INCREASE OF HEAT IN THE SOIL. 159 



the effect upon vegetation of heat in the soil : " It 

 has been proved, both by observation and experiment, 

 that heat acts locally upon the organs of plants 

 much more energetically than by transmission from 

 one part to another. The best proof of this is 

 afforded in the fact, that if the branch of a tree that 

 is growing in the open air be protected, or intro- 

 duced into a pit for exotic plants, its leaves will be 

 developed before those upon the other branches. 

 So, also, in the polar regions, the warmth of the air 

 causes the plant to leaf and flower while the ground 

 is still extremely cold. Nevertheless, since all 

 plants pump up their sap from the ground, the 

 temperature of the latter is an important element. 

 During great heat, it is cooler than the air ; during 

 great cold, it is warmer; and the more variable a 

 climate is, the greater the importance to the gar- 

 dener of this phenomenon. The heating or cooling 

 effect of the soil will be most sensibly felt in those 

 organs which are nearest the root, in those to which 

 the sap flows rapidly and copiously, and in those 

 which are bad conductors of heat. Thus the tem- 

 perature of the interior of the cocoa-nut differs little 

 from that of the earth around its roots, because of 

 the thickness of the husk, which conducts heat 

 badly ; whilst, on the other hand, the young shoots 

 and flowers of trees acquire the temperature of the 

 surrounding air. Plants whose roots penetrate the 

 earth deeply will resist vicissitudes in the surround- 



