SPECIFIC HEAT. 173 



It does not appear tliat observations liave been made on the 

 special point of the development of heat in forest trees during 

 florification, or at any other period of intense vital action ; and 

 hence an important element in the argument remains undeter- 

 mined. The " circulation of the sap " commences at a very early 

 period in the spring, and the temperature of the air in contact 

 with trees may then be sufficiently afiected by heat evolved 

 in the vital processes of vegetation, to raise the thermometric 

 mean of wooded countries for that season, and, of course, for the 

 year. The determination of this point is of much greater im- 

 portance to vegetable physiology than the question of the winter 

 temperature of trees, because a slight increment of heat in the 

 trees of a forest might so affect the atmosphere in contact with 



cause the vital processes of trees of persistent foliage are less interrupted during 

 winter than those of trees which annually shed their leaves, and that therefore 

 more organic heat is developed ? 



In crossing Mont Cenis in October, 1869, when the leaves of the larches on 

 the northern slope and near the top of the mountain were entirely dead and 

 turned brown, I observed that these trees were completely white with hoar- 

 frost. It was a wonderful sight to see how every leaf was covered with a 

 delicate deposit of frozen aqueous vapor, which gave the effect of the most 

 brilliant silver. On the other hand, the evergreen coniferae, which were grow- 

 ing among the larches, and therefore in the same conditions of exposure, were 

 almost entirely free from frost. The contrast between the verdure of the leaves 

 of the evergreens and the crystalline splendor of those of the larches was strik- 

 ingly beautiful. Was this fact due to a difference in the color and structure 

 of the leaves, or rather is it a proof of a vital force of resistance to cold in the 

 living foliage of the evergreen tree ? 



The low temperature of air and soil at which, in the frigid zone, as well as 

 in warmer latitudes under special circumstances, the processes of vegetation 

 go on, seems to necessitate the supposition that all the manifestations of vege- 

 table life are attended with an evolution of heat. In the United States it is 

 common to protect ice, in ice-houses, by a covering of straw, which naturally 

 sometimes contains kernels of grain. These often sprout, and even throw out 

 roots and leaves to a considerable length, in a temperature very little above 

 the freezing-point. Several years since I saw a lump of very clear and ap- 

 parently solid ice, about eight inches long by six thick, on which a kernel of 

 grain had sprouted in an ice-house, and sent half a dozen or more very slender 

 roots into the pores of the ice and through the whole length of the lump. The 

 young plant must have thrown out a considerable quantity of heat ; for though 

 the ice was, as I have said, otherwise sohd, the pores through which the roots 

 passed were enlarged to perhaps double the diameter of the fibres, but still not 

 BO much as to prevent the retention of water in them by capillary attraction. 



