The importance that has within the last few years attached to the Dahlia would render it easy to fill a volume with descriptions of its various species and varieties, and the details of their culture. Its history is also somewhat curious, as, strange to say, though it has become so great a favourite, and is so universally cultivated, the history of its introduction is very obscure. It is generally said to have been introduced by Lady Holland in 1804; but the fact is, it had been introduced many years before that period, and was only brought from Madrid, in 1804, by Lady Holland, who apparently did not know that it was already in the country. The first kind of Dahlia known to Europeans (D. variabilis, Dec.) was discovered in Mexico by Baron Humboldt in 1789, and sent by him to Professor Cavanilles, of the Botanic Garden, Madrid, who gave the genus the name of Dahlia, in honour of the Swedish professor Dahl. Cavanilles sent a plant of it the same year to the Marchioness of Bute, who was very fond of flowers, and who kept it in the greenhouse. From this species nearly all the varieties known in the gardens have been raised; as it seeds freely, and varies very much when raised from seed.

In 1802, a second species (D. coccinea) was introduced from France, in which country it had been raised from Mexican seeds. A few varieties have been raised from this kind, but they are much smaller than the others. It is rather remarkable, that the two species do not hybridise together; and the first kind produces flowers of colours so different as crimson, purple, white, yellow, orange, and scarlet, without hybridisation. Among all the colours, however, displayed by these varieties, no flowers have yet appeared of blue, and comparatively few of a pure white. These two species, and their varieties, were the only Dahlias known in English gardens for many years; as though a few kinds were introduced from time to time from France and Spain, yet as they did not hybridise with the others, and were rather more tender, they were not generally cultivated, and appear to have been soon lost

The Dahlia is a tuberous-rooted plant, which is propagated either by seeds, or by division of the root The seeds are chiefly used for raising new sorts; and they should be treated like tender annuals, being sown on a slight hotbed, in February or March, and planted out in May. The plants rarely flower the first year, but the tubers will form in the course of the summer, and may be taken up in autumn with those of the old plants. When the plants are propagated by division of the root, care must be taken that each piece has a bud attached to it. These buds, or eyes, as gardeners call them, are not scattered all over the tubers, like those of the potato, but collected in a ring round the collar of the root. These eyes, when the tubers are in a dry state, are sometimes scarcely perceptible; and to discover them nurserymen often plant their Dahlia-roots in a hotbed,"to start the eyes," as they call it; that is, to force the latent buds sufficiently forward to show where they are situated, before they divide the tubers for the purpose of forming new plants. Sometimes the eyes do not form a ring round the collar or crown of the root, but a considerable portion of it is without any buds.

These parts, when divided from the rest, are called blind tubers; and, though, if put into the ground, they will live for several years, sending out abundance of fibrous roots every year, no gardener has yet been able to induce a blind tuber to form an eye, or to send up a shoot. Dahlias are also propagated by cuttings of the stem, taken from the lower part of the plant; or young shoots slipped off the tuber, with part of the woody fibre attached. The cuttings should be struck in sand, or very sandy loam, under a bell-glass, and with bottom-heat Great care should be taken to shade them from the direct rays of the sun, till they have thrown out roots; as the leaves are easily withered, and when this is the case they cannot be recovered, and the cutting will perish for want of a due circulation of the sap. The roots will generally form in a fortnight, or, at most, three weeks. The best soil for Dahlias is a compost of equal parts of sand and loam, with a little peat; which may be enriched with part of an old hotbed, or decayed leaves. Manure of any kind should, however, be used very sparingly; as too much will cause the plant to produce strong, coarse-growing leaves and stems, instead of fine flowers. Striped flowers are never either bright or distinct in their colours in very rich soil.

Dahlias will not grow well in the richest clayey soil without sand; and though they will grow freely in sand without loam, the flowers will be poor and only semi-double. Though they flower so late in the year, Dahlias are killed by the slightest frost; and thus their beauty, great as it is, is generally rather short-lived. As soon as the leaves turn brown from frost, which is generally in October, the stems should be cut down; and in November the tubers should be taken up. A dry day should be chosen, if possible; and the tubers should be carefully taken up, and laid on boards in an open shed, or some similar place, to dry. While drying they should be turned every day, and the earth that falls from them should be swept away. They should be dried in an open shed, if possible, where they will be only sheltered from the rain; for if dried suddenly by fire-heat, or exposure to the sun, the tubers are apt to wither up; and if dried too slowly, without the admission of plenty of air, they will rot. They generally do best kept during the winter in a dry cellar, in sand or sawdust; but any dry place will do, which is not too hot.

In spring, the tubers are replanted, either in pots plunged in a slight hotbed, about the middle of February, or the beginning of March, or in the open ground in May or June; but the dwarf early-flowering kinds may be planted in the open air in April. When the tall kinds are wanted to flower early, they may be forced rapidly forward by being plunged into stronger heat, and kept in the hotbed till just ready to flower. If, however, the summer should prove hot and dry, the plants thus forced are frequently attacked by a disease called the curl, which is caused by an insect known by the name of the green bug, that perforates the young leaves, and occasions them to wither and shrivel up.

569. The operations of gardening will be found in various parts of the twin volume, The Horticulturist.