DIRECT AND INDIRECT STIMULATION II7 



the distance traversed and by the conducting capacity of 

 the tissue ? To these questions an answer is obtained from 

 the study of the effects of Direct and Indirect Stimulation : 

 by Direct stimulation is imphed appHcation of stimulus at 

 or near to the responding organ ; by Indirect stimulation, 

 application of stimulus at some appreciable distance from 

 the responding organ. 



Direct stimulation. — Taking the case of an excitable 

 shoot of Mimosa, direct stimulation of the pulvinus of a 

 leaf at once causes it to fall. The same effect is produced 

 when the stimulus is applied to the petiole, for the petiole 

 is a fairly good conductor. 



Indirect stimulation. — Continuing the experiment with the 

 identical shoot, the same stimulus is now applied to the 

 stem, with the result that, instead of faUing as previously, 

 the nearest leaf shows an erectile movement, which may 

 or may not be followed by a fall. 



These different effects suggest that stimulation generates, 

 in the stimulated tissue, two impulses. The one, which 

 induces the fall of the leaf, has been shown to be excitatory, 

 that is, of the nature of protoplasmic excitation, and since 

 the response which it evokes is designated negative, so the 

 impulse itself may conveniently be termed negative also. 

 The other, which induces the rise of the leaf, that is a positive 

 response, may likewise be termed the positive impulse. 

 But what is its nature ? On grounds which have been fully 

 discussed in previous works, more especially in the ' Ascent 

 of Sap ' (p. 247), I have come to the conclusion that it is 

 an hydraulic impulse, of the nature of a wave of increased 

 hydrostatic pressure which, originating in the contraction 

 of the cells at the point of stimulation, travels to the pulvinus, 

 causing it to expand. The two impulses produce opposite 

 effects : the excitatory nervous impulse causes contraction 

 of the responding cells, and is attended by galvanometric 

 negativity ; the hydraulic impulse causes expansion of the 

 cells, and is attended by galvanometric positivity. More- 

 over, the hydrauHc impulse resulting from stimulation 



