Artificial darkness induces a closure of the leaflets, the closure being completed in the course of an hour. On readmission of light, the leaflets become fully expanded in the course of one hour and a quarter. The leaflets are extremely sensitive to light, closure movement being induced by the transitory passage of a cloud. The effect of rise of temperature is antagonistic to the action of light. The movement of opening during the course of the day is due to the effect of light overpowering the effect of rise of temperature. Under daily variation of light and darkness, the movement of closure is initiated at about 5 p.m., when the light is undergoing a rapid diminution. The movement of closure is complete by 9 p.m. The leaflets remain closed till about 5 a.m. next morning, after which they begin to open and become fully expanded by 9 a.m. The terminal leaflet of _Desmodium gyrans_ exhibits a diurnal movement which is very similar to that of _Cassia_. It begins to open early in the morning and remains outspread during the whole day; the leaflet exhibits a rapid down-movement after 5 p.m. and becomes closely pressed against the petiole in the course of about two hours. The midday sleep of leaflets of _Mimosa_ and _Averrhoa_ is due to the excitatory action of strong sunlight on the pulvinule, the more excitable half becoming contracted under excitation. In _Mimosa_ leaflets it is the upper, and in _Averrhoa_, it is the lower half of the pulvinule that is the more excitable. It is in consequence of this that the diffuse excitation of strong sunlight causes the leaflets of _Mimosa_ to fold upwards, those of _Averrhoa_ to fold downwards. XLVIII.--DIURNAL MOVEMENT DUE TO VARIATION OF TEMPERATURE AFFECTING GROWTH _By_ SIR J. C. BOSE, _Assisted by_ LALIT MOHAN MUKERJEE. It has been stated that there are two classes of diurnal movements caused by variation of temperature; one of these is due to differential growth induced on two sides of the organ, and the other is brought about by the induced variation of geotropic curvature. The former may be distinguished as _Thermonastic_, and the latter as _Thermo-geotropic_ movement. Before laying down the criteria to distinguish the one class of phenomenon from the other, it would be advisable to refer to the somewhat arbitrary distinction that has been made between nastic and tropic reactions. TROPIC AND NASTIC MOVEMENTS.