102 APPLICATION OF PEINCIPLES. 



CHAPTBE I. 



OF BOTTOM HEAT. 



This term is, in common practice, made use of only 

 in those cases where the temperature of the soil in 

 which plants grow is artificially raised considerably 

 above that which we are acquainted with in England ; 

 and there seems to be a general idea that such an artifi- 

 cial elevation of temperature is only necessary in a few 

 special instances. It has, however, been shown (116), 

 that the mean temperature of that part of the soil in 

 which plants grow is universally somethinghigherthan 

 that of the air by which they are surrounded, and con- 

 sequently it appears that nature, in all cases, employs 

 some degree of bottom heat as a stimulus and protec- 

 tion* to vegetation. At the same time, it must be ad- 



* That the warmth of the soil acts as a protection to plants 

 may be easily understood. A plant is penetrated in all direc- 

 tions by innumerable microscopic air passages and chambers, so 

 that there is a free communication between its extremities. It 

 may therefore be conceived that if, as necessarily happens, the 

 air inside the plant is in motion, the effect of warming the air in 

 the roots will be to raise the internal temperature of the whole in- 

 diyidual ; and the same is true of its fluids. Now, when the tem- 

 perature of the soil is raised to 150° at noonday by the force of the 

 solar rays, it will retain a considerable part of that warmth 

 during the night ; but the temperature of the air may fall to such 

 a degree that the excitability of a plant would be too much and 

 suddenly impaired, if it acquired the coldness of the medium 

 surrounding it; this is prevented, we may suppose, by the 



