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 



BIS— ^— i[i^..=__ consequent concavity, which is 



^^P concomitant to the negative 



ll ^ f^ 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, i 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 frictionless 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, 



Fig. 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. 



