CHAPTER XII 



HOW PLANTS ARE ADJUSTED TO THEIR 

 ENVIRONMENT 



The Environment. — No plant or animal can live its 

 life entirely to itself. The sum total of all the external 

 influences acting on any organism constitute its environ- 

 ment. No organism can escape the influence of tempera- 

 ture and moisture. The duration and intensity of light 

 is very important to any green plant. The amount and 

 distribution of rainfall and the fertility of the soil are 

 vital factors. The competition of other plants or the 

 abundance of plant-eating animals may determine 

 whether a species can occupy a particular habitat or not. 



Animals are somewhat less directly dependent on the 

 physical factors of their surroundings than plants but are 

 nevertheless subject to them indirectly. Temperature 

 usually proves an important factor limiting the spread 

 of animals. The kind of plants occupying any region 

 determines the food supply and the nature of shelter 

 available to animals. A very important part of an ani- 

 mal's environment is its relation to other animals. It 

 must compete with some of them for food and avoid 

 becoming food for others. 



The Necessity for Adjustment. — With so many 

 elements of the environment playing important roles in 

 the life of plants and animals it is obvious that adjusting 

 mechanisms are required to enable them to secure or 

 maintain favorable relations with their surroundings. 

 Plants need to be able to adjust the growth of roots, 

 stems, and leaves so as to place these organs in a favorable 

 position to perform their several functions, for if these 



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