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Flower lifespan
Flowers usually start to fade after they are fertilised but for example I refer to two pot plants I have which produce spectacular flowers, do not get fertilised, yet one lasts a couple of days and the other several months. The short lived one, a cactus (Echinopsis), produces beautiful trumpet shaped flowers some 6" in length which sadly last no more than a couple of days. The other example is an orchid, the popular Moth Orchid, whose flowers last unchanged for months.
The plants have spend much energy in producing such complex flowers - why the difference in lifespan. Maybe the question needs to be addressed to the great Gardener up above!!
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Interesting Q's you've raised DrMac. I think it largely has to do with the given plant's natural habitat.
Take for example the cactus and orchid comparison that you put forward as an example.
The cactus most likely comes from a desert, which is incredibly dry for most of the year or sometimes for years at a stretch. Conditions suitable for flowering therefore do not come around very often. It is usually after a good downpour that all of the cacti and other flowering plants will suddenly burst forth, perfectly synchronized, which is why deserts often erupt into a kaleidoscope of dazzling color stretching as far as the eye can see after rain events. Two great examples of this are the flower fields of Namaqualand in the western part of South Africa and similarly the western parts of Australia both of which peak in spring (September) after good winter rains. There is a very narrow window of opportunity for these plants to bloom as the suitable conditions won't last long. This is why the flowers of plants from these areas typically only last very briefly.
The orchid on the other hand comes from a much more stable environment in terms of humidity, rainfall, warmth, etc. In the jungles of tropical Asia which moth orchids call home their pollinators, large carpenter bees from the genus Xylocopa, are active and on the wing far more regularly so it makes sense to have your flowers hanging around for longer periods of time. As you said it costs the plant a great deal of energy to produce flowers, they are valuable and vital organs to ensure survival. The moth orchids flower is not delicate, on the contrary it is quite solid and built to last unlike a daisy, cactus or poppy flower with flimsy petals.
That is a perfect explanation, Hippophae!
Thanks Sue!
Also to be remembered those flowers that are not flowers at all, like the mophead hydrangeas, which are actually modified leaves (the real flowers being the tiny pinprick-sized starry flower in the middle).