12 



the leaf is attained in i . 5 seconds. Complete recovery takes place 

 in 6 minutes in summer, and in 18 minutes in winter. In a leaflet 

 oi BiopJiytum the maximum fall is attained in .5 of a second and 

 full recovery is reached in 3 minutes. The excitatory fall of the 

 leaf takes place when stimulus is applied at or near the respond- 

 ing point. Seen from different points of view, this reaction will 

 appear as a diminution of turgor in the pulvinus, constituting a 

 negative turgidity -variation ; or a shortening or contraction of the 

 more excitable lower half of the pulvinus. Electrically speaking, 

 this reaction will have its concomitant in an electrical variation of 

 galvanometric negativity. It is convenient to include all these 

 excitatory symptoms together, under the single term negative 

 response. Here, however, we may describe a responsive change 

 of precisely opposite character, which takes place under definite 

 conditions. This positive response consists of an erectile move- 

 ment of the leaf, a positive turgidity-variation, expansion, and an 

 electrical change of galvanometric positivity. The occurrence of 

 this positive response may be denionstrated, in Mimosa, by apply- 

 ing stimulus at a point distant from the responding organ. In a 

 certain experiment this positive or erectile response occurred .6 of a 

 second after the application of the stimulus, and was followed, 2.8 

 seconds later, by the normal excitatory fall of the leaf Here we 

 have a response which is diphasic, positive followed by negative. 

 When stimulus is moderate, and applied at a still greater distance, 

 the response evoked is positive alone. These facts obtain uni- 

 versally, and from them we derive the following law of direct 

 and indirect stimulation : 



The effect at the responding-region of a strong stimulus trans- 

 mitted to a short distance, or throiigJi a good conducting channel, 

 IS negative. The effect transmitted to a great distance, or tliroiigh 

 a semi-conducting channel, is positive. 



Responsive movements, like those of the "sensitive" plants 

 so-called, can be detected also in ordinary plants. It will be no- 

 ticed, in Mimosa, that the responsive movement is made possible 

 by the unequal excitability of the upper and lower halves of the 

 pulvinus, the movement being determined by the greater shorten- 

 ing or contraction of the lower. If now we take a hollow tubu- 



