Propagation by Cuttings. 



197 



366. The Greenhouse is an expansion of the hotbed, 

 i. e., a structure sufficiently large so that it may be 

 entered, and arranged for heating by fire.* In temperate 

 climates, greenhouses are usually constructed 12 to 22 

 feet wide, with a gable or M roof, having a slope of 35 

 to 40, covered with glass and with the ridge or ridges 

 extending north and south (Fig. 95); but in very cold 

 climates a shed roof facing the south is preferable. 

 Greenhouses are often built with one slope of the roof 

 longer and less steep than the other, and with the ridge 

 extending east 

 and west. Such 

 a roof is called 

 a "two- thirds" 

 or "three-quar- 

 ters span, ' ' ac- 



cording as the 



lono-er Slope FlG ' 95 * Cross - section of greenhouse. (After Greiner). 



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 advan- 

 tages 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 (241). These may be of wood, but a wall of brick, 

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

 is preferable, since this better economizes heat. The 

 furnace and potting rooms obstruct the light least, and 



* Hotbeds are now being heated by fire to some extent. 



