108 APPLICATION OF PEINCIPLES. 



flower unless in a high bottom heat ; and that Palm 

 trees, planted in the soil of conservatories which it is 

 impracticable to heat sufficiently, soon become un 

 healthy. 



The reason why it is necessary to plants in a grow- 

 ing state, that the mean temperature of the earth 

 should be higher than that of the air, is sufficiently 

 obvious. Warmth acts as a stimulus to the vital 

 forces (17), and its operation is in proportion to its 

 amount, within certain limits. If, then, the branches 

 and leaves of a plant are stimulated by warmth to a 

 greater degree than the roots, they will consume the 

 sap of the stem faster than the roots can renew it ; 

 and, therefore, nature takes care to provide against 

 this by giving to the roots a medium permanently 

 more stimulating, that is, warmer, than to the branches 

 and leaves. 



Such being the fact, it is obvious that one of the 

 first of a gardener's cares should be, to secure the 

 means of insuring a proper temperature to the soil in 

 which he grows his plants, and that this is requisite 

 for hardy as well as tender species ; and I entertain 

 little doubt that the time is at hand when it will be 

 considered quite as necessary to furnish heat for the 

 soil as for the air ; not, however, heat without moi» 

 ture, for that would evidently produce much greater 

 evils than it was intended to cure, as has indeed been 

 found by inconsiderate experimenters. I quite agree 

 with Mr. Writgen in believing that it is the tempera- 

 ture and moisture of a soil, much more than its mine- 

 ralogical quality, that determine its influence upou 



