496 Plant Physiology 



the stimulus required is analogous to the pressure of the 

 finger on the electric button which sets at work powerful 

 dynamos. The pressure upon the button has no relation 

 to the amount of work which will be accomplished by the 

 machines thus released. When a stimulus affecting the 

 plant is external, its relations to response may be worked 

 out with a fair degree of success ; but the methods of ac- 

 tion of internal stimuli are almost entirely unknown. 

 In the action of any stimulus upon a sensitive organ there 

 are to be distinguished primarily (1) perception, (2) trans- 

 mission, and (3) reaction or growth response. Commonly 

 the perceptive region is at no great distance from the motor 

 or responsive part ; yet in certain cases the stimulus may 

 be transmitted through considerable intervening tissue. 

 Practically nothing is known regarding the mechanism of 

 transmission. Perception often resides in the terminal 

 portion of the organ, but not always in the formative 

 region. 



In order to produce response a stimulus must act 

 usually for a certain interval of time, and this interval 

 (presentation time) depends upon temperature and other 

 growth conditions. The visible response to the stimulus 

 may be prompt, as in the case of many tendrils, or it may 

 be delayed for several hours. The interval between stimu- 

 lation and response (reaction time) is usually longer than 

 that of presentation. 



301. Tropic curvatures. - - Every plant exhibits a 

 normal form and habit, and its members are arranged in a 

 definite manner with respect to one another and to en- 

 vironmental forces. Tropic curvatures are commonly 

 the results of irritable growth responses manifest when the 



