GREENHOUSE. In America the term greenhouse is loosely applied to all glass struc tures, except cold frames and hot beds, in which plants are grown. In Great Britain it is almost limited to those buildings in which living plants that do not demand high temper ature are stored or grown. Etymologically and as originally used the word included all struc tures, even storage `pits,* in which plants were placed to be kept alive or protected during winter without the intention that they should grow.
The modern greenhouse is the product of a long evolution from two sources. It appears to have started on the one hand as a window garden in the human residence, the first step toward the greenhouse as now known being an enlargement of the glass area. Next came the extension of the window into an oriel or a °bay.° When the glass roof was added the greenhouse may be said to have fairly begun its pedigree. The other line of descent traces its course back to the European practice of growing fruit trees and vines against brick or stone walls, partly as a protection against incle ment weather and partly because such walls absorb sun heat and thus help to hasten or insure the maturity or higher quality of the fruit. As far back as the 16th century this was a popular practice upon the estates of the wealthy. In 1699 Nicholas Facio Duilhier, a mathematician, wrote a book entitled (Fruit Walls Improved' in which he contended that by tilting the walls at right angles to the sun the heat would be increased. In 1724, however, Stephen Switzer proved and wrote that such walls gave no better results than ordinary perpendicular ones. Over some of the walls at Belvoir Castle glass sash were placed in front of the walls and over the vines as extra protection. Heating flues were built behind the walls to supply additional warmth. The success of this experiment led Switzer to build walls with glass fronts three and one-half feet away as described and pictured in 'The Prac tical Fruit Gardener' (1731). Switzer's claim that the introduction of these structures led to improvement of glassing and forcing grapes is more than supported by Johnson, who writes ((History of Gardening' ) that the use of such walls °led to the first erection of a regular forcing structure of which we have an ac count.° From the covered wall the first step toward a °greenhouse° was the ulean-to' and from that by many gradations to the modern forms.
In America improvements in greenhouse construction began to appear about 1800. This was mainly due to the necessity of better caring for the plants imported from foreign countries by John Bartram and his followers, to the improvements in heating appliances and to the general betterment of architecture. However, in the light of the present these improvements are crude, cumbersome and even ludicrous; for instance, the device described and illustrated by Loudon (Treatise on Several Improvements Recently Made in Hot Houses,' 1805), whereby fresh air was to be forced into the greenhouse by means of a bellows! The principal types of greenhouses may be described according to their function as fol lows: The conservatory, in which plants that have been grown elsewhere (generally in other greenhouses) are brought for display when they have reached perfection; the forcing house in which plants, especially certain vegetables are grown out of their normal season; the warm house, or as known in England the °stove house,' in which tropical and other heat-loving plants are grown; and the propagating-house, in which plants are grown from seeds, cuttings, layers, grafts, etc. The term °stovehouse° is derived from the original practise of heating such a structure by means of a brick stove. Hence it applies to the warmest part of the greenhouse. In America the term warmhouse is in general use though among foreign trained gardeners the term °stove° used alone is frequently heard.
Greenhouses may also be clissified accord ing to their forms of which the following are the most popular: The lean-to, which consists of one low and one high wall with one slope of glass connecting them; the even-span, which consists of walls of even height and two roofs of equal le. aszlii and slope; uneven span, in which the walls may be of unequal height but one of the roofs of both different length and different pitch from the other; the curvilinear, in which the walls are of equal height, the roofs of even pitch but curved or less. The first three are all in ial structures, the last, because expensive to erect and main tain, only for the pleasure oI the owners.