CHAPTER XI 

 ADJUSTMENT TO SURROUNDINGS 



125. Environment. — Not only must plants be nour- 

 ished, and respire in order to live; they must also be in 

 general harmony with their surroundings. The sum total 

 of these surroundings is called the environment. Among 

 the factors of environment are temperature, water, light, 

 gravity, air, electricity, soil, animals, and other plants. 

 It will not be possible, here, to study the adjustments of 

 the plant to all of these factors, but only to the more 

 important ones, such as gra"sdty, water, and light. 



126. Stimiilus and Response. — It is the nature of the 

 various parts of a plant to grow in a certain definite rela- 

 tion to their environment. Thus, for example, main stems 

 and roots normally grow parallel to the plumb-line, while 

 their branches grow at an angle to it; foliage-leaves grow 

 naturally in the light, while roots grow naturally in the 

 dark. Any change in the environment requires a re- 

 adjustment on the part of the plant, if the latter is to 

 remain healthy. If the readjustment cannot be made 

 the given organ, or the. entire plant, may become un- 

 healthy, or may die. The change in the environment, 

 considered from the standpoint of its effect on the plant, 

 is called a stimulus; the readjustment or attempt at re- 

 adjustment, a response. Thus, if a plant is growing at a 

 certain rate at a certain temperature, any change in the 

 temperature becomes a stimulus to which the plant 



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