DEVELOPMENT OF HEAT BY PLANTS. 787 



of heat also, in the process of flowering, has been alluded to 

 (p. 747). The rise of temperature which thus occurs in the 

 processes of germination and flowering, is due, without doubt, 

 essentially, to the production of carbonic acid. We have still 

 to inquire, whether the ordinary vital actions which are going on 

 in plants are calculated to raise or diminish their temperature. 



The experiments of Hunter, Schoepf, Bierkander, Maurice, 

 Pictet, and more especially of Schubler, lead to the conclusion, 

 that the trees of our climate with thick trunks exhibit a variable 

 internal temperature, being higher in the winter and at sunrise, 

 than the surrounding atmosphere, — that is, at periods of great 

 cold, or of moderate temperature ; and lower in the summer 

 or at mid-day, — that is, at periods of great heat. In no 

 observed cases were such trees noticed to possess exactly the 

 temperature of the atmosphere around them. The experiments 

 of Eeaumur on trees with slender trunks exposed directly to 

 the sun's rays, showed a considerable increase of temperature in 

 them over the external air. These experiments of Reaumur 

 are, however, by no means satisfactory. 



The temperature of trees under the above conditions depends 

 upon various causes, such as the sun's rays, the amount of eva- 

 poration, chemical changes which take place during assimilation, 

 &c., the conducting powers of the wood, and particularly upon 

 the temperature of the soil in which the plants are grown. In 

 the active periods of the growth of plants, when evaporation is 

 constantly going on, and the fixation of carbon taking place, 

 both of which processes are accompanied by a diminution of 

 heat, it is evident, that such changes must have some effect 

 in modifying the temperature, and hence if, at such periods, 

 their temperature be above that of the surrounding air, that it 

 is due to external influences, such as the sun's rays, and the 

 temperature of the soil, &c. This probably explains, to some 

 extent at least, why the temperature of thick trees exposed to 

 great heat, is lower than that of the surrounding air, for at such 

 a period vegetation is in avery active condition, evaporation and 

 assimilation being then in full play. Again, when the tempera- 

 ture of the air is low, as in winter or during the night, but little 

 or no evaporation or assimilation takes place, and hence we find 

 that the temperature is higher in them than the external air. 



The conclusions come to in the last paragraph do not, how- 

 ever, altogether agree with the published result of experiments 

 made by Dutrochet ; for he found, by operating with Becquerel's 

 thermo-electric needle, that when plants were placed in a moist 

 atmosphere so as to restrain evaporation, that a slight increase of 

 temperature took place, and thus seeming to prove that the che- 

 mical changes taking place in plants nroduced a rise rather than 

 a diminution of temperature. Probably this slight increase of 

 heat under such circumstances is due to an oxidation or com- 

 3e 3 



