84 PLANT RESPONSE 



long-continued stimulation diminishes the excitability of a 

 tissue. 



From these two experiments it will be seen that loss 

 of excitability may be produced in, amongst others, two 

 different ways. First, there is the molecular sluggishness, 

 induced, as we have seen, by cooling, which is, however, 

 only temporary, the original sensitiveness being restored on 

 warming. Secondly, we may have loss of sensibility due 

 to overstrain, owing to strong or long-continued stimulation. 

 This strain-effect, with its attendant loss of excitability, may, 

 if excessive, prove more or less permanent. It will thus be 

 seen that in consequence of unilateral stimulation, a mole- 

 cular differentiation is produced, in consequence of which an 

 organ originally radial becomes physiologically anisotropic. 

 The unstimulated portion of the organ is now the relatively 

 more excitable, and on diffuse stimulation becomes concave. 

 Molecular anisotropy induced under natural condi- 

 tions. — We shall next observe the induction of such mole- 

 cular anisotropy under natural conditions. The stem of a 

 }-oung Gourd is at first erect and strictly radial ; but later it 

 bends over, and then assumes a creeping habit. Under these 

 conditions, its upper surface is subjected to the unilateral 

 action of vertical light, the lower half being shaded and 

 protected. The organ is no longer radial, then, but plagio- 

 tropic, and from what has been said already, we shall expect 

 its upper or exposed surface, in consequence of the prolonged 

 action of stimulus of sunlight, to be less excitable than the 

 lower or shaded surface. This inference I have been able to 

 verify by means of the electric mode of investigation. I find 

 that on simultaneously exciting both sides of such a stem, 

 a current of response flows from the lower to the upper 

 surface ; hence it will be seen, according to the third law of 

 electrical response, as enunciated at the end of Chapter III., 

 that this lower side is the more excitable, and ought to 

 become concave under diffuse stimulation. 



Such induced concavity in response to diffuse stimulation 

 I have found in the case of various plagiotropic stems, for 



