CHAPTER VII. 



STIMULATION AND CORRELATION. 



120. General irritability of plants. The plant may 

 carry on its functions only under a certain range of conditions 

 or intensities of certain external forces or agencies, of which 

 light, moisture, temperature, food, substratum, and gravity 

 are the most important. Any change in the relations of any 

 one of these factors to the organism sufficient to alter the 

 behavior of the plant in any of its functions constitutes a 

 stimulus, and the response to the action of the stimulus, or 

 to stimulation, is termed a reaction. Reactions are not pro- 

 duced by variations in environmental forces alone, however. 

 The parts of the body are most intimately connected, and 

 any change in the manner of performance of a function in 

 one organ acts as a stimulus on other parts of the plant. 

 Thus the cessation of growth, the reception of an injury, or the 

 inception of a new activity in an organ stimulates tissues in 

 distant parts of the body to variations in their behavior. 

 This correlation of all the organs of an individual exists in 

 a much higher degree among plants than among animals. 

 Familiar and marked examples of irritability or reaction to 

 stimulation are to be seen in the movements made in bend- 

 ing toward the source of light rays, and the closure of the 

 pinnules in the pinnules of Mimosa. 



121. Sensory organization of the plant. Changes 

 in external or internal forces constituting a stimulus often do 

 not directly affect the organs or tissues in which the reaction 

 ensues. The stimulus may be received by the outer layer of 



