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 alkalis induce antagonistic reactions, abolition of excitability with alkali taking place in a contracted, and with acid in an expanded condition of the pulvinus. The responses exhibit fatigue when the period of rest is diminished. The passage of constant current is found to remove the fatigue. Response is enhanced on exposure to light, and diminished in darkness. Light is shown to exert a direct stimulating action on the pulvinus, independent of photo-synthesis. VI.--ON CONDUCTION OF EXCITATION IN PLANTS _By_ SIR J. C. BOSE. The plant _Mimosa_ offers the best material for investigation on conduction of excitation. With regard to this question the prevailing opinion had been that in plants like _Mimosa_, there is merely a transmission of hydro-mechanical disturbance and no transmission of true excitation comparable with the animal nerve. I have, however, been able to show that the transmission in the plant is not a mechanical phenomenon, but a propagation of excitatory protoplasmic change. This has been proved by the arrest of conduction by the application of various physiological blocks. Thus local application of increasing cold retards, and finally abolishes the conducting power. The conducting tissue becomes paralysed for a time as an after-effect of application of cold; the lost conducting power may, however, be quickly restored by tetanising electric shocks. The conducting power of an animal nerve is arrested by an electrotonic block, the conductivity being restored on the cessation of the current. I have succeeded in inducing similar electrotonic block of conduction in _Mimosa_. Conductivity of a selective portion of petiole may also be permanently abolished by local action, of poisonous solution of potassium cyanide.[K] [K] BOSE--“An Automatic Method for the Investigation of Velocity of Transmission of Excitation in _Mimosa_.” ‘Phil. Trans.’ ‘B, Vol. 204 (1913) and also “Irritability of Plants.” Longman’s Green & Co. (1913), p. 132. Having thus established the physiological character of the transmitted impulse in plants I shall now proceed to give some of the principal results of my earlier and recent investigations on the effects of various agencies on conduction of excitation in plants.