_Effect of variation of temperature: Experiment 201._--Special arrangement was made for gradual variation of temperature in the plant chamber. Two rectangular metallic vessels each 50 × 30 × 6 cm. were placed on opposite sides of the plant chamber, and warm water was made to circulate through them; this device ensured a steady rise of temperature. The flow of warm water was then stopped and the plant chamber was allowed to cool down; the fall of temperature was at first moderately rapid, but later on the rate of cooling became extremely slow; on account of this the temperature of the plant chamber, towards the end of the experiment remained higher than the normal temperature outside. The rate of rise and fall of temperature during the entire course is illustrated in the thermo-graphic (_a_) tracing (Fig. 187); the record (_b_) exhibits the movement of the leaf in a vertical plane, rise of temperature being attended by a diminution of geotropic curvature resulting in the fall of the leaf, the fall of temperature inducing the opposite effect. In record (_c_) is seen the responsive variation of geotropic torsion, rise of temperature inducing a diminution and fall of temperature causing an enhancement of torsion. The results obtained by diverse methods thus prove that the geotropic effect is diminished under rise, and increased under fall of temperature. SUMMARY. The position of equilibrium under geotropic action is not fixed but undergoes change with variation of temperature. The geotropic curvature and torsion are increased by lowering of temperature, and decreased by rise of temperature. This is equally true of apo-geotropic and dia-geotropic curvatures. PART IV. NIGHT AND DAY MOVEMENTS IN PLANTS. XLVI.--DIURNAL MOVEMENTS IN PLANTS _By_ SIR J. C. BOSE. The subject has long been a perplexing one, and its literature is copious. After a good many years of experimental investigation, I have succeeded in analysing the main factors concerned in the many phenomena which have been described as Nyctitropism. The results of the researches are given in a sequence of five papers, which may be read separately, yet will be seen as so many chapters of what has been a single though varied investigation. The different chapters are: 1. Daily movements in relation to Light and Darkness. 2. Daily movements due to Variation of Temperature affecting Growth. 3. Daily movements due to Variation of Temperature affecting Geotropic Curvature. 4. The Immediate and After-effect of Light. 5. Diurnal Movement of the leaf of _Mimosa_ due to combined effects of various factors. Nyctitropic movements are thus described by Jost[39]: