RESPONSIVE CURVATURE OF ANISOTROPIC ORGAN 83 



say A, is now immersed in ice-water, the other being dipped 

 in water at the ordinary temperature. The two halves are 

 next taken out of the water and bound together, from their 

 free ends upwards, so as to make once more a single tube. 

 The petiole is now held by the uncut part vertically, with 

 a long index projecting downwards from its lower end. 

 The petiole was, as will be remembered, originally radial; 

 but now, by the local application of cold to one half, a 

 certain molecular differentiation has been induced, and the 

 particles in the cooled, or A half, are now therefore more 

 sluggish and irresponsive than those of the B side. This 

 induced molecular differentiation, moreover, is invisible to 

 the eye, and had we not observed the process by which it 

 was brought about, it would have been impossible, from mere 

 visual inspection, to know which side had been subjected to 

 cold. The application of diffuse stimulus, however, reveals it 

 at once ; for on passing electrical shocks along the length 

 of the specimen, the relatively more excitable, or uncooled B 

 half, becomes concave. It is clear, then, that at this moment 

 the B half is the more excitable, and stimulus acts on 

 it preferentially. But after long-continued stimulation, the 

 15 half becomes overstrained, and its excitability undergoes 

 diminution or fatigue. At the same time, by means of 

 repeated shocks, the sluggishness of the A half has been 

 gradually made to disappear ; that side now regains its 

 excitability; and, the excitation of A becoming thus rela- 

 tively greater, the curvature of the specimen is reversed. 



I have said that excitability is diminished under the 

 molecular strain caused by over-stimulation. I shall now 

 demonstrate this by another and independent experiment. A 

 new specimen, slit like the last, is taken, and one half, say A, 

 is alone subjected to strong excitation, by sending electric 

 shocks along its length. The two halves are again brought 

 together, and on now subjecting the whole petiole to electric 

 stimulation, it is found that the fresh, or previously unexcited, 

 half is that which becomes concave, thus proving that 

 the fresh half is the more excitable, and that strong or 



