What biological process causes flowers to wilt?

Sarah - Well there are two different things that are going on here. There's the argument of why plants wilt, so in the case if you have a house plant, and why if you don't water it, it wilts. I'll address that first.

Usually in plants, the cells within the plant are what are known as turgid, which means that they're absolutely stuffed full of water which keeps moving into them by osmosis. They're very rigid which is how plants are able to support themselves. But if they're not able to get enough water, water will move out of the cells and the cells become sort of floppy or what's known as flaccid and that is why the plant is no longer able to support itself and the leaves go all floppy and wilted and soft. So that's what happens if you don't get enough water, but there are actually some other reasons behind cut flowers wilting. So obviously, if you don't water them, the same thing will happen, but also, it can be because they run out of nutrients because obviously, they're no longer attached to roots that are getting any nutrients as well and there can be a build up of bacteria, and fungi, and things on the end of the cut surface.

But also, when you cut flowers, you cut them on your work top or whatever and then you put them in water. Because of the water tension within the xylem vessels (which are the vessels that go up and down a plant, carrying the water around), if you cut the stem, it sucks in a bubble of air into the xylem. If you then put the stem in water, it stops more water from flowing up the xylem. So that actually can be a real reason why they wilt. Some florists recommend that you cut the stems of flowers underwater which will keep the water just a little droplet on the end whilst you put them in the vase and will help them to stay alive for longer.