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.
Experiment 209.—One of the perianth leaves was attached to one of the recording levers, the differential thermometer being attached to the other. It will be seen (Fig. 198) that the movement of the flower follows very closely the curve of variation of temperature. The flower was tightly closed in the day time; and the perianth leaves began to open out in the evening at first slowly, then very rapidly, and the flower becoming fully expanded by 10 p.m. at night. Though the temperature continued to fall, there was no possibility of further expansion beyond the maximum. The temperature began to rise after passing through the minimum at 6 a.m., and the movement of closure set in with rising temperature, the flower becoming completely closed by 10 a.m. That geotropism has little effect is seen from the fact that the inversion of flower does not interfere with the normal opening or closing of the flower.
The phenomenon of diurnal movement of Nymphæa is therefore thermonastic, the floral leaves exhibiting movement of opening at night owing to fall of temperature. Luffa acutangula, which opens in the afternoon, and closes early in the morning, gives a diurnal record similar to that of Nymphæa.
SUMMARY.
The flower of Nymphæa exhibits a movement of closure during rise of temperature, and of opening during fall of temperature.
It is shown further that the effects of light and of rise of temperature are antagonistic to each other. Light is shown to induce in Nymphæa the movement of opening, and darkness to cause the movement of closure. The diurnal movement of Nymphæa is not therefore due to periodic variation of light and darkness, but to the predominant effect of variation of temperature.
The diurnal record shows that the perianth leaves begin to open in the evening with falling temperature, and the flower becomes fully expanded by 10 p.m. The movement of closure sets in with rising temperature in the morning, and the flower becomes fully closed by 10 a.m.
[41] Strasburger—"Text-book of Botany" (1912), p. 300.
[42] Pfeffer—Ibid, Vol. III. p. 122.