CHAPTER III. THE PLANT 



Stimulus and Response 



general relations 



144. The nature of stimuli. Whatever produces a change in the func- 

 tions of a plant is a stimulus. The latter may be a force or a material ; it may 

 be imponderable or ponderable; effect, not character, determines a stimulus. 

 Consequently, reaction or response decides what constitutes a stimulus. 

 The presence of the latter can be recognized only through an appreciable 

 or visible response, since it is impossible to discriminate between an impact 

 which produces no reaction and one which produces a merely latent one. 

 From this it is evident that quantity is decisive in determining whether the 

 impact becomes a stimulus. Plants grow constantly under the influence 

 of many stimuli, all varying from time to time in amount. Small changes 

 in these are so frequent that, in many cases at least, the plant no longer 

 appreciably reacts to them. Such changes, though usually measurable, are 

 not stimuli. Futhermore, it must be clearly recognized that plants which 

 are in constant response to stimuli are stimulated anew by an efficient in- 

 crease or decrease in the amount of any one of these. As is well known, 

 however, such increase or decrease is a stimulus only within certain limits, 

 and the degree of change necessary to produce a response depends upon 

 the amount of the factor normally present. The entire absence of a force 

 usually present, moreover, often constitutes a stimulus, as is evident in the 

 case of light. The nature of the plant itself has a profound bearing upon 

 the factors that act as stimuli. Many species are extremely labile, and 

 react strongly to relatively slight stimuli ; others are correspondingly stable, 

 and respond only to stimuli of much greater force. Some light is thrown 

 upon the nature of this difference by the behavior of ecads. A form which 

 has grown under comparatively uniform conditions for a long time seems 

 to respond less readily, and is therefore less labile than one which is sub- 

 ject to constant fluctuation. Tn many cases this is not true, however, and 

 the degree of stability, i. e., of response, can only be connected in a general 

 way with taxonomic position. 



145. The kinds of stimuli. The factors of a habitat are external to 

 the plant, and consequently are termed external stimuli. Properly speak- 

 ing, all stimuli are external, but since the response is often delayed or can 



