206 Principles of Plant Culture. 



Heat is economized by placing the fermenting maL' 

 rial in a pit in the ground, but hotbeds are often made 

 above ground. The hotbed pit should be in a well- 

 drained and sheltered place, and two to two and one- 

 half feet deep. In this the heating material .should be 

 moderately packed, until the pit is nearly or quite full. 

 The frame may then be placed over the pit, after which 

 the heating material should be covered with soil and 

 the sash put on to confine the warmth. Within a few 

 days after covering with the sash, the fermenting mate- 

 rial usually generates a rather violent heat, which 

 should be permitted to decline to about 90° F. before 

 planting seeds or cuttings in the hotbed. The same 

 protection against excessive heat or cold is used as for 

 the cold-frame; but the hotbed requires much 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. 



366. The Greenhouse is an expansion of the hotbed, 

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

 tered, 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- 



♦ Hotbeds are now being heated by Are to some extent. 



