HEAT. 93 



independent of that of the air outside. If the external 

 heat rose above 63 Fah., the temperature of the tree 

 remained as it was before, and showed a lower degree 

 than that of the atmosphere, but if the air sank from 

 between 36 Fah. to 32, the tree remained always be- 

 tween 52 and 74 : but he especially remarked that 

 the warmth of the tree sank during rain. Numerous 

 other experiments, which were made upon this subject, 

 left no doubt that the temperature of the interior of a 

 living tree during low degrees of heat of the external 

 air, was always higher than that of the latter, but was, 

 on the other hand, always lower when the temperature 

 of the surrounding air was very high ; the difficulty, 

 however, was to explain this. The subject was much 

 canvassed, and Nau proceeded to examine the ques- 

 tion again, and came to the conclusion that there was 

 really an internal warmth of plants, but that it had its 

 origin only from the inability of the air within the 

 plant to change its temperature synchronously with 

 that of the external air, and which, according to cir- 

 cumstances, was sometimes warmer, and at others 

 colder than that of the plant. 



86. Of the power of plants to generate particular de- 

 grees of heat, there can be now not the least doubt, and 

 that this generation of heat depends upon the perform- 

 ance of certain phenomena taking place within the 

 plants themselves, is equally as evident, and receives its 

 support from the following facts now to be mentioned. 



87. In 1777 Lamark noticed the evolution of heat in 

 a strong- smelling flower, the Arum italicum. Sene- 

 bier afterwards described it more particularly in another 

 plant of the same genus, and showed that the evolution 

 of heat began in the flower so soon as the spatha ex- 

 panded, that the evolution of heat was noticeable about 

 3 or 4 P.M. daily, and that its maximum occurred be- 

 tween 6 and 8 o'clock ; but that the maximum of the 

 external air was not coexistent with that of the plant, 

 and that the difference between the two maximi was 

 7 degrees in favour of the plant. 



