CHAPTER TWENTY-THREE 



ENVIRONMENTAL FACTORS AFFECTING GROWTH AND 

 REPRODUCTION 



The environment of a plant is made up of many factors. 

 Moreover, the factors are more or less interdependent, and it 

 is very difficult, and often impossible, to change one factor with- 

 out altering related factors. Consequently, it is often difficult 

 to determine the underlying cause of a change in the form of a 

 plant that is undoubtedly produced by something in the envi- 

 ronment. 



Changes in the water content of a soil are complicated by the 

 effects of decreased oxygen content. The addition of lime to a 

 soil changes the permeability of the roots to water and mineral 

 salts, modifies the rate of transpiration, as well as alters the 

 chemical and physical qualities of the soil itself. The final effect 

 upon the plant is the combined result of all these changes. 



In the following paragraphs the more important factors of the 

 environment and some of their effects upon plant growth and 

 reproduction are discussed. 



Light an important environmental factor. The amount of light 

 available to a plant depends primarily upon the intensity of the 

 sunshine. This is greatest in the tropics and least at the poles. 

 The total amount of light is influenced also by the length of the 

 day. At the equator the daylight lasts 1 2 hours ; at the poles the 

 light continues all summer. So tropical plants have intense light 

 during half of each day, while arctic plants have weaker light 

 continuously through the growing season. 



Orange growers at the northern end of the Central Valley of 

 California are able to ripen their fruit for market 3 to 6 weeks 

 earlier than their competitors 400 miles farther south, due to the 

 longer daily period of sunlight and the protection from cool night 

 winds afforded by the surrounding high mountains. 



As much as 1000 bushels of potatoes have been grown on an 

 acre of land along the Mackenzie River, at the arctic circle. 



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