IRRITABILITY 37 



vantageous. Each of these consists of 

 certam processes which take place as the 

 result of the stimulus. In what respect 

 do they dilffer from the reactions found 

 in non-living matter? With this question 

 in mind let us consider some typical ex- 

 amples of irritability. 



One of the striking methods of re- 

 sponse is seen in the tropisms. A tropism 

 is a bending or moving toward, away 

 from, or at an angle to the source of the 

 stimulus. A good example is found in 

 the germinating seed. Under the influ- 

 ence of gravitation the stem grows up- 

 ward, the main root downward, and the 

 side roots grow out at an angle with the 

 vertical. 



If sprouting seeds are placed on a 

 wheel which is slowly revolving, the 

 roots are stimulated equally on all sides 

 and consequently do not bend, but if the 

 wheel revolves rapidly, they are stimu- 

 lated by centrifugal force and grow out- 

 ward from the center of the wheel. If 

 they are placed at the center of the 



