THERMONASTY in TULIP & CROCUS flowers
Crocuses exhibit a phenomena called
Thermonasty, which is the method by which they are able to open up in the daylight and close up at night. Tulips also exhibit this characteristic.
Like a bimetallic strip, it is based upon rising and falling temperatures, but un-like bimetallic strips, it is not due to any differential thermal expansion nor to ordinary thermal expansion. The tepals are able to sense temperature, and when they are warming up (sun rising) the tepals grow differentially more on the inside of the tepal than they do on the outside. This differential growth forces the tepal to open outwards. A temperature increase of only 0.36°C is sufficient to initiate the opening of the flowers.
Conversely, as the temperature drops, it is the outer parts of the tepal which grow faster than the inner parts, forcing them to close again. Note that the tepals can only grow bigger, they cannot 'grow shorter'. Tulips also exhibit thermonasty.
This differential growth rate is due to the cells on the inner surface of tepals having an optimum growth rate that is 10°C higher than the cells on the outer surface of the tepals. This difference in growth rate is due to differences in the internal carbon dioxide concentrations. Increasing the concentration of CO2 causes a marked extension in the cells and a lowering in the temperature for optimum growth. The effect is thus closely related to the plants respiration.
It does this to protect the pollen within from rain or snow.
Other flowers accomplish the opening and closing of flowers or other parts of the plant by differing means, such as by either
Tropic movements or by
Nastic movements. Nastic movements are initiated by various stimuli, thus
Chemonasty is initiated by soil chemicals or nutrients,
Epinasty (by gravity),
Geonasty (by gravity),
Hydronasty (by water),
Hyponasty (by the
growth hormone Ethylene),
Nyctonasty (by circadian clock and light), Photonasty (by light),
Thermonasty (by heat) and Thigmonasty (by touch).