Page:Life Movements in Plants.djvu/116

86 excitatory response of the upper half is also seen to be very sluggish.

Experiment 26.—It is obvious that the mechanical moment exerted by the weight of the leaf must help its responsive fall under excitation. But the relative importance of the factors of active contraction of the lower half of the pulvinus and of the weight, in the rate of the responsive down-movement, still remains to be determined. A satisfactory way of solving the problem would lie in the study of the characteristics of response-records taken under three different conditions: (1) When the leaf is helped in its fall by its weight; (2) when the action of the weight is eliminated; and (3) when the fall has to be executed against an equivalent weight. An approximation to these conditions was made in the following manner. We may regard the mechanical moment to be principally due to the weight of the four sub-petioles applied at the end of the main petiole. In a given case these sub-petioles were cut off, and their weight found to be 0.5 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 0.5 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 there is very