1367. The plant is also a totality in reference to the earths. It contains all the mineral classes, and from each of these indeed the principal or fundamental minerals. It can therefore only thrive in a soil which represents the whole mineral kingdom. Among the earths the siliceous earth is very frequently found in plants, and especially in the graminaceous kinds. This, having been dissolved in the earth by potash and the rich supply even of carbonic acid, appears to be absorbed or imbibed by the plant. The argillaceous earth is scarcely met with in the plant itself; but from imbibing and storing up water for the consumption of the plant, it is without doubt its best and most necessary soil. The talcose earth is rarely found contained in plants; it, however, keeps the soil slacker, by dividing into laminæ, and being present for the most part as mica in sand. The calcareous earth is a more essential constituent part of plants, and is found therein in tolerable quantity, usually combined with phosphoric or carbonic acid. Of the salts, all plants contain a fair proportion of common salt and potash, combined too with carbonic acid; soda with saccharic or oxalic acid; probably also ammonia. Of the acids, carbonic acid appears to be alone contained in a free state in vegetable sap; the other elemental and mineral acids are united to alkalies, talcose and calcareous earths. As regards the Inflammables, almost the whole plant consists of carbon, but contains also some sulphur. The metals are represented by iron, which occurs in all plants.
b. ORGANIC VEGETABLE BODIES.
1368. These must be regarded as the repetition of the inorganic bodies. The alcohol, which does not indeed occur ready formed in the plant, but is developed out of the sugar, certainly corresponds to the æther. The ætherial or volatile oils, and the balsams and resins that are thence formed, correspond with the air. The mucus, gelatine, albumen, and sugar correspond to water; the wood, gum, starch and vegetable mould, to the earth. Of the organic salts, plants contain tannin, with azetic, benzoic, mucic, gelatic, saccharic, tartaric, citric, malic, oxalic, tannic, oleic, isatic, and hydrocyanic acids. The alkaline bodies are pungent, bitter, stupefying, and saponaceous; the fixed or greasy oils, the wax and the vegetable butters, are to be regarded as organic Inflammables; the colouring matters, as the organic ores.
1369. These bodies intermixed form the compound vegetable matters. What has been called vegetable sap is for plants, what the blood is for animals. It consists for the greatest part of water and mucus, starch, sugar, acids, and salts. It passes over into vinous and then into acetous fermentation. The starch-granules appear to form in the cells.
1370. To the secreted saps belong the coloured milky juices present in particular vessels, and consisting for the most part of water with resins, as in the celandine and spurge. The particular saps, especially those of the fruits, are very composite, consisting for the most part either of mucus, sugar and acids, or occasionally of gelatine and albumen. Solid compound matters are almost universally made up of flour, which consists principally of starch and gum; or they are furthermore mucus in the roots and seeds. The excreted or separated matters, which no longer interpose in the vegetable process, are the etherial oils, resins, fixed oils, colouring matters, poisonous substances, gum, tannin, nectar-juices, and even water.
a. WROUGHT BY EXTERNAL INFLUENCE.
1371. The influence of the elements produces different phenomena in the plant. I have felt constantly more inclined to consider, that not merely the descent of the root, but even the ascent of the stalk was simply to be viewed as a mechanical event, or one forsooth effected by gravity. The roots obey under all circumstances the gravity and would grow as far as the centre of the earth, were they to meet with no impediment; and there they would follow the revolution of the earth, and consequently become spirally convoluted upon themselves. It is almost beyond doubt that the water, which sinks downwards and, as it were like that in stalactites, invariably rigidifies or hardens at the radical capillaries, is heavier in the root. The cause of this greater weight depends upon the mucus not being decomposed.
1372. The straight ascent of the stalk also depends upon nothing else than gravity. The upper drops of mucus become lighter by means of greater heat and by decomposition in light and air, and they are therefore compressed by the heavier in the upward direction. It is always such a small drop upon the summit, which hardens into its uppermost cell. The stalk, therefore, grows upwards through the same forces and in the same manner, as the air-bubbles ascend in a glass of beer. The cause of their becoming lighter resides certainly in the vital process, which nevertheless effects in this respect nothing else than the extension or increase; but yet the cause of the ascent is naught else than the gravity.
1373. The light likewise acts upon the direction of plants and especially that of their leaves; not simply from its promoting growth by elevation of temperature and by decomposition, but obviously in a mechanical manner also; for not only do the branches of plants in a green-house grow towards the window, but most leaves turn themselves the whole day in obedience to the course of the sun. This turning must, nevertheless, have one and the same kind of cause with the growing towards light; it also is only a conatus or effort unto growth. The upper leaf-cells, being illuminated by the sun, become lighter, and are therefore directed at once, like the apices of the branches, toward the influence of light. The cells that stand perpendicularly upon the surface of the leaf are to be regarded as branches conjoined by growth.