202 PRINCIPLES OF PLANT CULTURE 



a " two-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 (240). The walls may be of wood, brick or 

 concrete. Concrete 'is rapidly becoming more popular 

 as wall material and will doubtless largely replace all 

 other materials for this purpose. The furnace and pot- 

 ting rooms obstruct 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 greenhouses. Heating devices for the 

 greenhouse are of various kinds. Greenhouses of the 

 better class are now almost invariably heated with steam 

 or hot water, or with a combination of the two. Pipes 

 from a boiler located beneath the floor level extend nearly 

 horizontally about the house, below the benches, return- 

 ing to the boiler ; or the main feed pipe extends overhead 

 to the farther end of the house, where it connects with a 

 system of return pipes beneath the benches. Where the 

 pipes need to make many turns, steam is usually more 

 satisfactory than hot water. 



368. The propagating-bed. A certain part of the 

 greenhouse is usually set apart for propagating plants 

 from cuttings. The propagating-bed is made upon the 

 ordinary greenhouse bench, directly over heating pipes. 

 To furnish the bottom heat (362), the space beneath the 



