124 



THE TEMPERATURE OF THE SOIL [chap. 

 Root Growth of Maize in 24 Hours. 



The osmotic absorption of water by the roots of 

 a plant is much affected by the temperature of the soil ; 

 although some plants, like cabbage, will continue to take 

 in a little water even near freezing point, others require 

 a higher temperature; for example, Sachs has shown 

 that tobacco and vegetable marrow plants will wilt 

 even at night, when transpiration is very small, if the 

 temperature of the soil falls below about 40 F. 



The killing of plants like rose trees during frost is 

 generally due to drying out from this cause rather than 

 to the actual cold. As the air is often very dry during 

 a frost, evaporation continues, especially if a wind be 

 blowing at the same time ; thus the exposed shoots of 

 the plant are losing water which is not being replaced 

 by the roots, whose action is suspended by the low 

 temperature. A covering of snow or dead leaves, or 

 even the protection afforded by a little straw, bracken, 

 or spruce boughs, prevent the destruction of the plant, 

 not by keeping it so much warmer, but by pro- 

 tecting it from evaporation. 



The connection between soil temperature and vital 

 processes is perhaps most apparent in the case of 

 germination, for which not only is a certain minimum 

 temperature requisite, but for several degrees above this 

 minimum the germination is so slow and irregular that 

 the young plant is very liable to perish while remaining 



