_Mechanical Stimulus._--Mechanical blow, friction, prick or cut. _Chemical Stimulus._--Effect of certain acids and of other chemical substances. _Thermal Stimulus._--Sudden variation of temperature; application of heated wire. _Radiation Stimulus._--Luminous radiation of the more refrangible portion of the spectrum; ultra-violet rays; thermal radiation in the infra-red region. All these different forms of stimulus induce an excitatory contraction, a diminution of turgor, and a negative mechanical response or fall of a motile leaf. SUMMARY. A radial organ responds to stimulus by contraction in length; as all its flanks are equally excitable there is no lateral movement under diffuse stimulus. Physiological anisotropy is induced in an organ, originally radial and isotropic, by the unequal action of the environment on its different sides. Diffuse stimulus induces a greater contraction of the more excitable side. In a curved tendril the concave side is less excitable than the convex. Diffuse stimulus tends to straighten the curved tendril. In the pulvinus of _Mimosa pudica_, the lower half is eighty times more excitable than the upper, and the fall of the leaf is due to the predominant contraction of the more excitable lower half. A diminution of turgor takes place in the excited cells. Restoration of turgor brings about recovery of the leaf to its normal erect position. Independent experiments show that the fall of the leaf may be brought about by an artificial diminution of turgor, and the erection of the leaf by an increase of turgor. IV.--DIURNAL VARIATION OF MOTO-EXCITABILITY IN _MIMOSA_ BY SIR J. C. BOSE. Several phenomena of daily periodicity are known, but the relations between the recurrent external changes and the resulting periodic variations are more or less obscure. As an example of this may be cited the periodic variation of growth. Here the daily periodicity exhibited by a plant is not only different in varying seasons, but it also differs in diverse species of plants. The complexity of the problem is very great, for not only are the direct effects of the changing environment to be taken into consideration but also their unknown after-effects. Even in the case of direct effect, different factors, such as light, temperature, turgor, and so on, are undergoing independent variations; it may thus happen that their reactions may sometimes be concordant and at other times discordant. The nyctitropic movement of plants affords another example of daily periodicity. The fanciful name of ‘sleep’ is often given to the closure of the leaflets of certain plants at night. The question whether plants sleep or not may be put in the form of the definite inquiry: Is the plant equally excitable throughout day and night? If not, is there any definite period at which it practically loses its excitability? Is there, again, another period at which the plant wakes up, as it were, to a condition of maximum excitability?