488 The Physiology of Plants BOOK in 



The power of appreciating and responding to vibration 

 was demonstrated by several observers ; it was called seismo- 

 tropism by Pfeffer. It was first spoken of by Prillieux in 

 1868, when he showed that if a shoot is struck repeatedly 

 but gently near its apex, the latter slowly curves over to- 

 wards the opposite side, so that the side which is struck 

 becomes convex. Blows administered to the shoot low down 

 cause a curvature towards the stimulated side. Pfeffer 

 showed, in 1885, that this kind of sensitiveness is exhibited 

 by tendrils, motile pulvini, and certain leaves (Biophytum 

 sensitivum) . The mechanism of perception is still uncertain, 

 while the resulting movements are usually those of varia- 

 tion and not of growth curvature. 



GENERAL CONSIDERATIONS AS TO IRRITABILITY 



The true nature of irritability appears to have been first 

 hinted at by Frank in 1870, but was only slowly developed. 

 The great accumulation of evidence that we have traced 

 through the next decade served to establish it firmly in the 

 current opinion of the time, and in 1879 we find Sachs 

 saying that the living material of plants is internally 

 differentiated in such a way that different parts are supplied 

 with specific energies resembling those of the sensory nerves 

 of animals. The coexistence of various forms of sensitive- 

 ness in the same organ was shown first by Sachs, then by 

 Miiller in 1876, and again later by Wiesner. Heliotropic 

 curvature occurs in a plant rotating on a klinostat so 

 as to eliminate its natural geotropism. Not only their 

 coexistence, but their power of independent manifestation 

 was shown by Noll in 1892, when he found that during 

 marked heliotropic excitation the manifestation of geo- 

 tropic sensibility is inhibited. 



As we have seen, detailed observations of the manifesta- 

 tion of the various kinds of irritability or forms of sensitive- 



