The Chemistry of Plant Life

by Roscoe Wilfred Thatcher

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.