RESPONSE OF TIPS OF SHOOT ANT) ROOT 



519 



3. Transverse transmission of effect of stimulus : 



{a) Moderate stimulation. — We have seen that the direct appli- 

 cation of stimulus at a point has the local effect of a nega- 

 tive turgidity-variation. It is of great theoretical importance 

 that the effect of this stimulus on the diametrically opposite 

 point should be clearly demonstrated. As conductivity has 

 already been shown to be very feeble across a tissue (p. 250), 

 we might expect that it would be 

 the indirect effect, that is to say the 

 positive turgidity-variation, which on 

 stimulation of A would reach the 

 diametrically opposite, or distal, 

 point B. The experiment is carried 

 out, by making galvanometric con- 

 nections with B and L, and applying 

 stimulus at A (fig. 212, b). 



On now applying moderate sti- 

 mulus at A, we obtain a positive elec- 

 trical response, indicating positive 

 turgidity-variation, at P> (fig. 217) — 

 a result precisely the same as was 

 obtained by stimulating the distant 

 tip. The effect, then, of stimulating 

 any point is to induce a negative 

 turgidity-variation of the point itself, 

 and a positive turgidity-variation of the diametrically opposite 

 point. 



{b) Stronger stimulation. — When stronger stimulus is 

 applied, however, at A, the true excitatory effect is gradually 

 transmitted across the tissue, and we obtain at l> the neu- 

 tralisation of the preliminary positive effect, as in fig. 214. 

 And, lastly, when the point A is very strongly stimulated, the 

 responsive effect on the diametrically opposite point P. is a 

 transient positive, followed by a strong negative variation, 

 as in fig. 215. 



We have thus studied the separate effects produced at 

 A and P by stimulation of a point near A. The differential 



Fig. 217. Record showing 

 Positive Electrical Varia- 

 tion indicating Positive 

 Turgidity-Variation of Dis- 

 tal Point, under Moderate 

 Stimulation of Proximal 



