CHAPTER XI 
ADJUSTMENT TO SURROUNDINGS 
126. 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 gravity, water, and light. 
126. Stimulus 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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