420 PLANT RESPONSE 



be traversed, to produce the excitatory negative response. 

 In this case, nevertheless, the sudden expulsion of water at 

 a distance will give rise to a wave of increased turgidity, 

 which will reach the responding organ, and produce there 

 only that response which is characteristic of positive turgidity- 

 variation. 



Positive response as indirect effect of excitation. — 

 Numerous experiments have been described exhibiting the 

 negative turgidity-variation as the direct effect of stimulation. 

 I have already described the production of positive turgidity- 



Fig. 170. Responses of Leaf cf Artocarpiis to Thermal Stimulat 



ion 



Thick dots show points of application of stimulation. In a stimulus 

 was applied near the responsive pulvinoid and gave rise to normal 

 response of fall — here represented as up movement— preceded by pre- 

 liminary erectile twitch. In b stimulus was applied at a greater 

 distance. The true excitatory effect did not reach the organ, and we 

 obtain positive erectile response of positive turgidity-variation, here 

 represented as down. 



variation as the indirect effect of stimulation (p. 400). A 

 fuller demonstration of this, by the electrical method, will be 

 found in Chapter XXXVII. I shall here give an experiment 

 which establishes the fact by means of mechanical response. 

 In fig. 32 (reproduced in fig. 170, a) was shown a series of 

 normal mechanical responses of negative turgidity-variation 

 obtained from the leaf of Artocarpus. The conductivity 

 of the petiole in this case is relatively feeble, and these 

 normal responses, preceded by preliminary positive twitches, 

 were obtained when stimulus was applied at a distance of 



