l8 THE CULTURE OF POT-PLANTS 



winter, when they should be resting, they should not be 

 compelled to grow by means of an unnaturally high temperature 

 and the stimulation of food and water ; for as growth largely 

 depends on the chemical or actinic rays which at that season 

 are weak, any wood that is produced then must also be 

 weak. 



Of course a high temperature in winter is often maintained 

 with the object of forcing plants out of their natural season. 

 But when so treated they are rarely of much use afterwards. 

 If it is desired to keep them in good condition, they should 

 be allowed to rest when they ordinarily do so, and the 

 temperature should be low enough to let them remain 

 dormant. At the same time it should be high enough to 

 prevent their being injured by cold. The degree necessarily 

 varies with the species. In a greenhouse fitted with hot- 

 water pipes and stove which is kept burning in winter, this is 

 easily managed. It is in a cold house or in one which is only 

 heated occasionally that losses occur. 



It is a mistake to attempt to grow tropical plants in such 

 a structure, but it is a common practice among amateurs, 

 and failures are not Hkely to act as a deterrent. To minimise 

 the risks, the plants should be kept dry — quite dry during 

 severe weather — for it is not frost alone, but frost combined 

 with damp that does the damage, for which reason fresh air 

 should be admitted as often as possible. Much, too, can be 

 done by covering the glass with matting, though this is more 

 applicable to frames. In greenhouses newspapers laid over 

 the plants will protect them from several degrees of frost, 

 and those that are specially tender can be placed in boxes 

 and covered at night. 



In small greenhouses an oil lamp is sometimes used to 

 exclude frost. It may serve this purpose, but if it is not 

 perfectly clean, or is badly trimmed, or has a defective 

 draught, it is sure to do more harm than good ; for in this 

 case some of the oil escapes unburnt into the air, and, coming 

 in contact with the colder surface of the leaves, it is deposited 

 on them and causes them to wither. If a lamp produces the 

 slightest smell of oil, it should be regarded as unsafe; and 



