Petiole-Pulvinus Preparations of Mimosa pudica. 223 



moment to be principally due to the weight of the four sub-petioles applied 

 at tbe end of the main petiole. In a given case these sub-petioles were cut 

 off, and their weight found tp be 05 grm. The main petiole was now 

 attached to the right arm of the lever, and three successive records were 

 taken : (1) With no weight attached to the petiole ; (2) with 0"5 grm. 

 attached to its end ; and (3) with 05 grm. attached to the left arm of the 

 lever at an equal distance from the fulcrum. In the first case, the fall due 

 to the excitatory contraction will practically have little weight to help it ; in 

 the second case, it will be helped by a weight equivalent to those of the 

 sub-petioles with their attached leaflets ; and in the third case, the fall will 

 be opposed by an equivalent weight. "We find that in these three cases tbere 

 is very little difference in the time taken by the leaf to complete the fall 

 (fig. 6). 



It has been shown that the presence or absence of the upper half of the 



Fig. 6. — Effect of weight on rapidity of fall. N, without action of weight ; W, with 

 weight helping ; and A, with weight opposing. 



pulvinus makes practically no difference in the period of fall ; it is now seen 

 that the weight exerts comparatively little effect. We are thus led to 

 conclude that in determining the rapidity of fall, the factors of expansive force 

 of the upper half of the pulvinus and the weight of the leaf are negligible 

 compared to the active force of contraction exerted by the lower half of the 

 pulvinus. 



7. The Action of Chemical Agents. 



In connection with this subject it need hardly be said that the various 

 experiments which I had previously carried out with the intact plant can also 



