STIMULUS AND RESPONSE . 3 



3. The kinds of stimuli. The simplest grouping of stimuli 

 is with respect to the force concerned. The factors of a habitat 

 are water, soluble salts, humidity, light, temperature, wind, soil, 

 pressure, physiography, gravity, polarity, and biotic factors. Cer- 

 tain ones of these, namely, soil, physiography, pressure, and 

 biotic factors, can act upon plants only through the action of 

 other factors, as a rule. For example, the wind normally influ- 

 ences the plant only through humidity, and the soil through, 

 water content. Since a stimulus can be determined only by 

 the response of the plant to it, only those factors that act imme- 

 diately upon a function can be termed stimuli. These are the 

 universal forces, gravity and polarity, and the physical factors, 

 water, soluble salts, humidity, light, and temperature. With 

 respect to certain mechanical effects, wind may also act as a 

 stimulus, and the same is often true of biotic factors in the case 

 of sensitive, insectivorous, and gall-producing plants. Stimuli are 

 often distinguished as internal and external, but the distinction 

 is of little value. This is due to the fact that internal stimuli 

 are obscure in nature and effect: it is not improbable that they 

 are merely the latent results of external stimuli. In any event, 

 little can be done with them until more is known of the precise 

 action of external stimuli. It is with the latter alone that our 

 present study is concerned. 



4. The nature of response. Plants seem to have no special sense- 

 organs for perceiving stimuli, and no definite sensory tracts for 

 transmitting them. Consequently an external stimulus acting 

 upon a plant is ordinarily converted into a response at once. 

 The latter, as a rule, becomes evident immediately. In many 

 cases some time elapses before the final response becomes visible,, 

 and in rare instances the response remains latent or impercepti- 

 ble. A marked decrease in humidity calls forth an immediate 

 increase in the amount of water evaporated from the leaf, but 

 a final response is seen in the closing of the stomata. The re- 

 sponse to decreased light, on the contrary, is much less rapid and 

 obvious. This difference in behavior is largely due to the func- 

 tional response being more marked and more easily perceptible 

 in the first case. 



The first response of a plant to a stimulus is always functional. 

 The nature and intensity of the stimulus determine whether this 

 is followed by a structural response also. The amount of re- 



