2 PLANT PHYSIOLOGY AND ECOLOGY 



real test of the latter is therefore furnished by the plant, since 

 the presence of a stimulus can only be ascertained by the re- 

 sponse made by the plant. ^loreover, while it is possible for 

 the effect of a stimulus to remain invisible or latent for a time, 

 a factor which works in this way can never be recognized as a 

 stimulus until its effect becomes apparent. Stimulus and re- 

 sponse are consequently not only inseparably connected, but the 

 latter is the only obtainable evidence of the action of habitat 

 factors. Since plants grow constantly under slight fluctuations 

 in the habitat, it has come about that they do not respond to 

 minute differences of factors. Living plants are in constant 

 response to stimuli, and they are stimulated anew only by an in- 

 crease or decrease in the factor sufficient to bring about an appre- 

 ciable change in a function. Sometimes the total withdrawal 

 of a factor acts as a profound stimulus, as in the case of a plant 

 placed in darkness. The nature of the plant itself is of the utmost 

 importance in determining what differences are sufficient to con- 

 stitute stimuli. A species whose characters have been fixed b}' 

 heredity responds much less readily to external factors than does 

 one in which the structures are variable or plastic. In other 

 words, a difference sufficient to produce a change in the latter 

 has no effect upon the former. Such a difference constitutes a 

 stimulus for the one, but not for the other. Thus, while light 

 acts as a stimulus to all green plants, a certain change in the 

 intensity of the light is a stimulus only to those plants that are 

 plastic enough to show a response to it. 



It has been the practice to distinguish between the tonic action 

 of external factors, such, for example, as that of light upon the 

 chloroplast, and the stimulatory action of such forces, as seen 

 in the bending of leaves toward the light, or the movement of 

 sensitive leaves in response to a shock. In the one case the 

 energy of the impinging factors results in an immediate and usu- 

 ally proportionate amount of work being done. In the other 

 this factor brings about the release of stored-up energy in the 

 plant, which in many instances results in a disproportionate 

 amount of work. However, a careful analysis of these two proc- 

 esses shows that at the bottom they are essentially the same. 

 Furthermore, they are seen to differ only in degree, and not in 

 kind, when one examines the many processes intermediate between 

 the two. 



