CHAPTBK, I. 



OP 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 ahove that which we are ac- 

 quainted with in England; and there seems to be a general 

 idea that such an artificial elevation of temperature is only 

 necessary in a few special instances. It has, however, been 

 shown (p. 135) that the mean temperature of that part of the 

 soil in which plants grow is universally something higher than 

 that of the air by which they are surrounded, and consequently 

 it appears that nature, in all cases, employs some degree of 

 bottom heat as a stimulus and protection to vegetation. At the 

 same time, it must be admitted that, in some cases, the amount 

 is extremely small ; for Von Baer found Ranunculus nivalis and 

 Oxyria reniformis flowering in Nova Zembla, where the soil 

 was not warmed above 34^" ; and, in Jakutzsk, Erdmann states 

 that Summer Wheat, Rye, Cabbages, Turnips, Radishes, and 

 Potatoes are cultivated, although the ground is not thawed 

 above three feet in depth. 



How tie warmth of the soil may act as a protection to plants will be 

 easily understood. A plant is penetrated in aU directions by in- 

 numerable air passages and chambers, so that there is a free commxmi- 

 cation between its extremities however far they may be apart. 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 individual ; and the same 

 is true of its fluids. Now, when the temperature of the soil is raised to 

 150° at noonday by the force of the solar rays, it will retain a con- 

 siderable part of that warmth during the night : but the temperature 



