The Environment of Plants 



147 



Fig. 84. Map showing date of oat harvest in different parts of the United States ; 

 an example of the effect of temperature on the maturing of plants. 



wet soils, (i) because less heat is required to raise their tem- 

 peratures, and (2) because the temperature of a wet soil is 

 lowered by the constant evaporation of water. The most 

 valuable farm lands are those with dark-colored, well-drained 

 soils. On north slopes, soils do not warm up so rapidly in the 

 spring, and plants growing there start their growth later than 

 do those on the south slopes of the same hills. Peach growers 

 prefer not only uplands but north slopes. Why? 



