MOVEMENTS OF IRRITATION. 533 



a plant structure to the short leg h. The lever dynamometer 

 may be obtained from Albrecht, Tubingen. 



We select for examination a vigorous specimen of Phaseolus. 

 We fix a wire along the midrib on the upper side of a primordial 

 leaf, so that it reaches up to the pulvinus at one end, and at the 

 other extends beyond the tip of the leaf. The wire must be such 

 that it will not bend under the forces to be liberated. Usually, 

 in experiments with bean leaves, a wire 60-80 mm. in length and 

 say 0'2 gr. weight, formed by twisting together two pieces of fine 

 iron wire, yields good results. The wire is tied to the leaf at at 

 least three places, the thread in each case being taken round the 

 wire and midrib. The end of the wire projecting beyond the leaf 

 is laid on the short arm of the lever, and it is best to fix it there 

 by means of thread or wire. 



Before proceeding to the experiments it remains to be deter- 

 mined what relation exists between the force acting on the short 

 leg of the lever on the one hand and the corresponding deflection 

 on the other. If the deflection is not too great, this relation can 

 be very exactly determined by suspending from the short leg a 

 scale pan, and loading this with weights. The same deflection 

 will naturally correspond with a greater or less pressure accord- 

 ing to the loading of the vertical arm p (by wax balls or 

 otherwise). One degree of deflection will correspond, e.g. 

 according to circumstances, with O'l gr. up to even 0'3 gr. The 

 product of the weight corresponding with the observed deflection 

 and the length of the leaf lying on the arm of the dynamometer, 

 and acting as the arm of a lever, gives the moment of the result- 

 ant pressure exerted by the pulvinus. 



In the experiments with bean leaves the petiole must naturally 

 be tightly fixed as far as the pulvinus. When the apparatus 

 is carefully arranged and vibration-free, changes in the expansive 

 force of the opposed halves of the pulvinus due to variations 

 in brightness evoke movements of the dynamometer legs. If 

 we place the apparatus in the dark, e.g. in the afternoon, the 

 pointer may be deflected 10 in the course of a few hours. 

 If 1 corresponds with a pressure on the short arm of 03 gr., and 

 if the lever formed by the leaf was 70 mm. in length, then the 

 10 would indicate a moment of 210. 



Bean leaves also execute autonomous movements of moderate 

 amplitude, each swing occupying about two hours. For these 

 the moment often lies between 10 and 25. According as these 



