EVERGREEN TREES IN WINTER. 321 



Temperature of the soil. The temperature of the soil is to 

 be considered one of the most important factors affecting vege- 

 tation. The number of days, when the minimum temperature of 

 the earth's surface sinks below 0°C., is larger than that when the 

 temperature of the air falls below 0°C.; thus the former is 91-121 

 days, while the latter 61-79 days. 



It is obvious that in winter an herbaceous plant rooted in 

 shallow soil in the open ground, can not supply itself with a 

 sufficient quantity of water for transpiration and the whole of it 

 is destroyed, as we see in so-called annual plants ; but with 

 regard to evergreen trees this is not the case, since their roots 

 go deep into the soil where the temperature is not so low as to 

 hinder the absorption of water. Thus the Central Meteorological 

 Observatory gives the following observations^^: — 



Temperature of earth's surface. Dec. Jan. Feb. 



Mean 3.22°C. 3.C0°O. 4.08°C. 



Minimum 1.17 „ 1.37 „ 1.35 „ 



Maximmn 7.51 „ 8.24 „ 9.59 „ 



Temperature in soil, 

 m. 



0.05 deep 3.64 „ 4.00 „ 4.33 „ 



0.1 „ 4.29 „ 4.26 „ 4.48 „ 



0.2 „ 5.44 „ 4.80 „ 4.84 „ 



0.3 „ 6.45 „ 5.18 „ 5.07 „ 



0.6 „ 9.52 „ 7.51 „ 6.80 „ 



The warmer temperature, greater abundance of rainfall and 

 higher humidity during winter months in middle Japan, in com- 

 parison with the countries of central Europe, — Germany, for 

 example — , lead us to anticipate that transpiration goes on much 

 more actively in the former than in the latter. In this respect the 

 northern part of Italy is perhaps in harmony with middle Japan. 



1) Annual Report 1897. 



