22 PEINCIPLES OF GARDENING. [CH. I. 



doubt, for no frost -would injure the sets if a little 

 coal ashes were put over them in each hole, for coal 

 I ashes are an excellent non-conductor of heat, and 

 1 consequently opposed to the admission of cold, and 

 / are, at the same time, a good preservative from ex- 

 ' cessive moisture. 



The fact that the earth, in regions not eternally 

 ice-boimd, never is reduced in temperature, at a few 

 inches from the surface, so low as the exterior air in 

 winter, nor is elevated at a similar depth to an equal 

 degree of warmth in summer, suggests the necessity 

 for more attention to the temperature of the soil 

 in our horticultural houses than it has hitherto ob- 

 tained. 



Attention is more awakened to it now than for- 

 merly, and by hottom-heat our gardeners now intend 

 something more than a mass of fermenting matter 

 for forcing cucumbers or pine-apples. 



It is quite certain, that eveiy plant, when growing 

 in a favourite soil in its native climate, has its roots 

 growing in the temperatui'e which is best accordant 

 with that in which its branches are delighting. Under 

 no circumstances, if the plant is flourishing, will the 

 temperature in summer, at 12 inches from the sur- 

 face, be found to be less than 2"", nor more than 5° 

 loiver than the average temperature of the atmo- 

 sphere ; and in winter, that temperature at the same 



