5l6 PLANT RESPONSE 



at A on its own side. This transmitted effect of increased 

 turgidity we have already distinguished as the indirect effect 

 of stimulation. 



But all cells conduct stimulus more or less efficiently, the 

 difference being one only of degree. Hence an indifferently 

 conducting tissue, such as that of the tip, will conduct the 

 true excitatory state only if the stimulation be sufficiently 

 strong and long continued. The transmitted effect of this 

 true excitation may then be expected to produce negative 

 turgidity-variation, and concomitant galvanometric nega- 

 tivity, at A. 



When the stimulus is applied, however, at or near the 

 responding point A, we may expect to obtain the direct effect 

 of stimulation — that is to say, a negative turgidity-variation 

 and the concomitant galvanometric negativity. 



To sum up, then, it may be expected that moderate 

 unilateral stimulus applied at the tip T will give rise on the 

 same side of the responding region to the indirect effect of 

 stimulation, which is an increase of turgidity exhibited by a 

 positive electrical variation. The direct effect of stimulus, 

 whether immediate or transmitted, always produces a negative 

 turgidity-variation, evidenced by galvanometric negativity. 

 This effect may be obtained either by the local application 

 of moderate, or by the distant application of strong, stimulus. 



In making the electrical investigation 'which is now to be 

 described, I employed various forms of stimulation — thermal, 

 mechanical, chemical, and the stimulus of light. Mechanical 

 stimulus may be applied by friction of emery paper, or by 

 means of a pin-prick. Chemical stimulation is applied by 

 touching the point with a brush which has been moistened in 

 hydrochloric or sulphuric acid. Very dilute acid produces 

 moderate, strong acid a more intense, stimulation. The most 

 perfect mode of stimulation is by means of incident light, the 

 intensity of which may be varied at will. The effect ot 

 stimulus of light, however, will be fully described in the 

 chapter on heliotropism. Another form of stimulation which is 

 also very suitable is the thermal. A short piece of platinum 



