462 The Physiology of Plants BOOKIII 



tion. This was one of the most remarkable discoveries of 

 the period. 



Among the writers of the time, one especially must 

 be mentioned who exercised considerable influence on the 

 progress of scientific thought. This was Pfeffer, whose 

 many contributions were of the first importance to the 

 elucidation of the problems arising from the new presenta- 

 tion. Pfeffer carried the subject of stimulation much 

 further than previous writers, discussing the relations 

 between the stimulus and the response, and emphasizing 

 the purposeful character of the latter. He stated clearly 

 the view that light and gravitation act by releasing forces 

 in the interior of the plant, the particular manifestation 

 following being determined by the organization of the part 

 concerned. He laid great stress, in support of his position, 

 on the disproportion between the energy imparted in 

 the stimulation and that released in the movement 

 following. 



The purposeful character of the responses to stimulation 

 necessarily drew attention to the protoplasm as dominating 

 them. A recognition of the part it plays in them only 

 slowly made its way into current thought. The apprecia- 

 tion of protoplasm in this matter appeared very clearly 

 in the Vorlesungen of Sachs in 1882. In these lectures, 

 after emphasizing the idea of purpose in response, he 

 pointed out that it follows that protoplasm must be the 

 source at once of perception and action, the phenomena 

 being associated with active life. His views on the mani- 

 festation of protoplasmic excitation were, however, not so 

 clear. He confused the phenomena presented by the proto- 

 plasm with those which are the special characteristics of 

 the mechanisms by which it carries out its purpose. 



The special phenomena attending on excitation were first 

 demonstrated by Burdon-Sanderson in his researches on 

 the closing of the leaf of Dionaea, when stimulated. By 



