Stimulation and Correlation. 



113 



protoplasts, which thus serve a perceptive function, and some 

 form of energy constituting an impulse is transmitted to the 

 organ concerned, in which the reaction takes place. In some 

 instances the reaction is a movement, and special structures 

 for producing the motion are differentiated, being known as 

 motor organs. The method by which impulses are trans- 

 mitted is not known, nor have any transmitting organs been 

 definitely located. Perceptive tissues or organs are diifer- 

 entiated in a few instances only. 

 The action of a stimulus is sup- 

 posed to be received by the ecto- 

 plasmic layer of the cells, and 

 transmission takes place through 

 these layers into, adjacent cells 

 connected by interprotoplastic 

 threads. Nearly all reactions have 

 for their direct purpose the placing 

 of the organ in a condition of 

 better adjustment with the other 

 organs of the plant or with the 

 environment, either for the better 

 performance of the functions or 

 the avoidance of injury. The 

 mechanism developed, however, may be set in action by 

 other forces than those to which they are designed to 

 react. 



122. General reactions of a plant to light Ger- 

 minate seeds of mustard, pea, or bean, and when the 

 roots are a few millimeters long, place on a piece of net- 

 ting fastened over the mouth of a bottle or dish, as in 

 Fig. 82. Cover with a cardboard box, and set near a 

 window in a room kept at ordinary temperatures. Cut a 

 slit in the box on the side nearest the window. A few 

 days later lift off the box, and note the position of the root 

 8 



Fig. 82. — Curvatures of a seed- 

 ling of mustard in response to 

 rays of light from left. After 

 Detmer. 



