526 



PLANT RESPONSE 



Stimulus. I performed a similar experiment on the tip of 

 the root of Bindweed, the response being precisely the same. 

 The rate of movement was in this case somewhat slower, but 

 persi-sted for eight minutes. The application unilaterally of 

 dilute sulphuric acid brought about exactly similar results in 

 both cases. The unilateral application of thermal stimulus of 

 moderate intensity, again, to the tip, induced movement away 

 from the source of stimulation in 

 both shoot and root, as will be seen 

 from the record given below (fig. 219). 

 {b) Stronger stimulation. — A 

 somewhat stronger stimulation of the 

 same character caused a movement 

 away, followed by movement towards, 

 the source of stimulus, the resultant 

 effect being neutral. We now pass 

 on to the effects induced by still 

 stronger stimulation. I have already 

 explained how, when very strong 

 stimulus is applied unilaterally at the 

 tip of an organ, its first and transitory 

 effect is to induce a positive turgidity- 

 variation on the same side of the 

 growing region. From this we in- 

 ferred the occurrence of a convexity on that side, which 

 would carry the tip away from the source of stimulus. The 

 direct effect of stimulus next reaches the responding region, 

 reversing the first effects and causing a negative turgidity- 

 variation, which would, pari passu, induce a concavity, and 

 carry the tip towards the source of stimulation. 



In carrying out the experimental verification of these 

 mechanical movements on a bud of Crocus, I found that an 

 application of strong sulphuric acid on one side of the bud 

 caused it to move first away from, and then very energetically 

 towards, the direction in which the application was made. I 

 next tried to determine the effect of a strong unilateral 

 application of thermal stimulus on the root-tip of Bindweed. 



Fig. 219. Mechanical Re- 

 sponses of Shoot, s, and 

 of Root, R, to Unilateral 

 Stimulus applied at the 

 Tip 



In this and following records 

 the down curve indicates 

 negative movement or away 

 from source of stimulus. 

 The up curve indicates 

 positive or movement to- 

 wards source of stimulus. 

 The time-marks represent 

 minutes (cf. fig. 213). 



