2] THIGMOTROPISM 383 



through 45 as a result of contact irritation. The tendril con- 

 tinued to bend at the previously irritated point even after the 

 irritant had been removed. Thus a stimulated condition per- 

 sists after the removal of its inciting cause, and this gives a 

 chance for the building up of a powerfully stimulated condition 

 by the accumulation of slight stimuli. 



The persistence of the stimulated condition is revealed by 

 another set of changes. PFEFFER found that the tendrils of 

 Sicyos when repeatedly rubbed at short and regular intervals 

 first coil and then gradually straighten out. DARWIN" ('82, 

 p. 155), who had previously shown this same thing, found also 

 that after a tendril of Passiflora gracilis had been stimulated 21 

 times in 54 hours it finally hardly responded at all. Similar 

 results have been obtained with Drosera hairs (PFEFFER, '85, 

 p. 514). Thus the constantly repeated stimulation produces 

 such a modification of the protoplasm that it eventually fails to 

 respond. On the one hand, this phenomenon is the same as 

 that of fatigue ; on the other, it resembles closely the condition 

 seen in stimulated Protista referred to in Chapter IV, 3, and 

 there designated as acclimatization. 



7. Explanation of Thigmotropism. Concerning the cause of 

 the bending of plants as a result of contact, I know of no better 

 explanation than that offered by SACHS ('87, pp. 697-699). 

 He ascribes the bending to a difference in the rate of growth 

 on the two sides of the bending cylindrical organ. This is 

 indicated by the fact that it is chiefly in the region of 

 " stretching" that the response occurs ; i.e. shortly behind the 

 apex. At the very tip, growth by assimilation is chiefly occur- 

 ring ; below the region of stretching, growth is occurring only 

 slowly. The measurements of DE VRIES ('73) have, however, 

 shown directly that the convex side of the tendril grows faster 

 than the straight tendril and the concave side less rapidly. 

 The convex side thus pushes still farther over the concave, pro- 

 ducing the coiling. As the region of stretching is one of 

 imbibition of water, we conclude that more water is taken in on 

 the convex side than on the concave, where even a loss of water 

 may occur ; or perhaps there is a movement of water from the 

 irritated towards the opposite side. The irritant then produces 

 a chemical change on one side of the organism such as to cause 



