RESPONSE OF TIPS OF SHOOT AND ROOT 



521 



the distant responding organ. This will give rise to retarda- 

 tion of growth and consequent concavity of that side, the tip 

 being carried, by the responsive 

 movement, towards the source of 

 stimulation. 



2. (a) Stimulation of a point 

 at or near the responding region 

 of growth will induce a negative 

 turgidity-variation as the direct 

 effect of stimulus, and at the 

 diametrically opposite point a 

 positive turgidity-variation as the 

 indirect effect of stimulus. This 

 will give rise to concavity of the 

 proximal with convexity of the 

 distal sides. The mechanical 

 effects of negative turgidity- 

 variation on one side and posi- 

 tive turgidity-variation on the 

 other side are thus additive. In 

 this way, the concavity of the 

 proximal and convexity of the 

 distal conspire to bring about the 

 resultant curvature. 



(b) When the stimulus applied 

 at A is stronger, the effect is a 

 negative turgidity-variation at A, and later, owing to the 

 transmission of stimulus across the tissue, the positive is 

 succeeded by a negative turgidity-variation of B. The 

 resultant effect obtained by algebraical summation thus tends 

 to become zero. When the stimulus at A, however, is still 

 stronger and of longer continuance, the negative response of 

 A will be found to undergo a gradual diminution owing to 

 fatigue, while the transmitted effect at the diametrically 

 opposite point undergoes an increase. Under these circum- 

 stances the responsive negative change at B will predominate 

 over that at a. The final result may thus be a relative 

 negativity of B, that is to say an effect opposite to that seen 



Fig. 218. The Relative Elec- 

 trical and Turgidity Variations 

 of two Diametrically Opposite 

 Points, A and };, when Strong 

 and Long-continued Stimulus 

 is applied near A (cf. fig. 

 212, c). 



We see here (1), in the up curve, 

 the negative variation of the 

 proximal, followed by (2) 

 neutralisation, followed by (3) 

 reversal, that is to say, rela- 

 tive negativity of B. Note the 

 multiple response which here 

 makes its appearance. 



