Page:Life Movements in Plants.djvu/125

Rh darkness there is a great depression of excitability. Blinds were next pulled up and the records now obtained exhibit the normal excitability under light. The sky had by this time become brighter, and this accounts for the slight enhancement of excitability. This experiment proves conclusively that light has a direct stimulating action on the pulvinus, independent of photo-synthesis.

On isolation of a petiole-pulvinus preparation, the shock of operation is found to paralyse its sensibility. After suitable mounting the excitability is restored, and remains practically uniform for nearly 24 hours. After this a depression sets in, the rate of fall of excitability becomes rapid 40 hours after the operation, sensibility being finally abolished after the fiftieth hour.

Experiments carried out on the effect of weight, and the influence of selective amputation of the upper and lower halves of the pulvinus, show that in determining the rapidity of fall of leaf, the assumed factors of the expansive force of the upper half of the pulvinus and the weight of the leaf are negligible compared to the force of active contraction exerted by the lower half of the pulvinus. The excitability of the lower half is eighty times greater than that of the upper.

Chemical agents induce characteristic changes in excitability. Hydrogen peroxide acts as a stimulant. Barium chloride renders the recovery incomplete; but tetanisation temporarily removes the induced sluggishness. Acids and