The flower of _Nymphæa_ remains closed during the day and opens at night. Figures 195 and 196 are from photographs of the day and night positions of the flower. The closure and opening movements of this flower have been regarded as being mainly due to recurrent variations of light and darkness.[42] If the opening be due to darkness, closure of the flower should take place in the morning with the appearance of light. But the flowers often remain open till ten or eleven in the forenoon. I have sometimes succeeded in keeping the flower open for greater part of the day by lowering the temperature of the plant-chamber. The movement of the flower thus appeared to be associated with variation of temperature rather than of light. [42] Pfeffer--Ibid, Vol. III. p. 122. _Action of light: Experiment 208._--I investigated the effect of light on the movement of opening or of closing of the flower. One of the petals was attached to the recording lever; light from an arc lamp was made to act diffusely on the petal; this was done by means of two inclined mirrors by which the divergent horizontal beam of light was thrown on the upper and lower sides. The record in figure 197 shows that light induced a movement of opening, followed by closure in darkness. Since light induces a movement of opening, and darkness brings about a closure, the opening of the flower at night could not be due to darkness. We have therefore to look for a different cause for the diurnal movement of the flower. _Effect of variation of temperature._--I have already described an experiment which proves that rise of temperature induces a movement of closure of the floral leaves of _Nymphæa_, lowering of temperature producing the opposite effect (p. 311). From the study of the action of light and of variation of temperature, it will be seen that the flower of _Nymphæa_ is acted on in the evening by two antagonistic forces; darkness induces a movement of closure, and fall of temperature gives rise to a movement of opening. Since the flower opens in the evening, the predominant effect is that of falling temperature. The above conclusions are fully borne out by the diurnal record which I obtained with _Nymphæa_. [Illustration: FIG. 198.--Diurnal record of _Nymphæa_. Upper record gives variation of temperature; the up-curve representing fall, and down-curve rise of temperature. The lower record exhibits the movement of the flower, up-curve representing the opening, and down-curve the closure of the flower.]