OF POTTING. 273 



a dry air in motion, unless in those rare cases where 

 the air is kept by artificial means shaded, and uni- 

 formly damp. By these means, in a dry summer daj'', 

 when the leaves are perspiring freely, and therefore 

 requiring an abundance of water from the roots, the 

 latter are placed in contact with a substance whose 

 moisture is continually diminishing ; or in a green- 

 house where the pots are syringed, the heat of the 

 earth in contact with the roots is lowered by a copious 

 evaporation from the sides of the pot, just when, in 

 nature, the bottom heat should be the greatest. The 

 evil consequences of this are well known to gardeners, 

 who however seldom take any sufficient precautions 

 to prevent it. Greenhouse plants exposed to the 

 open air in summer always suffer severely from the 

 irregular condition of the sides of the pots ; whence 

 the common practice of plunging them in the earth, 

 for the purpose of bringing them into the condition 

 of plants growing in the open ground. 



This is, however, attended with some disadvan- 

 tage ; for the plants root, through the bottom of the 

 pots or over the edges, among the earth in which 

 they are plunged ; and, when taken up in the autumn 

 for removal into the greenhouse, they must have all 

 such roots cut off again ; for there are no means of 

 bringing them within the limits of a pot. For these 

 and similar reasons, no good gardener will expose his 

 greenhouse plants to the open air in summer, if he can 

 help it, unless they are duplicates, or unless there is 

 some object to be attained very different from the 

 strange notion that ihey are hardened by this pro- 

 12* 



