The culture of the plants contained in the dry stove requires considerable care. The Cáctus family may be arranged in three groups; first, the Tree Cacti, which are included in the genus Cèreus, and have long slender stems without a single leaf, sometimes thirty or forty feet high, and not thicker than a man's arm. These plants grow on the summits of the mountains in Brazil, and only require greenhouse heat in England. Secondly, the Mammillàriæ and Echinocácti, which grow in the valleys of the temperate parts of South America, and should be kept in a warm greenhouse in this country; and thirdly, the Melocácti, the Epiphýllum, the Opúntiæ, and the Rhípsalis, which grow in the tropics, and require a dry stove in England. These plants should be grown in pots well drained with cinders, and they should be kept almost without water from October till March, and then watered profusely till they are about flowering, when the quantity of water given to them should be gradually diminished. Some cultivators keep a few of their plants in a bark stove, and plunge the pots in the tan, and they are said to thrive on this treatment, if carefully managed; but as it requires a great deal of care to prevent them from damping off, the ordinary way is the safest for an inexperienced gardener. Mesembryanthemums, which are also kept in a dry stove, require the same treatment as the Cácti.
The orchidaceous plants grown in a damp stove are all epiphytes, which, in their natural state, grow either on the branches of trees, or on exposed rocks. The former of these are found in their wild state with their roots hanging down in the air, and growing in dense forests, where shade, moisture, and excessive heat are essential to their existence. Most of these plants, in a state of culture, are grown in the husks of cocoa-nuts, half filled with moss, from which the roots hang down, or they are tied with wire to pieces of wood hung from the rafters. The wood generally preferred for this purpose in England is the robinia, or false acacia, on account of the roughness of its bark, and the softness of its wood: and moss is generally put between the epiphyte and log so as to make it quite compact. Some genera of orchideous plants, such as Dendròbium, Epidéndrum, and Cattlèya, are always grown in pots, which are filled with turfy peat, chopped moss, and lime rubbish. Others, such as Stanhòpea and Catasètum, should be grown in baskets half filled with moss, or in a curious kind of frame, made of pieces of turf fastened between four upright pegs of wood; as the flowers of these plants come from their roots, and they must be allowed abundance of room to enable them to protrude properly. The baskets or frames for the Stanhòpea and other root-flowering plants should be from three to six inches deep, and from six to ten inches wide; and the frames should be filled with strips of turf, two or three inches wide, piled up on one another so as to fill the frame, and yet leave a sufficient space between to admit the passage of the flower stems which protrude downwards from the root. When orchideous plants are grown in pots, the pots are drained with cinders, and then filled with chopped turf mixed with lime rubbish to keep it open. The exotic ferns grown in the same house require no particular care, except that of potting them so as to insure perfect drainage, and keeping them frequently syringed overhead.
There may be said to be five kinds of greenhouse; viz., the Australian house, the common greenhouse, the heath house, the conservatory, and the orangery, to which is sometimes added, the camellia house, though these plants are generally kept in the conservatory.
The Australian house contains all the curious New Holland plants, such as Bánksia, Dryándra, Grevíllea, Melaleùca, Callistèmon, Metrosidèros, and various genera belonging to the Leguminòsæ, together with the Cape plants belonging to the Proteàceæ and Compósitæ. These plants require a considerable degree of heat, and also as much air and light as can be given to any plants which require shelter from the open air. On this account they are generally grown in curvilinear houses, that is, houses that have glass on all sides, like that in the garden of the London Horticultural Society at Turnham Green. All the plants contained in this house are extremely difficult to grow, and they require the greatest care in watering, so that they may never have too much, and yet never be suffered to become too dry. Houses of this kind are generally heated by pipes of hot water, and kept at a temperature of from 40° to 50° or 60°. There is no pit in the centre, and the plants are either planted in the ground as in a conservatory, or kept on a stage formed of wooden shelves.
The common greenhouse has a brick wall at the back, with a glass roof at an angle of about 45°, and upright glass at the front and sides. The plants are grown in pots placed on a stage or range of wooden shelves rising one above another, with a path all round, and a shelf for the plants under the glass in front. All the sashes are made to open, as it is essential that there should be a free circulation of air; and so little fire is necessary, that one fireplace will be sufficient to heat a greenhouse from thirty to forty feet long, and from twelve to fifteen feet wide. The house may be heated either by hot-water pipes or by flues; in the latter case the flue should go round at the front and return at the back, being about twenty inches high and twelve inches wide. The heat of a greenhouse of this nature need never be more than 50° in the day, and it may be allowed to sink even as low as 35° at night, the object being merely to keep out the frost.
The heath house, like the Australian house, requires to be as transparent, and as thoroughly ventilated, as possible. A heathery, however, is generally a span-roofed house with a walk down the centre, and shelves for the plants on each side. It is usually heated by hot water, the pipes for conveying which are placed in the centre of the house. The width of the house should be about ten feet, and the height in the centre of the span should never exceed nine, as it is of great importance to have the plants as near the glass as possible. The floor of the house should stand one foot above the level of the ground: and, where expense is not an object, the house is sometimes built on arches to insure perfect dryness. All the windows are made to move in every possible direction so as to admit of a current of air through the house, whenever the state of the atmosphere will permit it.
The conservatory has all the plants growing in the soil, instead of being in pots and placed on shelves. It is generally more lofty and architectural than a greenhouse, and of much greater extent; and it is frequently attached to the house, being so contrived that it can be entered from one of the living-rooms. The temperature of the conservatory is generally rather warmer than that of the common greenhouse, as it is kept at 60° during the day, and seldom allowed to fall lower than 40° at night.
The orangery is a still more architectural-looking building than the conservatory, and it has an opaque roof. It is used only for preserving orange trees and other plants that remain in a dormant state during the winter, and it requires no more heat than is absolutely essential to keep out the frost.
The camellia house is generally formed like a common greenhouse, but so contrived as to allow the whole of the sashes to be taken off during the summer. The plants are kept on stone shelves, raised one above another; and there is generally no walk at the back.
Of the culture of the plants in the Australian house I have already spoken; and of those in the greenhouse I need only detail the management during the winter months, as your greenhouse will, I suppose, be in the reserve-ground, and will be only used to preserve those plants during winter which you have kept in other more conspicuous situations during the summer.