162 TEMPERATURE OF THE SOIL. ' * 



Degrees. 



Mean of the year, Oi 



Winter, +2! 



Spring, —3i 



Summer, — 2| 



Autumn, +11 



" Hence it appears that from the middle of 

 autumn to the end of winter the temperature of the 

 soil is warmer than that of the air at the mean 

 depth of the roots ; and that, on the contrary, at 

 the season when the plant is at it greatest vigor, it 

 is colder than the air. In the one case, the maxi- 

 mum difference between the soil and the air occurs 

 in January, and in the other during one of the 

 summer months. In no case does it exceed six and 

 one-half degrees. At the end of winter and begin- 

 ning of autumn, periods occur when there is no 

 diiference between the temperature of the air and 

 soil. This fact, combined with that of the resuscita- 

 tion of vegetable life in spring and its withdrawal 

 in autumn, seems to indicate some direct adaptation 

 of the cooler soil to the wants of plants at that 

 season of the year. The difference is always greater 

 in summer than in spring, and vegetation is not 

 then more active than in April or May in the lati- 

 tude of Brussels. During winter, a difference of 

 four or five degrees can have but very little influ- 

 ence in diminishing the effects of cold ; for the sap 

 scarcely moves at that season, and the effect of 

 conduction from the roots to the branches must be 

 extremely small. 



