18-56.] Effects of Frost on Dormant Vegetation. 185 



this, says — " Salt and water freeze at different temperatures according 

 to the density of their mixtures, from four degrees to twentj-seven 

 degrees ; oil of turpentine freezes at fourteen degrees ; oil of burgamot at 

 twenty-five degrees ; milk at thirty degrees ; and water, the standard, at 

 thirty-two degrees ; olive oil at thirty-six degrees ; oil of anise at fifty 

 degrees ; and it is not to be doubted, that in like manner the fluid con- 

 tents of plants, which we know are differently constituted, and therefore 

 variously modified, will resist the action of cold in very different degrees." 



Entirely unsatisfactory, also, is the statement that ice when brought 

 into contact with living plants is found to melt ; for the same result was 

 readily found when the ice was placed in contact with portions of the 

 dead plant. Again, the higher temperature of the exterior of trees, as 

 compared with the surrounding atmosphere, can not be caused by heat 

 from within ; but it must be governed by the simple physical capacity 

 of the wood to conduct the heat or cold. The very accurate observations 

 of Prof. Nau, have shown that the temperature of a tree must be entirely 

 dependent on the temperature of the soil, and the degree of cold or 

 warmth previous to the time the observation was made. He observed 

 that the temperature of a tree, immediately after a sudden falling of the 

 temperature without, was warmer than the air ; and he invariably 

 found it colder than the air, immediately after a sudden rising of the 

 atmospheric temperature. This statement is fully accounted for by the 

 fact, that wood, in general, is a slow conductor of heat; more or le.^s 

 time must elapse, therefore, before the temperature of the tree and the 

 temperature of the air can be set in equillhrio. In conformity with the 

 above. Prof. Nau found that the temperature of trees partakes of the 

 moderate changes of the day and night-time in summer. At from nine 

 to ten o'clock in the morning they retained the lower temperature 

 imparted to them during the previous night ; while, at from eight to 

 nine o'clock in the evening they still retained the higher temperature 

 imparted to them by the warmth of the day-time. In all c;ises of a 

 rapid sinking of the temperature in winter, he found the trunk of a tree 

 to be warmer than the air, the root warmer than the trunk, yet colder 

 than the soil. 



In view of these and other cognate facts, Prof. Xau rejected the doc- 

 trine of the generation of independent heat in plants, and pronounced it 

 as his opinion, that their temperature depended solely on the temperature 

 of the surrounding air and soil. 



The observations of another well known and eminent botani- 1, Prof. 

 Schubler, seconded and supported Nau's opinion on the subject. He 

 found trees both in winter and in summer, to be warmer than the air. 



