These structures are what we call plant-houses; and they are not only divided into hothouses for tender plants, and greenhouses for half-hardy plants, but subdivided into various kinds to suit the various climates in which plants are found. These climates, however, are not so numerous as might at first be supposed; as it is a curious fact in the history of plants, that many of the most ornamental grow in patches in some parts of the world without being found anywhere else, as, for example, the pelargoniums or shrubby geraniums at the Cape of Good Hope. Even when the same plant is found in different parts of the world, it is generally in the same climate, though in different countries; and thus pines and firs, oaks and birches, spread like belts or zones round the globe, from Asia, through Europe, to America, almost in the same degree of latitude, making an allowance for islands being warmer than continents, and mountains colder than valleys; as you must always remember that knowing the degree of latitude from which a plant comes is not sufficient to teach its culture, unless we know also whether it grows on the mountains or in the valleys, and whether the climate of the locality is moist or dry. A plant will be soon killed by a dry atmosphere, if it requires a moist one; and it will be as much injured by being kept too hot as too cold. Furze and heath will not grow within the tropics; and the first camellias introduced into England were killed by being kept in a hothouse.
From what I have said, you will perceive that as plants will only thrive in climates suitable to them, it is not enough to have a hothouse for tropical plants, and a greenhouse for those of moderately warm countries, but that you must have three or four houses imitating different climates, if you wish to grow different kinds of plants to perfection. Philosophers who have written on the subject describe sixteen distinct kinds of climate, including our own; but, as these would be too many to imitate, gardeners are obliged to content themselves with the following kinds of plant-houses and pits.
The dry stove is generally kept at a heat of from 70° to 84° in the day, and never allowed to fall below 65° at night, even in winter. It should be placed in a situation sheltered from cold winds, but quite open to the sun; as the plants grown in it require a strong light, most of them being natives of dry sandy plains, on which there is no tree high enough to cast a shade. The plants are grown in pots, which are generally placed on a frame or stage of wooden or stone shelves, so as to have abundance of air around them; and the stove is best heated by flues. The plants suitable for a stove of this kind are some of the kinds of Cactàceæ, such as the genera Melocáctus, Epiphýllum, and Cèreus, with the tender kinds of Euphórbium, Mesembryánthemum, Stapèlia, Crássula, Sèdum, Sempervìvum, and Agàve, and some kinds of bulbs.
The bark stove has a brick pit filled with tan or dead leaves in the centre; and it is generally heated with pipes of hot water or steam, from 60° at night to 80° in the day; the pots in which the plants are grown being either plunged in the tan, or placed on the walls of the pit, or on a stone shelf near the glass in front. Sometimes the trees of hot climates are grown in the bark bed in the centre without pots; and sometimes there is no bark pit, but the space in the centre of the house is filled with boxes containing tropical trees. This last kind is frequently called the botanic stove, as it is most common in botanic gardens; and it is best adapted for growing palms, and other monocotyledonous plants with large leaves, such as bananas, which require abundance of air and light.
The damp stove, or orchideous house, is only suited for orchideous plants and exotic ferns. The heat should be from 70° at night to 90° in the day, or even more; and the house should be heated with hot flues, on which water should be thrown twice or three times a day, and hot-water pipes with open tanks; as all the plants to be grown in this kind of house require excessive heat and constant moisture. As they are also plants that love the shade, the house should have only a subdued light; and though I cannot say that I approve of the coloured glass adopted by some cultivators, as it decomposes the rays of light, and deprives the plants of a portion of the heat they would otherwise derive from the sun, yet I would certainly advise that some of those climbing plants which will grow in a moist warm climate should be trained close under the glass, to produce shade.
Forcing-houses for grapes and early stone fruits are of the nature of the bark stove; and in them the pit in the centre is frequently filled with pine-apple plants, in pots plunged up to the rim in the tan; but these houses belong to the kitchen-garden, as do pits for growing pines, cucumbers, and melons.
You will thus observe that there are only three distinct kinds of hothouses in use in British gardens, viz. the damp stove, or orchideous house, which is the hottest; the dry stove, or house for succulent plants, which is the rarest; and the bark stove, which is the most common, and which may be said to have two varieties, viz. the botanic stove and the forcing-house.
The culture of hothouse plants in the bark stove requires more care than can be expected from any one not a regular gardener; and, as most tropical plants are valuable in this country, I would not advise you to try to manage them yourself, as you would be very much vexed if you should chance to kill them. I will, however, give you a few general hints on the subject, if you should like to have a house of this kind.
All bark-stove plants require a great deal of water when they are in a growing state, and, as it is necessary that the water should be of the same temperature as the house, there should be either an open cistern in the house, or a cistern in the shed behind, near the furnace, and communicating with the house by a pipe. The best plan is to have a cistern in the house, as it can be used as an aquarium; and there are many beautiful tropical aquatics, such as the different species of Nymphæ'a and Nelúmbium, which deserve growing for their beauty, while others are interesting for their curiosity, such as the Papyrus. In summer, bark-stove plants require very little care, except to prevent them from receiving any sudden check, as, if the heat be not kept up regularly, the plants are very liable to stop growing, and, when the heat is renewed, to shoot a second time, and thus to waste their strength in sickly and imperfect growth. Great care is also required in autumn to increase the fire heat in proportion as the weather grows cold, so as to prevent the plants from receiving any check from the decrease of temperature in the atmospheric air, as tropical plants may be said to have only two seasons, viz. summer and winter; and thus they should be kept as nearly as possible at the same heat as long as they are in a growing state; and then have a complete change to a season of rest, by never letting the heat rise higher than from 60° to 65° during the dark days of winter. During the winter months very little air can be admitted, on account of the great difference between the air of the atmosphere and that within the house; but in the spring, advantage should be taken of every warm day, even in March and April, to open the sashes a few inches wide for half an hour or an hour in the middle of the day, when the sun shines; but the house should be shut up immediately if the sun should go in and the air become chilled. As the summer advances, air should be admitted freely, and continued till the beginning of September, when it should be gradually reduced till the cold of winter prevents any being given at all. Most gardeners repot, or shift, as they call it, all their bark-stove plants about the middle or end of April; but this is too indiscriminate a practice, and, therefore, only those should be repotted that appear to want such an operation.
One of the greatest difficulties attending the culture of plants in a bark stove is, guarding against the ravages of the immense number of insects that are engendered by the heat; and one of the most troublesome of these is the active little mite called by gardeners the red spider (Àcarus telùrius). This little pest breeds in the bark, and when first hatched it is so small as to be scarcely perceptible; particularly as it is of a pale green, nearly the colour of the under side of the leaf, to which it fixes itself, and there spins a web. As it gets older it becomes of a brownish red, and having eight legs, it runs with the greatest rapidity. It is also furnished with a proboscis, with which it sucks the juices of the leaves, making them wither and shrivel up; and thus the flowers and the fruit of the trees are both spoiled, as neither can attain perfection unless the sap that is to nourish them be first properly matured in the leaves. Tobacco smoke, and most of the other usual remedies against insects, have no effect on the red spider; and, though sprinkling it with very cold water will kill it, it is difficult to apply without injuring the plants. The best remedy is allowing plenty of air to pass through the house, whenever the weather is hot enough to allow the atmospheric air to be admitted with safety.