530 



PLANT RESPONSE 



I have been at some pains to make these examples of 

 the direct and indirect effects of stimulus clear ; for all the 

 complex curvatures of growth, which appear at first sight so 

 anomalous, are ultimately resolvable into these. And since 

 the subject is so important, I shall add still another demon- 

 stration, which is capable of easy repetition, and will be 

 found to be striking and convincing. 



Experiments on the direct and indirect effects of 

 stimulus on Mimosa. — We have now seen that all growth- 

 curvatures may be analysed 

 into, (i) that contraction, with 

 consequent concavity, which is 

 concomitant to the negative 

 turgidity-variation that consti- 

 tutes the direct, or transmitted 

 direct, effect of stimulus ; and (2) 

 that expansion, with consequent 

 convexity, that is concomitant to 

 the positive turgidity-variation, 

 which constitutes the indirect 

 effect of stimulus. Now, the 

 negative turgidity-variation is 

 exhibited in the case of the 

 motile leaf of Mimosa by depres- 

 sion, and the positive turgidity- 

 variation by erection. 



I shall now explain the 

 experimental arrangements by 

 which the plant itself may be 

 made to record these opposite effects. The indicating leaf is 

 attached to the short arm of a long writing lever. This lever 

 consists of the quill of a long tail-feather of a peacock. Its 

 short arm, 1 cm. in length, is tied by a thread to the petiole 

 of the leaf. A fine needle is passed through the quill, and 

 rests on frictionlcss supports which may be glass tubes. 

 The longer arm of the lever, 10 cm. in length, has a piece 

 of bent aluminium, with a sharp point, tied to the end, 



s-Vl 



FlG. 223. Experimental Arrange- 

 ment for obtaining Records 

 on Smoked Drum of Responses 

 given to Direct and In- 

 direct Stimulation by Leaf of 

 Mimosa 



Thermal stimulator at S produces 

 direct stimulation, and conse- 

 quent fall of leaf. Moderate 

 stimulation, at a distant point, 

 S„, gives rise to indirect effect 

 of erection. 



