Propagation by Cuttings. 



207 



thirds" or "three-quarters span," according as the 

 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, or brick, ten 

 inches thick, with a two-inch air-space in the center. 



fnion' _ '-^ 



Fig. 95. Cross-section of greenhouse. (After Greiner.) 



Walls of concrete are rapidly becoming more popular 

 and will doubtless largely replace all other materials 

 for this purpose. The furnace and potting rooms ob- 

 struct the light least, and afford the most protection, 

 when located to form the wall opposite 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. Greenhouses of the better class are now 



