324 LECTURE XIV. 



no currents could be detected in leaves of Dionaea which had 

 either died slowly, or had been suddenly killed. Velten was 

 however able to detect currents in leaves and portions of 

 stems which had been suddenly killed by immersion in 

 boiling water or in alcohol. Though these results are contra- 

 dictory, yet this one fact is clear that, in certain organs at 

 least, when death is suddenly induced in such a way that the 

 organisation is not materially injured, the currents persist. 



Some further light is thrown upon this point by the 

 observations which have been made on the currents in mo- 

 tile organs when quiescent and when stimulated. Burdon- 

 Sanderson and Munk have found that when the leaf of Dionaea 

 is stimulated, any current which may be observed between 

 any two points when the leaf is unexcited exhibits a variation. 

 For instance, let us suppose the electrodes to be placed on 

 opposite points of the upper and under surfaces of an un- 

 excited leaf, and that the point on the lower surface is then 

 found to be positive with regard to the point on the upper 

 surface. On stimulation, either mechanical or electrical, of 

 the irritable upper surface, the under surface becomes suddenly 

 negative to the upper, and then gradually becomes positive to 

 the upper, more positive than in the unexcited leaf, the first 

 (negative) variation being less considerable than the second 

 (positive) variation. Kunkel obtained similar results in his 

 experiments on the pulvinus of the leaf of Mimosa pudica. 

 When we come to study the mechanics of these organs, we 

 shall learn that stimulation of them gives rise to the passage 

 of currents of water through their tissues. The passage of 

 currents of water through the tissues leads, as we have seen 

 above, to a disturbance of electrical equilibrium, and Kunkel 

 and Burdon-Sanderson agree in referring to this cause the 

 positive variation which they observed : currents of water pass 

 from the cells near the point of stimulation to others more 

 remote, and the latter become positive to the former. With 

 regard to the negative variation which follows almost imme- 

 diately upon stimulation, their opinions differ. Kunkel con- 

 siders it to be due to alterations in the protoplasm which 

 cause a disturbance of the diffusion-currents in the resting 



