190 Principles of Plant Culture. 
same protections against excessive heat or cold are used as 
for the cold-frame; but the hotbed requires much the more 
care in ventilation, since the heating material generates 
vapor and carbonic acid, as well as heat, and these, when 
present in excess, are detrimental to plant growth. 
266. The Greenhouse is an expansion of the hotbed, 
i. e., a structure sufficiently large so that it may be entered, 
and with arrangements 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. 94); 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-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 
Fic. 94. Cross section of greenhouse. (After Grenier). 
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 plac- 
ing a certain number of movable sash in the roof or else- 
* Hotbeds are now being heated by fire to some extent, 
houses have | 
