156 RESEARCHES ON IRRITABILITY OF PLANTS 



indiarubber tube having a large bore. In the former case 

 a considerable mechanical disturbance would be necessary 

 to start the hydrostatic wave which can effectively reach a 

 distant point. Haberlandt supposes such a mechanical 

 disturbance to be brought about by deformation of the mass 

 of parenchyma in the stimulated pulvinus or by injury of 

 the stem or petiole. But it is not at all necessary to 

 initiate the excitatory impulse in Mimosa by stimulating the 

 pulvinus ; such an impulse may be originated in the thin 

 petiole where there is no turgid mass of parenchyma to be 

 deformed. Excitation may be caused, moreover, by the 

 agency of a physiological stimulus which does not cause any 

 injury or give rise to any mechanical disturbance. 



Again, as regards the question as to whether the trans- 

 mitted variation of pressure would always form an efficient 

 cause of excitation, it was found by Macdougal that sudden 

 artificial variation, whether by increase or diminution of 

 hydrostatic pressure, brought about no responsive fall of 

 the leaf of Mimosa.'^ 



We now return to the detailed consideration of Pfeffer's 

 experiment on anaesthetics and Haberlandt's on scalding. 

 As regards the former it has been assumed that the conduct- 

 ing-power was arrested under chloroform. It has, however, 

 been pointed out by Vines that though a narcotised pulvinus 

 certainly loses its motile excitability, it does not necessarily 

 follow that its conductivity likewise is completely abolished. 

 In fact, instances are known to physiologists in which a 

 tissue whose excitability has been abolished will still persist 

 in maintaining its conducting-power. This circumstance 

 may be demonstrated in the case of plants by taking a 

 specimen of Biophytum and applying a strong stimulus to 

 an old leaf the motility of whose leaflets has been abolished 

 on account of age. Though its own leaflets do not afford 

 any motile indication, the excitation is found conducted 

 through the petiole of the old leaf, inducing the fall of the 

 leaflets in a neighbouring young leaf. 



* PfefifQr: Physiology of Plants, vol. iii. p. 95 (Cla,rendon Press), 



