OF BOTTOM HEAT. 105 



our practice of exposing it during summer to the open 

 air, in tubs, where the soil scarcely rises in tempera- 

 ture above 66°, and preserving it during winter in 

 cold conservatories, the soil of which often sinks to 

 36° ; under such circumstances the Orange exists in- 

 deed, but where are the perfume and juiciness of its 

 fruit, and where the healthy vigour of its noble foliage? 

 The Vine cannot be grown in the open air of this 

 country to any useful purpose, except when trained 

 to walls, in soils and situations unusually exposed to 

 the beams of the sun ; it is only then that it can ob- 

 tain for its roots such a permanent warmth of 75°, 

 which it will have at Bordeaux, or 80° in Madeira. 



It may hence be considered an axiom in horticul- 

 ture, that oilplants require the soil, as well as the at- 

 mosphere, in which they grow, to correspond in 

 ternperature with that of the countries of which they 

 are natives. It has also been already shown, that the 

 mean temperature of the soil should be a degree or two 

 above that of the atmosphere (119). 



This explains why it is that hardy trees, over whose 

 roots earth has been heaped or paving laid, are found 

 to suffer so much, or even to die ; in such cases, the 

 earth in which the roots are growing is constantly 

 much colder than the atmosphere, instead of warmer. 

 We have here,* also, the cause of the common circum- 



* Mr. Knight long eince mentioned an important fact con- 

 nected with this subject: — "It is well known," he said, "that 

 the bark of Oak trees is usually stripped ofli in the spring, and 

 that in the same season the bark of other trees may be easily 

 detached from their alburnum, or sap-wood, from which it is, at 



5* 



