causing the boiled fruits to set into jelly. Gums and pectins do not color with iodin; they reduce copper only after inversion.
Sugar.—The various forms of sugars have a wide distribution in the vegetable kingdom. Some reduce copper in alkaline solution; others do so only after inversion. All turn the plane of polarized light. They are soluble in water; much less so in alcohol. The most important types are:
Glucose (Dextrose and Levulose), ; Maltose and Saccharose, ; Mannite .
Glucosids.—These are ester-like combinations of sugars with various other substances, frequently with phenols. Acids or appropriate ferments hydrolyze them into sugar and the other substance. Many do not contain nitrogen. Most glucosids are neutral or weakly acid; a few are alkaloidal. They are widely distributed in plants, and include active principles such as the digitalis constituents, saponins, tannins, etc.
Saponins.—These are neutral, non-nitrogenous substances distinguished by foaming with waters, emulsifying oils, and laking red blood corpuscles. Many have the formula . Some are glucosids. Those which are markedly toxic are sometimes called sapotoxins. The saponins are also of wide occurrence.
Tannins.—This term is applied to a group of phenol derivatives, distinguished by giving a bluish or greenish color with ferric salts. The greater number also form insoluble compounds with other metallic salts, with alkaloids, proteins, etc. This precipitation leads to an astringent action.
Tannins occur in many plants, especially in the leaves and bark, and in pathologic formations (nut-galls). They are non-nitrogenous; some are glucosids. They are soluble in water and in alcohol; but since they form insoluble compounds with so many substances, they often occur in plants in granular form.
Phlobaphens.—Tannins are easily decomposed into resin-like substances called phlobaphens. These exist naturally in plants, but are usually formed as artificial decomposition products in all extracts. They are dark-colored, soluble in alcohol and in alkaline liquids, insoluble in water.
Tannins and phlobaphens, as well as most other plant constituents, are easily converted into a group of substances called humins. These do not exist in living tissues, and arise on the death of the cells, by the action of air and moisture. They cause the brown color which plants assume on drying; they are also present in the brown bark.
Tannins, phlobaphens, and humins form a series, without sharp demarcation. They are rather resistant to the action of bacteria, and in this way protect plants against putrefaction.
Alkaloids.—These comprise many of the most active and important plant constituents. They may be defined as natural nitrogenous organic bases (generally tertiary amins); i.e., they are organic substances, containing nitrogen, of basic character, combining with acids without the elimination of hydrogen, forming well-defined and usually crystalline salts. The salts with halogens are called hydrochlorids, hydrobromids, etc. (not chlorides, etc.).
The names of alkaloids are often spelled with a final e, to distinguish them from neutral principals (thus morphine, an alkaloid; salicin, a glucosid). This distinction is rather arbitrary, and is not followed here.
Discovery of the Alkaloids.—This is an achievement of the 19th century (Morphin, by Sertuerner. 1805 to 1817; Strychnin, 1818; Quinin and Caffein, 1820; Nicotin, 1829; Atropin. 1833).
