226 PLANT RESPONSE 



that while moderate intensity of stimulus, applied on the 

 etherised area, failed to evoke a responsive movement of the 

 distant leaflet, a powerful stimulus was able to do so. 



Receptivity versus motile excitability. — At the begin- 

 ning of the present chapter, I drew attention to the necessity 

 of discriminating between the functions of receptivity and 

 motile excitability. It is only by carefully distinguishing these 

 that we can possibly come to an understanding of certain 

 apparent contradictions. Let us suppose that stimulus is 

 applied on a motile organ, say the pulvinus of Mimosa. In 

 this particular case, the areas of receptivity and motile excit- 

 ability are coincident. By the reception of stimulus the motile 

 machinery is eventually set in motion. The mobility of the 

 superficial particles will thus determine the receptivity and 

 the inner mechanism of the organ, the motile excitability. 

 The motile excitability is measured by the amplitude of 

 response. Receptivity, on the other hand, may be partially 

 discriminated by (i) the length of the latent period, and (2) 

 the.value of the minimally effective stimulus. 



When a tissue is cooled, say to 7° C. or lower, its recep- 

 tivity and motile excitability both undergo diminution. Hence 

 the latent period is prolonged (p. 268), and the stimulus 

 which was formerly effective becomes ineffective. In such a 

 case, where the two factors conspire, it is difficult to distin- 

 guish between the relative effects of receptivity and motile 

 excitability. But when, on the other hand, the temperature 

 is raised, say to 35° C, the amplitude of contractile response, 

 by which we are in the habit of gauging the motile excitability, 

 is generally speaking diminished (fig. 79). Hence we are 

 apt to infer that excitability in general is decreased at 35° C. 



But if we test this question by means of the minimally 

 effective stimulus, we arrive at a very different conclusion. 

 For example, taking a specimen of Biophytum at 30° C, 

 I found that the minimally effective stimulus was given by 

 a condenser charged to twenty-two volts, whereas when the 

 temperature was raised to 35° C. the minimally effective 

 timulus was a charge of fourteen volts. It is clear from 



