542. Hothouses differ from green-houses, chiefly in requiring more heat, as they are intended for the growth of tropical plants; whereas green-houses are intended for the plants of climates only a little warmer than our own. Hence the temperature of a hothouse should be several degrees higher than that of a green-house; the lowest heat being from 55° to 60° even at night, and rising in the middle of the day, partly by sun heat, to 80° or 90°, or higher during summer. The great difference between the heat of the night and the day, is one of the late improvements in horticulture. Formerly gardeners kept their stoves at nearly the same heat night and day, though it was clearly different from the natural habits of the plants, as the nights in tropical climates are well known to be very much colder than the days. There are many varieties of hothouses, but the most useful are the stove, the orchideous-house, and the aquarium.

543. The moist stove has generally a lean-to roof, as shown in fig. 376.; and those which were built some years ago had always a brick pit for tan or earth in the centre of the house. Into this pit the pots were generally plunged; but sometimes the bed was covered with a slate on which the pots were set. The pots were also occasionally set on - brick flues. Now, when there is a pit in the centre, it is generally covered with brick, and either contains tanks of hot water, or hot water pipes, or flues. Sometimes it is a hot air-chamber, furnished with gratings that can be closed by slides, so that hot air can be admitted into the house at pleasure; observing that whenever hot air is admitted into a plant house, by the Polmaise, or any other mode of heating, there must be open tanks or cisterns introduced to give moisture to the air. There should also always be two or three ventilators, or more, as near the roof as possible, to let off the air that has passed through the house; and if these ventilators are only of moderate size, there is no danger of any cold air entering by them; as the volume of hot air which is continually rising to the roof, will either be sufficient to repel the cold air, or at least it will mix with it, and warm it sufficiently to prevent any danger resulting to the plants.

544. The dry Hove only differs from the moist stove in having leas moisture, as it is intended for the growth of the cactacae, and other succulent plants. In some cases there is a kind of stage for the pots, like a green-house; but generally they are kept on shelves and flues.