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and displace the warm air there. In consequence frost often 

 visits the bottom lands long before it touches the hilltops, 

 because the latter are nightly bathed in the warm air crowded 

 up from below. For this reason farmers usually plant the late 

 crops of buckwheat on the hillsides. In certain valleys there 

 is a zone part way up the slope, called the verdant zone or 

 thermal belt, in which late spring and early autumn frosts are 

 almost unknown. This zone is due to the movement of the 

 warm air out of the valley at night. Other inclosed valleys 

 drained by a stream may be nearly exempt from frost because 

 the cold air flows away over the stream. This distribution of 

 temperature has a curious effect upon the distribution of plants. 

 Northern plants are usually found farthest south in the val- 

 leys, and southern plants farthest north on the hillsides, exactly 

 the opposite of what at first glance one would assume to be 

 the natural occurrence. 



How cold kills plants. Some tropical plants are so sensitive 

 to cold that they may be killed by exposure to temperatures 

 several degrees above the freezing point, but usually plants 

 are killed by the freezing of the protoplasm or the sap within 

 the cells. Freezing of the cell sap takes place at a tempera- 

 ture somewhat lower than 32, since water containing dissolved 

 substances requires a greater degree of cold to congeal it than 

 does pure water. Often it is not the mere cold that kills plants, 

 but rather the withdrawal of moisture from the cell by the 

 formation of ice crystals in the intercellular spaces. In such 

 cases the effects of cold are exactly the same as those of dry- 

 ing. Otherwise hardy plants are often killed in winter by the 

 heaving due to the alternate freezing and thawing of the soil 

 and the consequent breaking of the roots. 



Other effects of cold. In plants that are not killed outright 

 by the cold, the lowered temperature may injure the less 

 resistant parts. Some plants, such as the catalpa, grape, rasp- 

 berry, and sumac, continue to grow until stopped by the cold, 



