DOMAIN OF EVOLUTIONARY HYPOTHESIS 113 



According to Pfeffer l the stimuli are associated 

 with the whole of the vital action, and there is perhaps 

 no single action ' in which these do not and must not 

 play a role . . . they are thus a general quality of 

 all living substance/ 



Haberlandt calls ' all organisms, animals and plants, 

 excitable/ 3 Then follows almost word for word the 

 definition given above by Strasburger. Similar opinions 

 were already held by Treviranus ; Haberlandt says 

 of him, 3 ' that he had correctly grasped the signification 

 of vital excitation/ Treviranus, however, attributes 

 the whole vital process to stimuli. 



The term ' excitability ' may, however, be more 

 narrowly defined, as is done by Haberlandt in his 

 work ' Sinnesorgane im Pflanzenreich/ He discusses 

 therein the special arrangements of many plants where- 

 by ' the sudden deformation of the sensitive proto- 

 plasm which is essential to the " excitation " becomes 

 particularly easy and marked. This is also the most 

 general building principle of this apparatus/ 4< 



Many plants have thus --and this Haberlandt has 

 described in a masterly fashion and determined by 

 experiment special apparatus for receiving definite 

 stimuli for instance, mechanical contacts. Such 



1 In Pflanzenphysiologie, I, Leipzig, 1897, p. 10, Pfeffer objects 

 * emphatically ' to the assumption that only certain striking phenomena 

 of motion were due to excitation, as, for instance, the sudden movement of 

 the sensitive plant Mimosa pudica (p. 11). 



2 G. Haberlandt: Physiologische Pflanzenanatomie,Lei$zig, 1909, p. 520. 



3 Sinnesorgane im Pflanzenreich zur perzeption mechanischer Reize, 

 Leipzig, 1906, p. 4. 



4 Parenthesized in the original. 



