The Environment of Plants 



147 



PATE WHEN HARVEST OF SPRING OAT3 BEGl] 



MAY 



Fig. 84. Map showing date of oat harvest in dilTerent 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 rapidlx- in the 

 spring, and plants growing there start their gro^^'th later than 

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

 prefer not only uplands but north slopes. Why? 



