528 PLANT RESPONSE 



(b) Strong or long- continued stimulus. — I have already 

 explained that with moderate stimulation the negative 

 turgidity-variation and concomitant contraction of the proxi- 

 mal, and the positive turgidity-variation and concomitant 

 expansion of the distal, conspire to induce a positive respon- 

 sive curvature ; but when the stimulus is strong or long- 

 continued the excitation is conducted across the tissue to 

 the distal side, which, now contracting, antagonises and 

 reverses the action of the proximal. We have seen this ex- 

 emplified in the electrical response given in fig. 218, where the 

 first positive electrical response of the distal, indicating positive 

 turgidity-variation, was afterwards neutralised and converted 

 into negative by the transverse conduction of excitation. 



In the case of responsive growth-curvature we obtain 

 results precisely similar. Long-continued unilateral stimula- 

 tion is here often found to neutralise the first or normal 

 effect. Or if, again, the unilateral stimulation be very strong, 

 the proximal side is liable to become fatigued, and the 

 response of the distal to transmitted stimulus being thus 

 predominant, a responsive movement occurs which is reversed 

 or negative^ that is to say, away from the source of stimula- 

 tion. Examples of this will be seen in Chapter XLII. 

 I shall also presently give a demonstration of similar 

 preliminary effects with subsequent transverse conduction, 

 giving rise to reversed effects, in the case of the motile 

 response of Mimosa. 



We have thus seen that growth-curvature is induced by 

 unequal variations of turgidity, on the diametrically opposite 

 sides of the growing region. We have seen that the effect 

 of indirect stimulation is a positive turgidity-variation. When 

 a feebly conducting tissue is unilaterally subjected to moderate 

 stimulation, the direct excitatory effect cannot be transmitted 

 far, and it is the indirect effect which reaches the responding 

 region, R (fig. 222), and induces convexity. Hence we obtain 

 the typical examples of this effect by stimulating the tip, T, 

 of cither root or shoot. The sensibility of these regions is 

 itself in no way different from that of any other portion of 



