CHAP, xiii TEMPERATURE OF SOIL 53 



heat. In the deeper layers the temperature extremes are great, whereas 

 in loose soils only the superficial layers are heated. 1 



Frozen soil, which extends more or less deeply below the surface in 

 Polar lands and on high mountains, naturally plays an important part 

 in relation to vegetation, partly because roots bend away from it as from 

 rock soil (also perhaps by reason of the thermotropism of the roots), 

 and partly because the cold depresses the functional activity of the roots. 



8. The texture of vegetation, in particular its density, influences the 

 temperature of soil, because it more or less screens this from direct insola- 

 tion and evaporation, and intervenes in radiation from the soil. 2 



9. Internal heat of the Earth. According to Tabert's estimation the 

 mean temperature of the earth's soil is raised by conduction from the 

 internal heat of the earth by 0-1 C. an indifferent quantity. Krasan's 

 view of the great importance to vegetation of conduction of the earth's 

 internal heat, is based upon an inadequate appreciation of the climatology 

 of soil. 



In this connexion it may be mentioned that at Zwickau, owing to 

 the heat liberated from the anthracite which undergoes slow subterranean 

 combustion, it has been found possible to cultivate sub-tropical plants 

 in the open. 



10. The cooling of soil by wind is in many cases capable of playing 

 an important part on vegetation. For instance, the vegetation on the 

 coasts of the North Sea may suffer the greatest injury from the north- 

 west wind, and this must be partly due to depression in the activity of 

 the roots, caused by this cold wind cooling the soil. 



Concerning the relations between the heat of the soil and of the atmosphere, 

 it may be stated that in winter the surface of the soil is warmer than 

 the air only during a few hours at the middle of the day, but at other 

 hours it is a little colder than the air. Nevertheless the daily mean 

 temperature of the soil is higher than that of the air. Only where there 

 is a covering of snow is the temperature of this surface, also its daily 

 mean temperature, lower than that of the air. In summer, the tempera- 

 ture of the soil during the day is considerably higher than that of the 

 air, but during the night a little lower or rarely higher. Consequently 

 the annual mean temperature of the soil greatly exceeds that of the 

 air. On mountains the maximal temperatures of the soil are nearly 

 as high as in the lowland at their base, whilst the minima are not corre- 

 spondingly lower, so that the excess of the temperature of the soil over 

 that of the air increases with the altitude. 



The daily fluctuations of the temperature affect the soil to a depth 

 of one metre, descending most deeply in compact kinds of soil that are 

 good conductors of heat. Annual fluctuations penetrate much deeper : 

 in Denmark, for example, to a depth of twenty-five metres, where approxi- 

 mately the mean temperature of the soil remains constant. 



Thus it follows that the temperature of the soil undergoes greater 

 fluctuations than does that of the air. The fluctuations are greater 

 in warmer kinds of soil. But plants rapidly adjust their vital processes 

 to variable temperatures. A variable temperature that often approxi- 

 mates to the optimum is more beneficial to plants than is a constant 

 temperature that remains far below the optimum. 



1 See Homen, 1897. * See Chapters XII and XIX. 



