EFFECTS OF STIMULATION 125 



of these reactions on a growing organ ? Diminution of 

 turgor has been shown to induce a retardation of growth, 

 the incipient contraction culminating in a marked con- 

 traction (pp. 67, 68, 74). Enhancement of turgor due to 

 indirect stimulation, on the other hand, has been shown to 

 induce expansion and acceleration of growth (p. 91). The 

 following experiments w^ere so devised that the growing 

 organ should itself record the responsive variations of 

 growth under direct and indirect stimulation. 



Experiment 73. Effect of direct and indirect radio-thermal 

 stimulation. — The experimental specimen was a flower-bud 

 of Crinum, held by a clamp a little below^ the region of growth 

 {cf. fig. 48). Radio-thermal stimulus was apphed below the 

 clamp, so that the transmitted impulse had to pass through 

 the securely held intervening tissue. The stimulus was 

 unilaterally apphed at a point about 30 mm. below the 

 region of active growth. The first stimulation was feeble, 

 and brought about an acceleration of growth on the same side 

 with expansion and convexity, the resulting movement being 

 negative, or away from the stimulus ; this effect is attri- 

 butable to the positive impulse transmitted to the region of 

 growth. The latent period was 5 seconds, and the maximum 

 negative movement was completed in the further course of 

 10 seconds, after which there w^as a recovery in the course of 

 a minute and a half. A stronger stimulus S' gave a larger 

 response; but when the intensity was raised still higher 

 to S", the positive was overtaken by the excitatory nega- 

 tive impulse within 15 seconds of the commencement of the 

 positive response ; the convex w^as then succeeded by a 

 concave curvature, the response being therefore diphasic 

 (fig. 68). When the stimulation was direct, that is, applied 

 at or near the region of growth, the response was a cur- 

 vature towards the stimulus. 



I will briefly summarise the results obtained with growing 

 organs under direct and indirect stimulation. The effect 

 induced by feeble stimulus applied at a distance from the 

 growing region is a positive variation or acceleration of 



