CHAPTER XX 

 TEMPERATURE AND LIGHT 



Environmental Factors. — The living plant is influenced hy 

 its environmental factors, such as water, food supply, heat, light 

 and many others too numerous to mention. It is doubtful if 

 any of these factors are ever exactly right for the best growth 

 for any considerable time. Therefoa-e, the plant may be con- 

 sidered as having made the best possible growth under an en- 

 vironment which is never absolutely perfect. Very naturally, 

 the more satisfactory the environment with regard to all its 

 many factors the better the plant that will be produced. We have 

 already considered some of these environmental factors, such as 

 water and the 'food supply. 



Plant Geography. — The natural distribution of plants over 

 the surface of the earth is controlled by these environmental fac- 

 tors. This is very noticeable in the north and south boundaries 

 of certain plants, which are evidently due to temperature. It is 

 equally true where the limiting factors are due to water supply 

 and character of the soil. When crop plants are introduced 

 from one part of the world into another part these environmental 

 factors must be taken into consideration if the introduced crop 

 is to prove successful. A study of the native and agricultural 

 plants of the country enables us to map out certain regions which 

 are referred to as the pine regions, the hardwood regions, the 

 corn belt, the cotton belt and the rice districts. 



Temperature. — Plants grow under a very wide range of 

 temperature from the equator to the polar regions; from the 

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