The structural arrangement of the characteristic groups in these glucosides has not yet been definitely worked out.
Cymarin, the active principle of Indian hemp (Apocynum cannabinum), is similar in type to the digitalis glucosides. When hydrolyzed, it yields a sugar known as "cymarose," C7H14O7, which seems to be a monomethyl derivative of digitoxose, and cymarigenin, C23H30O5, a compound which is either identical or isomeric with the organic residue obtained from other members of this group.
THE SAPONINS
The saponins constitute a group of glucosides which are widely distributed in plants, whose properties have been known since early Grecian times. They have been found in over four hundred different species of plants, belonging to more than forty different orders.
The most characteristic property of saponins is that they form colloidal solutions in water which produce a soapy foam when agitated, and are peculiarly toxic, especially to frogs and fishes. In dry form, they have a very bitter, acrid taste, and their dust is very irritating to the mucous membranes of the eye, nose, and throat.
On hydrolysis, the saponins yield a variety of sugars,—glucose, galactose, arabinose, and sometimes fructose, and even other pentoses—and a group of physiologically active substances, known as "sapogenins."
The more toxic forms of these glucosides are known as "sapotoxins."
The chemical composition of the saponins varies so widely that it is scarcely possible to cite typical individuals. Sarsaparilla, the dried root of smilax plants, contains a mixture of non-poisonous saponins, from which at least four individual glucosides have been isolated and studied. Corn cockle contains a highly poisonous sapotoxin which, on hydrolysis, yields four molecules of a sugar and one of sapogenin, C10H16O2. Other sapotoxins are obtained from the roots of soapwort and from several species of Gypsophila. Digitonin and digito-saponin are glucosides of this type which are found in the extracts from various species of Digitalis.
THE PHYSIOLOGICAL USES OF GLUCOSIDES
It is scarcely conceivable that substances which vary so widely in composition as do the different types of glucosides can possibly all have similar physiological uses in plants. The cyanophoric glucosides, the pigment glucosides, the mustard oil glucosides, and the saponins, for example, can hardly be assumed to have the same definite relationships to the metabolism and growth of the plant. To be sure, they are alike in that they all contain one or more sugar molecules, and it is probable that the carbohydrates which are held in this form may serve as reserve food material, especially when the glucoside is stored in the seeds; but it is obvious that the simpler and more normal form of such stored food is that of the polysaccharides which contain no other groups than those of the carbohydrates. It seems much more probable that the physiological uses of glucosides depend upon their ability to form temporarily inactive "pairs" with a great variety of different types of organic compounds which are elaborated by plants for a variety of purposes.