectively. The pith of dry corn stalks is a good illustration of their general character.

MUCILAGES

These are characterized by forming slimy masses when moistened with water. They are secreted by hairs on the skin of many plants, so that the external walls of the leaves, fruit, and seeds are often mucilaginous when damp. This is particularly true of aquatic plants. The chemical composition of the mucilages is unknown. When hydrolyzed, they yield arabinose and a hexose; the latter is sometimes galactose and sometimes mannose.

When present on the surface of plant tissues, the mucilages probably serve to prevent the too rapid diffusion of materials through the skin, in the case of the aquatic plants, and too rapid transpiration, in the case of young vegetative tissues or in other plants when growing under extremely dry conditions. When found in tubers, or other storage organs, it has been supposed that they may serve as reserve food materials, but it seems that such difficultly hydrolyzable compounds as these can hardly function as normal reserve foods.

PECTINS

Many fruits, such as currants, gooseberries, apples, pears, etc., and many fleshy roots of vegetables, such as carrots, parsnips, etc., contain substances known as _pectins_. These are readily soluble in water, and when dissolved in concentrated solutions in hot water, they set into "jellies" when the solution is cooled. These jellies carry with them the soluble sugars and flavors which are present in the fruits, and constitute a familiar article of diet.

There are undoubtedly several different modifications of the pectins, to which the names "meta-pectin," "para-pectin," "pectic acid," "meta-pectic acid," and "para-pectic acid," have been applied. These all seem to be products of hydrolysis of a mother substance known as "pectose," which constitutes the middle lamella of unripe fruit, etc. As the fruit ripens, the pectose is hydrolyzed into the various semi-acid, or acid, bodies mentioned above. The intermediate products of the hydrolysis are the pectins, which swell up in water and readily form jellies; while the final meta-pectic acid is easily soluble in water and resembles the true gums in its properties. When the middle lamella reaches the pectic acid stage, the fruit becomes soft and "mushy" in texture.

The pectins more nearly approach to the composition, properties, and functions of the celluloses than do any of the other groups of organic compounds. They have been extensively studied in connection with the parasitism of certain fungous diseases which cause the soft rots of fruits and vegetables. These parasites usually penetrate the tissues of the host plant by dissolving out the middle lamella material, which may sometimes serve as food material for the fungus; but more often the parasite secures its food supply from the protoplasm of the cell contents. In such cases, the parasite secretes both a pectose-dissolving enzyme, known as "pectase" and a "cellulase" which attacks the cell-wall material in order to provide for the entry of the fungus into the cells. Other enzymes, known as "pectinases," which coagulate the soluble pectins or pectic acids into insoluble jellies in the tissues of the plants seem to aid the plant in resisting the penetration by the parasite.

CELLULOSES

Used in its general sense, this term includes all those substances which are elaborated by protoplasm to constitute the cell-wall material. Cellulose proper is a definite chemical compound, whose properties are well established. In plants, however, this true cellulose is nearly always contaminated by various encrusting materials; and in the process of wood-formation, the cell-wall material continually thickens by the conversion of the cellulose into ligno-cellulose and the protoplasm of the cell as continuously diminishes in volume. Thus the protoplasm of the cell produces a number of different kinds of material which are deposited in the walls of the cell. All of these, taken together, constitute the general group known as the celluloses.

These may be divided into three classes: namely, (1) the hemi-celluloses, (2) the normal celluloses, and (3) the compound celluloses.

The =hemi-celluloses= (pseudo-, or reserve celluloses) include a series of complex polysaccharides which occur in the cell-walls of the seeds of various plants. They are found in the shells of nuts, rinds of cocoanuts, shells of stony fruits, etc., and in the seedcoats of beans, peas and other legumes. They are much more easily hydrolyzed than the other members of this group, and when hydrolyzed yield various sugars, chiefly galactose, mannose, and the pentoses. They bear the same relation to these sugars that starch does to glucose, and are generally supposed to serve as reserve food material, although it is difficult to conceive how the shells, etc., in which they appear can be utilized by a growing seedling. They differ in structure from the fibrous celluloses and are probably not cell-wall building material. They appear to be a form of reserve carbohydrates, which differ from the glucose-polysaccharides in being condensed in, or as a part of, the external structural material rather than in the internal storage organs. They are soluble in water and exhibit the properties of gums, and are often classified with the gums and described under the names "galactans," "mannosans," "pentosans," etc.

The =normal celluloses=, of which the fibers obtained from cotton, flax, hemp, etc., are typical examples, are widely distributed in plants and form the commercial sources for all textile fibers of vegetable origin. Ordinary cotton fiber contains 91 per cent of cellulose, about 7.5 per cent of water, 0.4 per cent of wax and fat, 0.55 per cent of pectose derivatives, and 0.25 per cent of mineral matter; or a total of only 1.2 per cent of non-cellulose solids. Filter paper is practically pure cellulose.

Pure cellulose is a white, hygroscopic substance, which is insoluble in water and in most other solvents. If heated with water under pressure to about 260° C., it dissolves completely without decomposition. If boiled with a strong solution of zinc chloride, or treated in the cold with zinc chloride and concentrated hydrochloric acid, or with an ammoniacal solution of copper hydroxide (Schweitzer's reagent), it dissolves to a clear solution from which it may be reprecipitated without chemical change by neutralizing or diluting the solution.

Cellulose has the formula (C_{6}H_{12}O_{5})_{n}. When hydrolyzed under the influence of the enzyme _cytase_, it breaks down, first into cellobiose, an isomer of maltose, and then into glucose. It is, therefore, chemically like, but not identical with, starch; and structurally it is arranged in fibrous form instead of in granules. Under the action of fermentative enzymes, as when vegetable matter decays under stagnant water, in swamps, etc., cellulose breaks down into carbon dioxide and marsh gas, according to the equation

(C_{6}H_{12}O_{5})_{n} + {n}_H_{2}O = 3_{n}CO_{2}+3_{n}CH_{4}.

