EFFECT OF FROST ON PLANTS. 67 



rod equal in length whatever the changes from heat 

 to cold. All resinous woods, such as the pine, 

 the fir, etc., are of the class so protected by na- 

 ture. Hence, though found in almost all habitable 

 latitudes, these are under no necessity of shedding 

 their foliage. 



The orange tree approximates in the character of 

 its sap to this order of plants, and is therefore, though 

 a tropical plant, able to stand the changes of a semi- 

 tropical climate. 



