The Chemistry of Plant Life

by Roscoe Wilfred Thatcher

HORMONES

The term "hormone" was first used to designate certain stimulating substances which are supposed to exist in the intestinal tracts of animals and to cause the glands to elaborate and secrete their characteristic enzymes. The supposed "hormones" are not themselves active in performing the digestive functions of the glandular secretions, but are the exciting, or stimulating, agents which cause the glands to secrete their active enzymes.

The same term has been used, by certain plant physiologists, to designate any agency, either external or internal, which stimulates plant protoplasm to abnormal activity. It has been pointed out that there are a variety of substances, which are themselves chemically neutral, that are powerful stimulants of vital activity if used in only minute proportions, but are powerful poisons if present in larger amounts. Many of the alkaloids act in this way upon the animal organism; while chloroform, toluene, and even some of the more complex hydrocarbons, act similarly upon the tissues of plants, and ether vapor is known to be a powerful stimulant in accelerating the flowering of plants and the ripening of fruits. It has been shown that the vapors of all such substances readily penetrate the protoplasm of leaves, seeds, etc., even when the same parts are impermeable to most mineral salts, sugars, etc.; and that upon entrance to the protoplasm of a leaf, or a seed, they tremendously stimulate its metabolic activity. These hormones, as a class, are chemical substances which have very little attraction for, or power of combination with water; and it has been suggested that the ease with which they penetrate the protoplasm is due to the fact that they are not held at the surface by combination with the active water molecules which are present in the surface layer.

The principal effect which is supposed to be produced by these "hormones" is the stimulation of the enzymic activity, particularly that of the degenerative processes which take place late in the plant's life, at the flowering or ripening periods. Many of the changes which take place normally at ripening time, such as the change in color from green to yellow or red and finally to brown or black, when the fruit or vegetable is fully ripe, can be greatly accelerated by treatment with these substances. Hormones are similar in type to the ethereal salts, or esters, which constitute the natural essential oils that develop in many plants at this stage of their growth. Hence, it seems probable that these changes in plants which are maturing naturally may be hastened by the hormone action of the esters and similar bodies which are developed in largest quantities at that stage. It has been pointed out that the characteristic group which is present in many natural glucosides is of the same general type as the "hormone" substances which are used in the artificial stimulation of the flowering or ripening changes. This fact, together with the possibility of the liberation of greater percentages of these aromatic compounds from their glucoside combinations at the later periods of plant growth, is assumed, by some plant physiologists, to account for the change from synthetic to degenerative processes at this stage of the plant's development.

Further, it has been suggested that the autumnal coloration of leaves, and their dropping from the stems of the plant, as well as the ripening of seeds, is probably determined by the liberation in the plant, at that stage of its growth, or as a result of changed climatic conditions at that particular season of the year, of the hormones which either initiate or hasten the special enzymic changes which distinguish the degenerative from the synthetic processes of the plant.

Similarly, it has been suggested that parasitic fungi are able to penetrate the host plant by first excreting "hormones" which bring about degenerative changes in the tissues of the host plant and so make it more easily penetrable by the hyphae of the parasite.

It will be seen that, in general, "hormones" are a type of substances (possibly often present in plants in the form of glucosides) which are supposed to stimulate the degenerative (or katabolic) vital processes in contrast to the synthetic (or anabolic) changes. It has been suggested that they do this in either one of two ways; namely, by favoring the introduction of water into the protoplasm and so diluting the cell contents, changing the osmotic pressure, etc.; or by bringing about a separation of the colloidal layers, or films, of the protoplasmic complex, producing a result similar to that produced by freezing the tissues. These ideas have been suggested by studies of the changes in the equilibrium of protoplasm when foreign substances are introduced into it. These studies have not as yet been brought to the stage of final conclusions, and the ideas presented must be considered as suggestive rather than as conclusive.

VITAMINES

"Vitamines," as contrasted with "hormones," are supposed stimulants of synthetic metabolic processes, or accelerators of growth, rather than of degenerative processes.

The term "vitamine" was first used to designate the substance, or substances, which must be present in the diet of animals in order that the animal organism may grow. Absence of these substances from the food of the animal results in the stoppage of growth of young animals and in various so-called "deficiency diseases" (such as beri-beri, scurvy, polyneuritis, etc.) of adults. This means that the animal organism is altogether unable to elaborate its own vitamines, and extended investigations have indicated that the vitamines necessary for animal uses are wholly of plant origin. The name "vitamine" was first used because it was supposed that these substances are chemical compounds of the amine type and, since they are necessary to normal life processes of animals, the name "vitamine" seemed to represent both their chemical character and their functions. Later investigations have caused doubt as to the accuracy of the first belief as to their chemical nature, and various other names have been suggested for the general group of substances which have the observed beneficial effects; while such specific names as "fat-soluble A," "water-soluble B," etc., have been used to designate individual types of these accessory food substances. However, the term vitamine is such a convenient one and is so generally recognized and accepted that it will probably continue to be used, at least until some more definite knowledge of the nature and composition of these growth-promoting, disease-preventing, and reproduction-stimulating food constituents is obtained.