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.