THE SOIL 335 



tects the soil below. At the same time it reduces evapora- 

 tion, so that a hoed soil is cool and moist and therefore 

 better adapted to plant-requirements. When the ground 

 is covered with vegetation this receives the sun's heat, the 

 soil is screened, and its temperature does not rise so much 

 as that of bare soil. 



Factors affecting the water-supply in the soil. From our 

 previous observations we have seen that any factor which 

 tends to modify the water-supply of a plant will affect its 

 growth. The more important factors are : 



i. Temperature. A high temperature favours rapid trans- 

 piration, a low temperature renders root-absorption 

 difficult. Much more heat is absorbed by a dark 

 soil than a light soil. Water is a bad conductor of 

 heat, and a wet soil is a cold soil. 



2. Air is important to plants in several ways : 



(i) Its temperature determines the rate of absorption. 



(2) The amount of moisture present determines the 



rate of transpiration. 



(3) The oxygen of the air is necessary for respiration 



both by roots and shoots, and aeration of the 

 soil is essential. 



(4) Carbon dioxide is necessary for photosynthesis by 



green plants. 



3. Wind. Moving air, by increasing the volume affecting 



a given surface, increases evaporation ; hence the 

 drying effect of winds. The rate of transpiration is 

 great in plants having large thin leaves with sto- 

 mata on both surfaces, while it is reduced to a 

 minimum in plants with small rolled leaves with 

 stomata only on the enclosed surface, and therefore 

 in a ' still air ' chamber. 



4. Precipitation. As plants must obtain their mineral 



food in a weak solution, water is essential as a solvent 



