206 



Principles of Plant Culture. 



longer slope covers two-thirds or three-quarters of the 

 width of the house. The long slope usually faces the 

 south, but houses have recently been built with the 

 shorter and steeper slope facing the south, a plan 

 thought to possess advantages for growing certain 

 plants, as carnations. 



Provision is made for ventilation in glass houses by 

 placing a certain number of movable sash in the roof or 

 elsewhere. In order that the glass may not be far above 

 the plants, the side walls should not exceed five feet in 

 height (240). These may be of wood, but a wall of 

 brick, ten inches thick, with a two-inch air space in the 



FIG. 95. Cross-section of greenhouse. (After Grelner.) 



center, is preferable, since this better economizes heat. 

 The furnace and potting rooms obstruct the light least, 

 and afford the most protection, when located to form 

 the wall opposite to the sun. In houses extending north 

 and south, the south end is usually glazed above the 

 height of the side walls. 



367. Heating Devices for the Greenhouse are of 

 various kinds. The "smoke flue" is simplest and cheap- 

 est in first cost. It consists of a flue extending from the 

 furnace, which is placed somewhat below the floor level, 

 lengthwise through the house, preferably rising gradu- 

 ally to a chimney at the opposite end; or the flue may 



