DEVELOPMENT OF HEAT BY PLANTS. 803 



is more especially evident, -wlien a number of seeds germinate 

 together, as in the process of malting. The development of heat 

 also, in flowering, has been alluded to (p. 764), The rise of 

 temperature which thus occurs in the processes of germination 

 and flowering, is due, without doubt, essentially, to the produc- 

 tion 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 temperatui-e. 



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 sun-ounding air, that it is due 

 to external influences, such as the sun's rays, and the tempera- 

 ture 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 tlie surrounding air, for at such a period 

 vegetation is in a very active condition, evaporation and assimi- 

 lation being then in full play. Again, when the temperature 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 than the external air. 



The conclusions in the last paragraph do not, however, alto- 

 gether 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 atmo- 

 sphere so as to restrain evaporation, a slight increase of tempera- 

 ture took place, thus seeming to prove that the chemical changes 

 taking place in plants produced a rise rather than a diminution 

 3f 2 



