THE PROPAGATION OF PLANTS 



201 



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 ven- 

 tilation, since the heating material generates vapor and 

 carbonic acid, as well as heat, and these when present 

 in excess are detrimental to plant growth. 



FIG. 98. Cross-section of greenhouse. 



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. They are built 

 in various sizes depending largely upon the use to which 

 they are to be put. Ordinarily they extend north and 

 south. 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 



