IMPORTANCE OF TEMPERATURE. 117 



A very extensive and valuable series of temperatures, observed aU 

 over the world, has been compiled by Mr. Thompson, and published in 

 the Journal of the Horticultural Society, Vol. IV., to which the reader 

 is referred. 



As plants have little power of generating heat, like animals, 

 except in particular cases, and very locally,* they are princi- 

 pally dependent upon the media that surround them for the 

 heat which they require. Consideriug the great importance of 

 heat in their economy, it is, for the purposes of gardening, 

 necessary to ascertain what proportion is usually borne to each 

 other, in different countries, by the temperatures of th« earth 

 and atmosphere, the chief media by which plants can be affected. 

 Upon the temperature of the atmosphere there are numerous 

 observations in many countries ; upon that of the earth much 

 fewer. It has been considered that the temperature of springs 

 affords sufficient evidence of the temperature of the, earth ; but, 

 so far as vegetation is concerned, tliis evidence. is .unsatisfactory. 

 Springs, deriving their origin from considerable depths, have a 

 nearly uniform temperature all the year round : but the tempe- 

 rature of the earth's surface varies with the seasons ; is 

 extremely different in summer and winter ; ' and ' is affected by 

 the quality of the soil, in proportion as that is more or less 

 absorbent and retentive of heat. What we want to know, as 

 respects vegetation, is, not the mean temperature of the earth 

 at some distance from its surface, but the temperature imme- 

 diately below the surface ; i. e. of that part of the soil which 

 the roots of plants penetrate, and whence they derive their 

 food. It is also requisite that this should be ascertained 

 monthly, so as to fui'nish the means of comparing the terres- 

 trial temperature with the periodical state of vegetation. Such 

 being the case, the temperature indicated by springs will be too 

 high in winter, and too low in summer, a most material error. 



I am indebted to Mr. Robert Thompson for the following highly 

 important tables indicating the ascertained temperature of the earth in 

 various parts of the world. 



* AllusioD is here, of course, made to the extrication of leat during the periods of 

 flowering and gennination, phenomena which have no otvions connexion with cultiva- 

 tion. 



