CHEMISTRY, or the chemistry of plants, is so extensive a subject that ' it is impossible here to give much more than an enunciation of the most important propositions, without entering into full proofs or details. On submitting to incin eration a plant which has been dried at a moderate heat till it ceases to lose weight, we find that the residue, which consists of mineral salts and a little carbon, is much lighter than the original plant, the portion which is burned off, or apparently lost, correspond ing to the organic constituents of the plant. Hence every plant, like every animal, is composed of organic and mineral or inorganic constituents. While the mineral constitu ents of the plant arc also found in the crust of the earth, the organic constituents are primarily formed in the plant itself from inorganic matters, viz., from water, atmospheric air, and the soil, which collectively may be termed the food Of plants.
The following general principles may be laid down regarding the organic constituents which mainly.con-tribute to form the bulk of the body of the plant: (1.) All organic constituents of plants contain carbon. (2.) All such organic constituents contain hydro gen. Some of them, as, for example, many ethereal or volatile oils, consist solely of these two elements. (3.) The greater proportion of these compounds contain oxygen in addition to the two preceding elements. To this class belong those constituents of plants which are at the same time of the most general diffusion and of the greatest physiological and economic importance; namely, the so-called carbohydrates, which consist of car bon combined with hydrogen and oxygen in the exact proportion in which the last two elements form water. Under this title are included cellulose, starch, gum, etc, Other organic constituents contain not only carbon with hydrogen and oxygen in the above ratio, but an excess of oxygen. In this category may be placed almost all the organic acids, many ethereal oils, wax, the resins, many of the so-called glycosides, and the fats. (4.) With the above elements, nitrogen is associated, to form two very important groups of constituents, viz., the organic &mei or alkaloids, and the albunzinates or proteins bodies.
Although the nitrogenous groups never form more than a small part of the mass of a plant, nitrogen is never altogether absent from a plant. (5.) In association with all the above-named elements, sulphur in small quantity is present in the albuminates of all plants; in association only with carbon and hydrogen, it occurs in oil of garlic and oil of asafetida; and when combined with carbon, hydrogen, and nitrogen, it has been as yet only found in oil of mustard. Whether phosphorus in very minute quantity occurs in any of the vegetable albuminates, is still uncertain.
The inorganic constituents which are found in the ashes of all plants are: potash, soda. magnesia, and lime, in combination with phosphoric, sulphuric, hydrochloric, and carbonic acids, and additionally, iron, manganese, and silica, with traces of fluorine; while the marine plants or sea-weeds contain also appreciable quantities of bromine and iodine. Alumina and baryta are also 'occasionally found, as also are nitrates in certain plants. The carbonates almost always found in the ash are, as is well known, for the most part formed by the action of the incineration upon the salts of the vegetable acids, such as the acetates, citrates, etc., and probably in some other respects, the arrangements of the constituents of the ash are not precisely identical with those of the mineral ingredients while existing in the actual•plant. Among the most essential of the inorganic constitu ents is water, which acts as a solvent for the matters dissolved in the vegetable juices, and forms a very preponderating part of the mass, sometimes amounting to from 86 to 96 per cent of the whole plant. From the preceding remarks, it is obvious that the nutrition and development of plants is dependent on their absorbing carbon-compounds, hydrogen-compounds, nztrogen-compounds, sulphur-compounds, water, and such inorganic compounds as yield the necessary inorganic constituents in a form capable of assimilation; together with the presence of oxygen, which is required for the formation of organic oxygenous compounds.