EVOLUTION OF HEAT. 647 



CHAPTEE VI. 



MISCELLANEOUS PHENOMENA. 

 Sect. 1. EVOLUTION OF HEAT BY PLANTS. 



In examining the cases falling into this section, it is important to 

 separate them into two classes : (1) those relating to the proper 

 or specific heat of plants generally ; and (2) those more remarkable 

 instances of elevation of temperature occurring at certain periods 

 of development in particular plants. 



It being a well-ascertained fact that the chemical combination 

 or separation of various elements found in plants is accompanied 

 by increase or diminution of sensible heat, and that the process of 

 evaporation, constantly going on in dry weather in healthy plants, 

 is a cause of depression of temperature as the substance from the 

 liquid passes off in the form ot: vapour, it is evident that the proper 

 temperature of plants, and their organs and tissues, must vary 



freatly according to the circumstances in which they are placed. 

 et, as the evaporation, and the fixation of carbon in a solid form 

 from gaseous material both "cooling processes" are such prepon- 

 derating operations in the nutritive and assimilative processes of 

 vegetation, it seems scarcely possible, under ordinary circumstances, 

 that plants should have a specific heat rising above that of the sur- 

 rounding atmosphere. 



The different power of conducting heat possessed by the several tissues, 

 and, in the case of the woody tissues, the difference in the conducting- 

 power according to the varying directions of the fibres, together with the 

 disturbances arising from the unlike conductivity of the fluids and solid 

 matters constituting the cell-contents, render it very difficult to arrive 

 at any general conclusions as to the specific heat of plants. 



From Dutrochet's experiments, made with a thermo-electric apparatus, 

 it would appear that the specific heat of all parts of plants in their ordinary 

 condition is rather lower than the temperature of the surrounding atmo- 

 sphere. In cases, however, where evaporation was prevented by placing 

 plants in an atmosphere saturated with moisture, the temperature some- 

 times rose from -^ to ^ per cent, above that of the atmosphere. More- 

 over a rise and a fall took place in the course of twenty-four hours, the 

 maximum occurring between ten and two in the day, the minimum at 

 midnight. It was further observed that the specific heat was only dis- 

 coverable in the soft or green parts, not in the woody structures. 



The experiments, however, which have been made to determine these 

 points are by no means conclusive. It seems probable that the increased 

 neat observed in the structures of plants when evaporation is restrained 

 depends upon the slow combustion of carbon, which is enabled to manifest 

 itself when the cooling influence of evaporation is removed. 



The germination of seeds, in which carbonic dioxide is abundantly 



