SECRETIONS, VEGETABLE. In the vegetable kingdom the term secretion has a wider application than in the animal kingdom, and all substances which have been formed by the action of cells upon the compounds taken up as food (such as carbonic acid, water, and ammonia)—whether these substances form a part of the tissues of the plant, or are thrown out upon its surface—are equally considered as secretions. All the important vegetable secretions are compounds of carbon, hydrogen, oxygen, and nitrogen; sulphur being also present in some cases; and according to their functions they may he classed in two great divisions—viz., (1) nutritive or assimilable secretions, and (2) non•assimi lable or special secretions.
1. The nutritive secretions are those substances which, having been formed within the plant., are used in forming its structures'and constructing its general mass. The chief substances in this class are cellulose, the varieties of starch, the varieties of sugar, the oils, and the so-called protein or albuminous bodies. The composition.of these substances is extremely varied; thus many of the volatile oils or essences contain only carbon and hydrogen; the sugars, starches, and cellulose contain carbon, hydrogen, and oxygen, and are named tenary compounds; while the protein bodies contain carbon, hydrogen, oxygen. and nitrogen, and in some cases sulphur.
2. The non assimilable secretions are only found in certain parts of the plant, and =hey receive their name from their never being converted into the nutritive secretions. The principal members of this class are the coloring matter of plants (chlorophyle and its modifications); the substances which, when extracted from plants, are of service as dyestuffs (the chromogcns or color-formers of recent chemists); the organic acids, which constitute a somewhat numerous group, and of which oxalic acid (occurring in rhubarb. sorrel, etc.), tartaric and racemic acids (in the grape), malic acid (in the apple and goose berry), citric acid (in the orange, lemon, lime; and red currant), gallic acid (in the seeds of the mango), meconic acid (in the opium poppy), and tannic acid (in the bark of the oak, elm, etc.), may be taken as well known examples; the vegetable alkalies or alka loids, such as morphia, strychnia, quinia, etc.; the volatile oils; and the resins.