262 PROPER HEAT OF PLANTS. 



409. Some recent experiments, however, made with an 

 apparatus that would indicate extremely slight changes of tem- 

 perature, have proved that the process of Respiration in Plants 

 is accompanied by a disengagement of heat; but in order to 

 establish this, it was necessary to compare the temperature of a 

 living plant with that of a dead one, having the same proportion 

 of moisture at its surface; since in this way only, could the 

 true effect of respiration in producing heat be known, whilst the 

 evaporation was continually preventing the manifestation of it, 

 by cooling the surface. In this manner it was found, that the 

 heat of the surfaces of plants is raised by their respiration, from 

 1 to 2| degrees above what it would otherwise be. 



410. It has long been observed, that the interior of large 

 trunks possesses 'a temperature more uniform than that of the 

 surrounding air ; being cooler than the atmosphere in summer, 

 and warmer in the winter. There are at least two causes of this 

 occurrence. Wood is a slow conductor of heat ; thus, if a piece 

 of stick and a tod of iron of equal sizes have one end heated in 

 the fire, the farther end of the stick will be nearly cold, whilst 

 that of the iron is too hot to be handled. Further, the conducting 

 power of wood is still less across the grain (or through the stem), 

 than with the grain (or along the stem) ; so that changes in the 

 external air will not readily affect the centre of a large trunk ; 

 and, accordingly, it is found that, the larger the trunk on which 

 the observation is made, the greater is the difference between its 

 state and that of the air. The other reason is, that some motion 

 of the sap takes place even in winter ; and the fluid taken up by 

 the roots principally comes from a depth in the ground, at which, 

 from the bad-conducting power of the soil, the temperature is 

 nearly uniform throughout the year. 



41 1. The evolution of heat by Plants is most evident, at those 

 periods of their existence, in which an extraordinary quantity of 

 carbonic acid is formed and given off. This is the case during the 

 germination or shooting-forth of seeds ; and though the heat 



hand is cooled least by the water, more by the spirit, and most by the ether, in 

 proportion, in fact, to the rapidity with which these fluids respectively pass off in 

 vapour. 



