196 



Principles of Plant Culture. 



starts vigorously, as is indicated by vapor rising from 

 the heap, and the outer part of the mass should be placed 

 in the center of the new pile. Leaves ferment slower 

 than the other materials above named, and hence may 

 often be advantageously mixed with them to lengthen 

 the period of fermentation. 



Heat is economized by placing the fermenting material 

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



ground. The hot- 

 bed pit should be 

 in a well-drain- 

 ed and sheltered 

 place, and two 

 to two and one- 

 half feet deep. 

 In this the heat- 



FIG. 94. Cross- section of hotbed in pit. The frame ing material 

 is banked up a little with earth. (After Greiner). g^ould ^6 mO( j_ 



erately 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 011 to confine the warmth. Within a few days after 

 covering with the sash, the fermenting material usually 

 generates a rather violent heat, which should be per- 

 mitted 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. 



