434 



THE DICTIONARY OF GARDENING, 



Silica continued. 



access to the roots of plants contains an appreciable 

 amount of Silica, either pure or, more frequently, com- 

 bined 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, con- 

 taining 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 are 

 easily broken up by the acids formed in plants during 

 growth, e.g., Oxalic and Citric Acid, &c. : the metals com- 

 bine 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 pre- 

 sent 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 in- 

 terfered 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 com- 

 pletely as possible, and whose ash, in consequence, con- 

 tains 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 men- 

 tioned above; and the Silica remains as a deposit in 

 the cell walls after the alkalies have been made use of 

 in the nutrition of the plants. It has been suggested 

 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 de- 

 fence 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. 



SILICATES. See Silica. 



SIIiICLE. A siliqna as broad as it is long, or 

 broader. 



SILICULOSA. A Linnean artificial order of the 

 class Tetradynamia, having silicnlose pods. 



SILIQUA. The long, pod-like fruit of Cruciferae. It 

 consists of a pair of valves applied to a frame on which 

 the seeds grow. 



SILIQUOSA. A Linnean artificial order of Tetra- 

 dynamia, having siliqnose fruit. 



SILK-COTTON TREE. A common name for the 

 genera Bombax and Eriodendron* 



SILKEN SISSY. An old name for Asclepias. 

 SILK-TREE. A common name for Albizzia Juli- 

 brissin. 



SILK VINE. A common name for Periploca grceca. 

 SILKWOOD-TREE. A name applied to Muntingia 

 Calabura. 



SILKY. See Sericeous. 



SILKY OAK. A popular name for Grevillea robusta. 

 SILFHA. A genus of Beetles, the larvse of which 

 usually feed in the rotting bodies of animals, and are 

 especially numerous in dead moles, birds, and other so- 

 called " vermin," hung up as scarecrows. The insects 

 of this genus are, for the most part, considerably de- 

 pressed or flattened from above downwards, oval in 

 outline, about in. or fin. long, with a small head, and 

 furrowed wing-cases. They are almost entirely black, or 

 brown-black, with a dull yellowish, downy coating, which 

 is easily rubbed off. The grubs are more slender in 

 form than the adult insects, and all the rings, except 

 the three next the head, have the edges sharp, and pro- 

 longed forward into a tooth, and the tail ends in two 

 sharp points. They are usually entirely black, or black 

 with a narrow, tawny border. They are active in their 

 habits, running about by means of three pairs of short 

 but well-formed legs, situated on the front part of the 

 body. When full-fed, they form cocoons in the soil, in 

 which they become pupae, and in due time beetles As 

 long ago as 1844, larvae of a Silpha were found feeding 

 on Beets, in France; and in the same year they proved 

 seriously injurious to Mangel-Wurzel crops near London- 

 derry, in Ireland. They devoured the young leaves 

 almost as soon as the latter appeared, till only the larger 

 ribs remained, and the plants, in consequence, died off 

 largely. Mangel-Wurzel and Beets alone suffered ; and 

 suoh crops as Oats, Wheat, Potatoes, and Turnips escaped 

 unharmed in fields where the former plants were entirely 

 destroyed. From larvae feeding on Beets, in France, 

 there were reared beetles belonging to the species 

 S. opaca, known as the Beet Carrion Beetle ; and the 

 same insect was also identified as the culprit in Ireland. 

 It is rather under in. long, flattened, and brownish- 

 black, with the tip of the body dull red. There are 

 three ridges down each wing-case. This beetle had long 

 been known to frequent dead bodies of animals. It is 

 probable that other species of Silpha also injure Beets 

 and other garden produce. 



Treatment. For the sake of prevention, only manure 

 free from such substances (e.g., offal) as might attract 

 the beetles should be used for ground on which Beets 

 and Mangel-Wurzel are to be grown. Should the plants 

 be attacked, dressings of gas-lime and of sulphur or soot, 

 scattered over the wet leaves in dewy mornings, would 

 probably be of service in checking the evil, as would 

 also be paraffin dressings. All methods of strengthening 

 the plants and promoting the growth of new leaves are 

 of great importance, and manures may enable the crops 

 to pass through an attack without serious results. But 

 should the attack prove fatal, it will be well to utilise 

 the ground for some other crop, which may be sown or 

 planted with safety in the infested soil immediately 

 after the removal of the Beets or Mangolds. 



SILFHIUM (Silphion, the ancient Greek name used by 

 Hippocrates for a plant which produced some gum-resin, 

 perhaps asafoetida, and which was transferred by Linnaeus 

 to this genus). Rosin-plant. ORD. Composites. A genus 

 consisting of eleven species of tall, coarse, hardy, peren- 

 nial herbs, with a copious resinous juice, confined to 

 North America. Flower-heads yellow, large, corymbose- 

 panicled ; involucre broad and rather flat the scales 

 imbricated in many rows ; ray florets numerous, fertile ; 

 disk florets sterile ; achenes glabrous, surrounded by a 

 wing, which is notched at the top. Leaves alternate, 



