PART IV. 



VEGETATION IN RELATION TO ENVIRONMENT. 



CHAPTER XLVI. 



FACTORS INFLUENCING VEGETATION TYPES; 

 OR ECOLOGICAL FACTORS. 



900. In studying the life and growth of plants it becomes very 

 apparent that from the germination of the seed to the production 

 of flowers and fruit the plant is dependent upon the favorable 

 influence of certain conditions of environment. A dry seed kept 

 in dry air or dry soil will never germinate, or if it is kept in moist 

 soil, the temperature of which is below a certain minimum (from 

 6 C. to o C.), it will remain apparently lifeless. But with a 

 favorable amount of moisture and heat the life which was dor- 

 mant becomes active, growth begins, the embryo is nourished 

 by stored food, the root lays hold of the soil for support, and the 

 stem and leaves rise to the light. Light now exercises an influ- 

 ence on the form and position of the leaves as well as on color 

 and work. The root takes up watery food substances from the 

 soil. If the soil contains too much water or too little, the plant 

 suffers correspondingly; or if certain chemical substances are 

 too abundant in the soil, or if others are lacking, the plant suffers. 

 Still we know that some plants do better than others in wet soil 

 or even in water of lakes or ponds, while still other plants thrive 

 better in a comparatively dry soil. Some thrive better in rocky 

 situations, some in sandy soil, some in mud, and others in humus. 



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