From the record thus obtained, it was found that the excitability of the plant is not the same throughout the day, but undergoes a variation characteristically different at different times of the day. In a typical case in spring the excitability attained its maximum value after 1 P.M. and remained constant for several hours. There was then a continuous fall of excitability, the minimum being reached at about eight in the morning. The plant at this time was practically insensitive. The moto-excitability was then gradually enhanced in a staircase manner till it again reached a maximum next afternoon. The effect of sudden darkness was found to induce a transient depression, followed by revival of excitability. The effect of persistent darkness was to induce a depression. Exposure to light from darkness caused a transient depression, followed by an enhancement of excitability. Excessive turgor induced a diminished response. Lowering of temperature induced a depression of excitability, culminating in an abolition of response. The after-effect of excessive cold was a prolonged depression of excitability. Excitability was enhanced by rising temperature up to an optimum; above this point a depression was induced. Owing to physiological inertia the change of excitability induced by variation of external condition lags behind the inducing cause. The diurnal variation of excitability is primarily due to diurnal variation of temperature. The effect is modified in a minor degree by variation of light. V.--RESPONSE OF PETIOLE-PULVINUS PREPARATION OF _MIMOSA PUDICA_ _By_ SIR J. C. BOSE, _Assisted by_ SURENDRA CHANDRA DAS, M.A. The most suitable plant for researches on irritability of plants is _Mimosa pudica_, which can be obtained in all parts of the world. An impression unfortunately prevails that the excitatory reaction of the plant can be obtained only in summer and under favourable circumstances; this has militated against its extensive use in physiological experiments, but the misgiving is without any foundation; for I found no difficulty in demonstrating even the most delicate experiments on _Mimosa_ before the meeting of the American Association for the Advancement of Science held during Christmas of 1914. The prevailing outside temperature at the time was considerably below the freezing point. With foresight and care it should not be at all difficult to maintain in a hot-house a large number of these plants in a sensitive condition all the year round.