In the description of direction of responsive movements, confusion is likely to arise unless the point of view be carefully defined. An up-movement of a leaf or a petal means approach towards the growing point of the axis. This may be variously described as movement of closure or of folding. A down-movement may, on the other hand, be described as a movement of opening or of unfolding. If the plant be held inverted, two different effects will be noticed depending on the character of the movement, whether nastic or tropic. In the case of nastic movement, the former up-movement in erect position would appear, on inversion of the plant, to be a down-movement; but in relation to the plant the closure movement will remain closure movement, whether the plant be held in the normal position or upside down. If, on the other hand, the direction of movement be determined by the paratonic effect of external stimulus, gravity for example, an up-movement due to fall of temperature will continue to be an up-movement, whether the plant be held in its normal or inverted position. The responsive movement in relation to the plant will, however, be different; the closure movement will, on inversion, be reversed into a movement of opening. The reversal of closure into an opening movement or _vice versâ_ will thus be a test of the paratonic effect of external stimulus. We may thus distinguish thermonastic from thermo-geotropic action by the following tests: 1. Thermonastic movements are, generally speaking, due to differential growth, and are therefore characteristically present in growing organs. Thermo-geotropic action is independent of growth. 2. Thermonastic movements take place in relation to the plant, and is not determined by external force of a directive nature. Opening or closing movement will remain unchanged after inversion of the plant. But thermo-geotropic reaction being determined by the external stimulus of gravity, becomes reversed on inversion of the plant. Closure movement is thus converted into opening movement, and _vice versâ_. I shall now take up the diurnal movement due to variation of growth induced by change of temperature. Of this the flower of _Nymphæa_ furnishes an example. [Illustration: FIG. 195.--Nymphæa closed at daytime.] [Illustration: FIG. 196.--Nymphæa open at night.] [Illustration: FIG. 197.--Response to light applied successively for 1 minute. Down-curve shows movement of opening followed by recovery in darkness. (_Nymphæa_).] DIURNAL MOVEMENTS OF _Nymphæa_.