GENERAL PROPERTIES OF GLUCOSIDES
As a rule, glucosides are easily soluble in water. They are generally extracted from plant tissues by digestion with water or alcohol. In most cases, the enzyme which is present in other cells of the same tissue must be killed by heating the material, in a moist condition, to the temperature of boiling water, before the extraction is begun, as otherwise the glucoside will be hydrolyzed as rapidly as it is extracted from its parent cell. Maceration or otherwise bruising the tissue, after the enzyme has been destroyed, facilitates the extraction. The glucosides, after extraction and purification by recrystallization, are generally colorless, crystalline solids, having a bitter taste and levorotatory optical activity. This latter property is remarkable, as most of them are compounds of the strongly dextrorotatory d-glucose.
Many of the natural glucosides have marked therapeutic properties and are largely used as medicines; others are the mother-substances for brilliant dyes; for example, indican, from which indigo is obtained, and the alizarin glucosides.
Several hundred different glucosides have been isolated from plant tissues, and their properties described, and this number is being added to constantly, as the methods of isolation and study are improved. They may be classified into groups, according to the nature of the organic compound other than sugars which they yield when hydrolyzed. The following descriptions of the occurrence, constitution, products of hydrolysis, and special properties of typical members of each of the several different classes of glucosides will serve to illustrate their general relationship to plant growth.
THE PHENOL GLUCOSIDES
Arbutin, C12H16O7, is obtained from the leaves of the bear berry (Arctostaphylos uva-ursi), a small evergreen shrub. When hydrolyzed by mineral acids or emulsin, it yields glucose and hydroquinone.
C12H16O7 + H2O = C6H12O6 + C6H4(OH)2.
Hydroquinone has strongly antiseptic properties. Arbutin is both an antiseptic and a diuretic, and is used in medicine.
Phloridzin, C21H24O10, is found in the bark of apple, pear, cherry, plum, and similar trees. Mineral acids (but not emulsin) hydrolyze it to glucose and phloretin (C15H14O5), according to the equation