Chap, xi Sensitivity and Stimulation 483 



which he said is a vital rather than a chemical process, 

 The changes in the cells were scrutinized very minutely 

 in 1896 and 1899 by Miss Huie, and in 1899 by Rosenberg. 

 Both observers agreed that, as a result of their activity, the 

 protoplasm and nuclei diminish in amount while an increase 

 in the volume of the cell sap becomes apparent. 



In the same memoir to which allusion has been made 

 Darwin discussed very completely the mechanisms of 

 Dionaea muscipula, the leaves of which after contact stimu- 

 lation close after the manner of a pair of hinges. He 

 showed them to possess chemotropic as well as tactile 

 sensitiveness, though the latter is the more evident. The 

 leaves agree with those of Drosera in responding to stimula- 

 tion by secretion as well as by movement. Darwin was 

 able to make out a differentiation between the two forms of 

 irritability similar to that noticed in the case of Drosera. 

 The sensitive area for the motor effects he determined to 

 be the three large hairs on each lobe of the leaf, mechanical 

 excitation of which does not call into play the secretory 

 activity of the glands. The general surface of the leaf appre- 

 ciates chemical stimulation, which is always followed by 

 secretion and, when sufficiently prolonged, by closure also. 

 Munk found, in 1876, that the sensitiveness is not confined 

 to the large hairs, though these manifest it most strongly. 

 In 1877 Batalin found that the closure of the leaf is effected 

 by variation of turgor in a strip of tissue on each side of the 

 midrib and parallel with it, the midrib itself being unaffected. 

 The vital character of the changes and the electrical dis- 

 turbance associated with them were investigated by Burdon- 

 Sanderson in 1873, 1877, l882 > and l888 - He showed the 

 electrical variations to be essentially the same as those 

 associated with the stimulation of muscle. Kunkel made 

 a similar observation in 1878 with regard to electrical 

 changes following the stimulation of the leaf of Mimosa. 



Darwin showed that Pinguicula vulgaris, among the less 

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