Talking Plants sometimes has to head below the belt to make a point, so I hope you’ll excuse me noting that we humans try to protect our reproductive organs as best we can.
It depends on the culture of course but mostly we wrap them in some kind of cloth and do all we can to protect them from bumps and bruises. For a flowering plant, the flower needs similar protection.
It’s all very well to waive it in the breeze so that the wind, or pollinating animals, can fertilise take pollen from one flower to another, but you want to look after your assets.
A lot of energy goes into producing the spectacular blooms we enjoy in our gardens but whose role is primarily reproductive, so plants have evolved ways to protect their blooms when they are not being used for…sex.
A popular solution for flowers with daytime pollinators is to close the flowers at night. This way the delicate bits are protected from wind, rain and dew, and all important pollen is kept dry and ready for action the next day.
Closing at night (i.e. diurnal) also keeps out the riff raff – the insects that might eat the flower but not help in pollination, and the nasty diseases they might carry with them.
Plants growing in tough environments are more likely to do this than ones that have plenty of resources and can readily produce new flowers. A good example is the native Pigface (Carpobrotus glaucescens), a succulent adapted to dry sand dunes, which has purple daisy-like flowers that closes up every night.
Flowers that depend on night-time pollinators, such as bats, may close their flowers during the day (i.e. nocturnal). There are the famous night-flowering cacti (e.g. Cereus species, sometimes called Moonflowers) and some of the bromeliads.
How do they open and close their flowers? Either by actually growing new cells every day in the inside (to open) or outside (to close), or by expanding and contracting existing cells in the petals.
I remember reading there was a fashion in gardening for planting flowers that open and close at different times each day in the shape of a clock face. Some species open with such regularity, that you could tell the time by when the appropriately planted flower opened.
Image: Sadly I can't find my copy of Warwick Orme's beautiful images of a Moonflower, but here is an equally impressive image of Cereus hildmannianus (from The Cactaceae (1919-1923) by Britton et Rose, Vol. II, Plate 3; reproduced at Wikimedia Commons).*This posting is from the Radio Archive (March 2008).