Further, it has been fairly well established that the "chromogens," or mother-substances of the pigments, which, under the influence of oxidase enzymes, serve to regulate the respiratory activities of the plant are essentially glucosidic in character. This, and other, functions of the pigments, most of which are glucosides, will be discussed at some length in the chapter dealing with the Pigments (Chapter VIII). Many gaseous anæsthetics are known to have a marked effect in stimulating plant growth. In a number of cases, it has been shown that the contact of plant tissues with these anæsthetics brings about an interaction of the enzyme and glucoside which are present in the tissue, with the consequent hydrolysis of the latter, setting free its characteristic components. This observation has led to the supposition that many of the organic constituents of glucosides are definite plant stimulants, to which the name "hormones" has been applied. There is considerable experimental evidence to support this conception that glucosides may be the source of stimulating hormone substances, which will be discussed more in detail in the chapter dealing with these plant stimulants (Chapter XVII). Glucosides may also serve as the mechanism for putting out of action of harmful products which may appear in the tissues as the result of abnormal conditions. These harmful substances may be rendered soluble by combination with sugars and so transposed by osmosis to some other part of the plant. The abnormally large percentages of glucosides which are present in certain species of plants during unfavorable climatic conditions lends some support to this view. Finally, it may be assumed that easily oxidizable substances, such as aldehydes and acids, are possibly protected against too rapid, or premature, oxidation by being transformed into glucosides. In general, it may be said that the glucosides seem to serve as the regulatory, protective, and sanatory agencies of the plant mechanism. BIOLOGICAL SIGNIFICANCE OF GLUCOSIDES The bitter taste of glucosides and their almost universal presence in the bark of plants undoubtedly helps to prevent the destructive gnawing of the bark by animals.