FORCING AND RETARDING PLANTS 159 



which are grown elsewhere. Plant houses have glass roofs 

 and sides to admit as much light as possible during the short 

 days of winter, and are heated largely by the sun's rays, at 

 least by day. The hotbed has been described as " a trap to 

 catch sunbeams," and the hothouse is merely a larger trap. 

 The heat rays coming from the sun pass through the glass 

 easily, but when reflected back from the soil the glass pre- 

 vents their escape. The interior therefore warms up rapidly 

 when the sun shines. On bright days the heat may be so 

 greatly increased as to injure or kill the plants if the house 

 is not ventilated. At night and on very cold days the warmth 

 is maintained by some sort of artificial heating system. The 

 first hothouses were kept warm by quantities of fermenting 

 manure placed in pits beneath the benches upon which the 

 plants were grown, and such means may still be depended 

 upon to keep the frost out of small houses during the winter. 

 In general, however, some sort of hot-water heating system 

 is used. 



Hotbeds. Hotbeds are really small plant houses kept warm 

 by fermenting manure and the sun's heat, and are used for 

 growing plants before the weather will permit of their being 

 grown in the open. Early crops of lettuce, radishes, and other 

 vegetables are raised in this way, and long-season crops, such 

 as tomatoes, peppers, and eggplants, are started here and 

 carried along until transplanting time. To make a hotbed, a 

 pit should be dug 2 feet deep and about 1 foot wider than 

 the frame that is to be placed over it. This pit is to be filled 

 with manure to supply the heat, and should contain a good 

 share of straw or other material in order that the heat may 

 be long continued. The manure should be placed in a pile 

 and forked over at intervals for several days before using, to 

 insure that the fermenting material has been thoroughly mixed 

 through it. When the whole pile is steaming it should be 

 placed in the pit in layers about 6 inches deep, and each layer 



