1 6 PLANT RESPONSE 



The most important question with regard to the application 

 of suitable stimulus is, as has been said, that it should be 

 capable of exact measurement, of uniform repetition, and of 

 definite increase or decrease at will. Another point which 

 must be borne in mind is that the application of stimulus 

 should not, by causing injury, change the excitability of the 

 organ. As, moreover, a magnified record of the responsive 

 movement is to be made immediately after the application, 

 any stimulus which causes the slightest jar must necessarily 



Fig. 12. Plant Chamber and Recorder 

 The glass cover is not shown. 



be avoided. And for these reasons the mechanical form 

 of stimulation is inappropriate to the investigation. The 

 three most perfect modes of stimulation which I have 

 been able to render practicable are, then, the thermal, the 

 electrical, and the stimulus of light. The action of the last 

 will be described in detail in another chapter, and we shall 

 for the present confine our attention to the first two. 



Electro-thermic stimulator.— Thermal stimulus may be 

 applied very easily by touching the plant with a hot wire, but 



