RESPONSE OF LEAVES 237 



I shall now, however, proceed to describe a series of ex- 

 periments exactly parallel to the ' fundamental experiment ' 

 on Dioncea, carried out on ordinary plants. We have seen 

 that the inner or concave surface of the sheathing petiole of 

 Musa is relatively more .excitable than the outer or convex. 

 Thus it corresponds with the ' internal ' or upper surface of the 

 leaf of Dioncea. The more excitable internal surfaces of both 

 these, again, correspond with the more excitable lower half 

 of the pulvinus of Mimosa. In fig. 149 is shown an experi- 

 mental arrangement with a specimen of Musa which will be 

 seen to be parallel to that of Burdon Sanderson's funda- 

 mental experiment on Dioncea. In order to avoid any such 

 disturbance as might conceivably arise from current-escape, if 

 the electrical form of stimulus were used, I employed the 

 thermal mode of stimulation. A momentary heating-current 

 passed through a thin platinum wire gave the thermal varia- 

 tion required, and was found to furnish a very satisfactory 

 form of stimulus. The led-ofT circuit was at first placed at a 

 distance of 16 mm. from the point of stimulation. As the 

 stimulation was moderate, and as the conductivity of the 

 tissue was not great, the effect induced at the respond- 

 ing circuit was hydro-positive, the more excitable concave 

 surface becoming positive (fig. 155 (a) ). This response is the 

 same as the positive responses given by the * unmodifiable 

 leaf of Dioncea (fig. 150), as well as that of a vigorous leaf 

 which had been 'modified' by fatigue (fig. 152). On next 

 taking a second pair of led-off points, at the shorter distance 

 of 8 mm., the hydro-positive effect reached the led-off points 

 earlier, and was followed by the true excitatory wave. This 

 is seen as a preliminary positive response, followed by the 

 excitatory negative (fig. 155 (ft) ). This again is the same 

 as the di-phasic response of a Dioncea leaf in its 'prime' 

 (fig. 151). In the experimental arrangement with Musa the 

 led-off circuit was now brought still nearer to a distance of 

 4 mm. There was now little interval between the arrival 

 at the led-off points of the hydro-positive and true excitatory 

 effects ; and since the latter is of predominant electrical 



