92 THE NATUEE AND WORK OF PLANTS 



None of these devices may protect against freezing 

 in general cold weather. 



118. Drainage of cold air. The layer of cold 

 air on the surface of the soil is heavier than the 

 warm air above it, and in rough or broken country 

 this heavier air flows down hill as water would, 

 accumulating in the valleys, which thus become very 

 much colder than the hill-tops around them. The 

 temperature of the valleys is often ten to twenty 

 degrees lower than that of the hills near by. As a 

 consequence of this fact, vineyards, orchards, and 

 gardens, in which delicate varieties are cultivated, 

 are planted on ridges and hills in preference to 

 low-lying valleys. 



