90 VITAL ACTIONS. 



where winter is unknown, the requisite periodicity of 

 3timulus and rest is provided for by what are called 

 the dry and the rainy seasons ; the former being equi- 

 valent to the winter, the latter to the summer, of north- 

 ern latitudes. 



115. As plants have little power of generating heat, 

 like animals, except in particular cases, and very lo- 

 cally,* they are principally dependent upon the media 

 that surround them for the heat which they require. 

 Considering the great importance of heat in their 

 economy, it is, for the purposes of gardening, a most 

 necessary object to ascertain what proportion is usually 

 borne to each other, in different countries, by the tem- 

 peratures of the earth and atmosphere, the chief media 

 by which plants can be affected. Upon the tempera- 

 ture of the atmosphere there are numerous observa- 

 tions in many countries ; upon that of the earth, so few 

 as to afford no sufficient data for the solution of this 

 problem. It is usually considered that the tempera- 

 ture of springs affiards sufficient evidence of the tempe- 

 rature of the earth ; but, so far as vegetation is con- 

 cerned, this evidence is unsatisfactory. Springs, de- 

 riving their origin from considerable depths, have a 

 nearly uniform temperature all the year round : but 

 the temperature 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. 



* Allusion is here, of course, made to the extrication of heat dur- 

 ing the periods of flowering and germination, phenomena which 

 have no obvious connexion with cultivation. 



