28 ELECTRO-PHYSIOLOGY CHAP. 



20 C., about 1 sec. Under all circumstances the electrical 

 variation long precedes the excitatory movement. 



For more exact investigation two methods were employed by 

 Burdon-Sanderson. In the first " a light straw lever was cemented 

 to two of the marginal bristles of a lobe, the opposite lobe being 

 fixed to a support. The lobe thus attached was mechanically 

 excited in such a way that the time of the exciting impact was 

 recorded on a horizontally-moving smoked plate below the curve 

 drawn by the lever." 



In the second method the leaf was fixed in the same way, 

 but a small mirror was cemented to the lower surface of the free 

 lobe, by means of which the image of a horizontal slit was 

 thrown on to a vertical scale, so arranged that the movement of 

 the lobe could be exactly measured. 



The result showed that with a temperature of 15 20 C. the 

 closing movement of a lobe, consequent on a single adequate ex- 

 citation, lasted 5-6 sec., occurring rapidly at first, and then with 

 diminishing speed. 



With a succession of very weak mechanical stimuli (gentle 

 impact on one of the sensitive hairs), inadequate singly to 

 produce complete closure of the leaf, Burdon-Sanderson observed 

 an effect which to some extent recalled the " staircase " of direct 

 muscular excitation, the mechanical effect of the movement iu 

 each successive stimulation being greater than in those that 

 preceded it. This reaction must, however, be referred to the 

 fact that the resistance of the cells of the upper surface of the 

 leaf to the closing movement, due to turgor, diminishes with each 

 new excitation. " The magnitude of each diminution of resistance 

 produced by excitation grows with each repetition of the stimulus, 

 until the leaf finally -closes together " (Burdon-Sanderson). 



It may be assumed as certain that galvanic excitatory 

 manifestations, similar to those of Dioncea, are to be observed in 

 the pulvini of the no less sensitive leaves of Mimosa. Un- 

 fortunately there are, so far, few observations on this point. 

 Kunkel, who, as we have seen, found a strong " current of rest " 

 in the cushion of the primary leaf-stalk, the bristle being positive 

 to the upper surface of the joint, observed, with the capillary 

 electrometer, a variation of this current (consisting of a number 

 of alternating oscillations) at the moment when contact with the 

 most excitable point of the lower circumference of the pulvinus 



