CHAP, xi Sensitivity and Stimulation 487 



had been observed and to a certain extent explained by 

 Knight as long ago as 1811, was made the subject of very 

 careful research by Sachs in 1872. It was further investi- 

 gated by Darwin in 1880, and the sensitiveness was located 

 in the root apex, as had been other perceptive powers. 

 Wiesner and Detlefsen both opposed Darwin's view, but 

 Molisch confirmed it fully in 1883. Pfeffer, some ten years 

 later, came to the conclusion that not only is the apex the 

 sensitive area, but that the irritability is confined to it. If 

 the apex is uniformly wet on all sides, no curvature of the 

 growing zone occurs. Molisch failed to find this kind of 

 sensitiveness in the stem. Wortmann showed, in 1881, 

 that the sporangiophores of the Mucorini grow away from 

 moisture. Hydrotropic irritability was observed in 1883 in 

 the rhizoids of Marchdntia by Molisch, and in 1894 by 

 Miyoshi in pollen -tubes. 



Curvature under the influence of injury was first examined 

 by Darwin in 1880 in his studies of the effect produced 

 by amputating root-tips, and was studied at some length 

 by Spalding in 1894. It has been called traumotropism. 

 Massart, in 1889, found that concentrated solutions of 

 various salts affect several free-swimming organisms, 

 independently of their chemical nature; he introduced 

 the term tonotaxis for this peculiarity. Pfeffer wrote of the 

 phenomenon as osmotaxis, as did Rothert subsequently. 

 In 1883 Jonsson discovered that plants are able to execute 

 curvatures in response to the movement of the water in 

 which they live. He called this kind of irritability rheo- 

 tropism. Elfving in 1882, Miillerin 1883, and Brunchorst in 

 1884, noticed that the radicles of certain seedlings are 

 variously affected by the passage of an electric current, 

 the response being in some cases positive, in others negative. 

 To this property the term galvanotropism was applied. 

 But little attention was paid to it up to the end of the 

 century. 



