Sometimes greenhouse plants which are left in the open ground are preserved from the frost by coverings of wicker-work like beehives being put over them, or tin hoops over which mats have been stretched; or, where the plants are small, a flower-pot may be turned over them, or a hand-glass used for the same purpose. It is seldom, however, worth while to take much pains to preserve greenhouse plants that have flowered in the open air. The ordinary way is to make abundance of cuttings in autumn; to strike them in a hotbed, and then, after hardening them by degrees, to preserve them in a small greenhouse, or in a cold pit, till the time for planting out next year. LETTER VIII. USE OF PLANT-HOUSES.--NATURE OF CLIMATES--DIFFERENT KINDS OF HOTHOUSES.--THE DRY STOVE, THE BARK STOVE, AND THE ORCHIDEOUS HOUSE.--CULTURE OF PLANTS IN THE BARK STOVE.--AQUARIUM AND WATER PLANTS.--RED SPIDER.--CULTURE OF SUCCULENT STOVE PLANTS.--CULTURE OF ORCHIDEOUS PLANTS.--THE GREENHOUSE.--THE AUSTRALIAN HOUSE, AND CULTURE OF ITS PLANTS.--THE COMMON GREENHOUSE, THE HEATH HOUSE, THE CONSERVATORY, THE ORANGERY, AND THE CAMELLIA HOUSE.--THE CULTURE OF PLANTS IN THE COMMON GREEN HOUSE.--POTTING PLANTS.--HEATHS.--CULTURE OF PLANTS IN THE CONSERVATORY.--CULTURE OF ORANGE TREES.--APHIDES. Before I say any thing of the management of the plants in your greenhouse, I must remind you that, in order to grow plants well, it is not enough merely to preserve them from the frost, but we must imitate as well as we can their native climate: that is, the degree of heat, light, and moisture they have been accustomed to in their native country, together with the air and the soil. The latter is the easiest condition to fulfil; as, by combining different kinds of earth, we can, without much difficulty, produce a very tolerable imitation of any soil we please: but it is not so easy to give plants abundance of light and air in combination with heat; and, though we can readily give plants plenty of water, it requires some management to surround them with a moist warm atmosphere, like that they have been used to in their native woods. To meet these difficulties, buildings have been constructed, suited for the reception of plants, with various contrivances for producing heat and distributing air and moisture, and with a glass roof, front, and sides, to admit abundance of light.