Comparative Physiology. 343 



was given off, yet failed to keep up the heat. From the quan- 

 tity of heat evolved in the combustion of carbon, as experi- 

 mented on by Despretz, and the quantity of carbon given off 

 from the body, he thinks, by the average of calculations, the heat 

 generated in that way may be perfectly sufficient to replace the 

 waste from perspiration, respiration, &c. Others differ from this. 

 Dr. Carpenter thinks, in plants at least, that the absorption of 

 oxygen, and its subsequent conversion into carbonic acid, form the 

 principal source of heat. It is evident that a certain amount of 

 specific heat must be needed in plants, when life is most active ; 

 they cannot be altogether dependent on external heat, though, 

 from their vital parts being more exposed than in animals, it is 

 not so sensible to thermometers. The heat pi'oduced by a seed 

 in germination is not perceptible, but, when large heaps are put 

 together, it becomes evident ; and, could we keep young shoots 

 together and enable vital activity to proceed, the heat would be 

 more perceptible there also. Dutrochet thought plants possessed 

 of a specific heat, and found it greatest after noon; light he 

 found essential, and, though the heat did not totally cease on 

 the first day of darkness, yet by the third day it had disappeared 

 altogether. Exposure, however, even to simple diffused light, 

 he found to restore the heat in a rapid manner. Some have 

 contended for electricity being a cause of heat in animals ; others 

 say the electricity is only the consequence of the heat developed 

 by chemical action. Plants are evidently much affected by 

 electrical states of the atmosphere ; and as chemical action deve- 

 lopes electricity, so may electricity have a power of increasing 

 vital chemical action, in which heat may be developed. The 

 diffused excitability in plants when put in motion, as it must be 

 by vegetation, may also be a source of heat ; it is similar to 

 nervous power, though of an inferior kind, and may produce the 

 same effects in a less degree ; they are both also connected with 

 vital power. As nervous force seems to increase heat, as its 

 action is developed, so may the inferior action of diffused excit- 

 ability do the same by its action in plants, though in an inferior 

 manner. Nervous power may act by chemical affinity to produce 

 the effect wanted, but still be productive of an action which would 

 not have taken place independent of vitality, and be productive 

 of the developement of heat when needed, though done by che- 

 mical means. The vital activity of plants, though acting also 

 chemically, may pi'oduce the heat required to keep up vital 

 vigour. To these two, therefore, to chemical and vital power, 

 accompanied likely by the developement of electricity, and per- 

 haps often stimulated and set to work by it, are we probably 

 indebted for the specific heat necessary to carry on vegetation. 



Chemical action is supposed to commence as soon as the fluid 

 food is absorbed. The quantity of sugar, in the ascending sajD, is 

 found to augment, according to the height from the root where it 



