Silica
A very abundant mineral in the soil, both in the pure state, and in combination with various elements forming Silicates. Pure Silica is made up of the two elements Silicon, or Silicium, and Oxygen, in the proportion of twenty-eight parts, by weight, of the former to thirty-two of the latter. It occurs in several conditions, of which the most frequent are quartz and flint, and the less common rock-crystal and calcedony. Amethyst and cornelian are forms of Silica, tinged red with iron oxides. Silica, in the form of quartz, is the chief ingredient of all sands and sandstones, as well as of granites and other minerals of the same general structure; and veins or masses of pure quartz of considerable size also exist. But almost more plentiful than Silica are the Silicates, chiefly of Potassium, Sodium, Calcium, and Magnesium. These are familiar to everyone as clay, mica, felspar, and other substances that make up the mass of most soils.
Silica is not soluble in pure water; but a little Carbonic Acid gas, dissolved in the water, permits of a little Silica being dissolved; and almost all water that gains access to the roots of plants contains an appreciable amount of Silica, either pure or, more frequently, combined with alkalies. Silica exhibits the properties of a weak acid, in its power of combining with various metals. It combines with these, in more than one proportion, certain compounds, called normal or basic Silicates, containing a larger proportion of the alkali than do the others. These normal Silicates are the ones that dissolve in water.
Silicates (especially normal Potassic Silicate) pass into the root-hairs of the plants from the soil, dissolved in the water absorbed by the hairs, and are then carried up the stems, as described under Sap. They easily broken up by the acids formed in plants during growth, e.g., Oxalic and Citric Acid, &c.: the metals combine with these acids to form new compounds, and the Silica is set free in the cell-sap; but, not being so soluble as the Silicates, it is mostly added to the walls of the cells, and is peculiarly often found in the layer or cuticle on the outer surface of the stems or leaves. In some plants, this layer is so strong and continuous that all the vegetable substance may be destroyed by means of Nitric Acid, or by burning on platinum-foil over a spirit-lamp, without destroying the continuity and markings of the cuticle, which even extends over the hairs. Such a deposit of Silica in the epidermis is well shown in many grasses, and still better in the Horsetails or Equiseta; but it is met with also in many other plants, e.g., in Deutzia scabra, where it forms a beautiful object when all the vegetable matter in the cuticle has been destroyed.
The use of Silica to plants is very doubtful. It is present in the ash of almost all plants. In many it is so abundant as to seem of much importance to them; yet the results of experiments on growing plants in artificial soils from which it is almost absent, tend to the conclusion that the growth of plants is not greatly interfered with even when the supply of Silica taken in by them is far below that usually present in them. For example, the straw of grasses is usually very rich in Silica (frequently to the amount of one-half of the ash) when grown in ordinary soils; yet grasses grown in artificial soils, from which it has been excluded as completely as possible, and whose ash, in consequence, contains less than 1 per cent. of Silica, prove as healthy and vigorous in every way as if they had been grown in ordinary soils. Probably, a large proportion of the Silica is absorbed in the form of alkaline Silicates, as mentioned above; and the Silica remains as a deposit in the cell walls after the alkalies have been made use of that Silica may be of service in two ways, viz., in giving strength and rigidity to the stems, and in rendering the cuticle harder, so that when spores of parasitic Fungi fall upon it, and begin to germinate, the mycelium tubes find greater difficulty in piercing into the tissues of the plants. The Silica in the cuticle may thus be a defence against disease from this cause, but, necessarily, against only such Fungi as bore through the cuticle into the inner tissues, and not against those which push in their mycelium through stomata.