Cellulose is acted upon by caustic alkalies in a variety of ways. When fused with a mixture of dry sodium and potassium hydroxides, it is decomposed into oxalic and acetic acids. When heated with a 10 to 15 per cent solution of caustic soda, cellulose fibers thicken and become translucent, thus resembling silk fibers. This process, known as "Mercerizing," is largely used for the production of commercial fabrics.

Acids also act on cellulose in a variety of ways. When heated with nitric acid (sp. gr. 1.25), it is converted into _oxycellulose_; while dilute sulfuric acid, under similar conditions, yields _hydro-cellulose_, a substance having the formula C_{12}H_{22}O_{11}, which retains the fibrous structure of the original cellulose but which, when dry, may be rubbed up into a fine powder. Concentrated nitric acid, or better, a mixture of concentrated nitric and sulfuric acids, acts upon cellulose, converting it into various nitro-derivatives, several of which have great industrial value. The number of NO_{3} groups which unite with the cellulose molecule under these conditions depends upon the temperature, pressure, etc., employed during the nitration process; di-, tri-, tetra-, penta-, and hexanitrates are all known. _Pyroxylin_, or _collodion_, is a mixture of the tetra- and penta-nitrates, which is soluble in alcohol and is used in surgery, in photography, and in the manufacture of celluloid, which is a mixture of collodion and camphor. The hexanitrate, C_{12}H_{14}(NO_{3})_{6}O_{4}, is the violent explosive known as _gun-cotton_.

Gentler oxidizing agents, such as "bleaching powder," etc., have no effect upon cellulose, and hence are extensively used in the treatment of cotton and other vegetable fibers, in preparation for their use in the manufacture of textiles, paper, etc.

Cellulose is indigestible in the alimentary tract of animals, but the putrefactive bacteria which are generally present there ferment it, with the production of acids of the "fatty acid" series, carbon dioxide, methane, and hydrogen. Excessive fermentations of this kind are responsible for the distressing phenomenon known as "bloat."

The =compound celluloses= comprise the larger proportion of the material of the woody stems of plants. They consist of a base of true cellulose, which is either encrusted with or chemically combined with some non-cellulose constituent. Depending upon the nature of the non-cellulose component, the compound celluloses are divided into three main groups, known respectively as (1) ligno-celluloses, (2) pecto-celluloses, and (3) adipo-, or cuto-celluloses. As the names indicate, the non-cellulose component in the first group is lignin; in the second, pectic substances; and in the third, fats or waxes.

=Ligno-celluloses.=--In the young plant cell, the cell-walls consist of practically pure cellulose; but as the plant grows older, this becomes permeated with lignin, or woody fiber, until in the stem of a tree, for example, the proportion of cellulose in the tissue is only 50 to 60 per cent. In the preparation of wood pulp for the manufacture of paper, the lignin materials are dissolved off by means of various chemical reagents, leaving the cellulose fibers in nearly pure form for use as paper. The lignin material generally consists of two types of substances, one of which contains a closed-ring nucleus of unknown composition and the other is probably a pentosan. These materials are so extremely difficult to hydrolyze that their composition has not yet been definitely determined.

=Pecto-celluloses= are found in various species of flowering plants; those which are present in the stems and roots being true pecto-celluloses, while those which are found in fruits and seeds contain mucilages rather than pectose derivatives, and are generally designated as "muco-celluloses." The exceedingly inert character of these compounds makes their study difficult and their functions uncertain.

The term =cuto-celluloses= is applied to the group of substances, including suberin and cutin, which constitute waterproof cell-walls. These were formerly supposed to consist of true cellulose impregnated with fatty or wax-like materials. Recent investigations seem to indicate, however, that there is really no cellulose nucleus in such walls as these, but that they are compound glyceryl esters resembling the true fats (see chapter X) in composition. If this view should finally be established as a fact, this sub-group of supposed compound celluloses should be dropped from consideration as such.

PHYSIOLOGICAL USE OF CELLULOSES

There seems to be no question that the sole use of celluloses is to serve as structure-building materials. They are undoubtedly elaborated from the carbohydrates as the cell grows. In only rare cases, however, is there any evidence that they can be reconverted into carbohydrates to serve as food material. Certain bacteria can make use of cellulose as food, and secrete an enzyme, cytase, which aids in the hydrolysis of cellulose to sugars for this purpose. But this enzyme seems rarely, if at all, to be present in the tissues of higher plants. It has been reported that some cellulose is hydrolyzed during the malting of barley, indicating that this might have some food use for the growing seedling; but this observation has not been confirmed and later investigations seem to throw doubt upon its accuracy.

Bacteria of decay also act upon cellulose materials, converting them chiefly into gaseous products; but this seems to be a provision of nature for the destruction of the cell-wall material of dead plants, rather than an arrangement for the constructive use of it as food for the bacterium. When fibrous plant residues decay in the soil, the cellulose compounds are first converted into a series of complex organic acids, known as "humins," which undoubtedly have a significant effect upon the chemical and physical properties of the soil, but these have little interest or significance in a study of the chemistry of plant growth.

REFERENCES

ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 2, Gummisubstanzen, Hemicellulosen, Pflanzenschleimen ..." 729 pages, Berlin, 1911; and "Band 8--1 Ergänzungsband (same title as Band 2)--," 507 pages, Berlin, 1914.

SCHWALBE, C. G.--"Die Chemie der Cellulose," 665 pages, Berlin, 1911.