THE RECORD OF GROWTH-RESPONSE 419 



Yet there is a certain difference between these responses. 

 In the case of Biophytnvi, the response is due to a sudden 

 diminution, but in growth, to a sudden increase, of turgidity. 

 This might at first sight appear anomalous, but I shall 

 presently show that both are expressions of an excitatory 

 reaction ; for we have seen that when the responsive organ 

 of Biophytuni is directly excited there is an expulsion of 

 water, and the response is brought about by negative turgidity- 

 variation. This variation we shall for the sake of con- 

 venience distinguish as tJie direct effect of stimulation. We 

 saw in the last chapter, however, that an increase of the 

 internal energy of a plant gives rise to an opposite re- 

 sponse, that is to say, one characteristic of positive turgidity- 

 variation. The two responsive turgidity-variations, then, both 

 negative and positive, are alike in being expressions of the 

 excitatory reaction, though the negative variation is the effect 

 of external stimulus applied directly to the responding organ, 

 while the positive variation is to be regarded as the effect 

 of the internal energy of the plant. This internal energy 

 may itself have been derived previously by the plant from 

 external sources of stimulation, or the internal energy of a 

 given point may result from the application of a stimulus at a 

 distance. The pumping-in of water by the stimulated root is 

 an example of the latter case ; the cells are thus made tense, 

 and the potential energy of the tissue is raised above par. 

 Again, we may conceive of another interesting instance as 

 follows. When stimulus is applied at a distance the excita- 

 tory expulsion of water gives rise to a wave of increased 

 turgidity, which produces an abnormal positive response, and 

 this positive turgidity-variation we shall designate as the 

 indirect effect of stimulation. The wave of true excitation 

 may in this case reach the organ after that of positive 

 turgidity-variation, and give rise to the direct effect of stimu- 

 lation, that is to say, depression of the leaflet. We may next 

 imagine that the seat of stimulus is at so great a distance 

 from the responding organ, that the transmitted excitation 

 becomes too much enfeebled, by the long tract which has to 



