Why do plants close their leaves at night?

Helen - It's a lovely question...

Nyctinasty is the word for the process by which plants close their leaves and their petals at night, and they do it in different ways.

I don't know if you've ever seen clover? If you went out and looked at some clover at night, you would see that they've raised their leaflets up and pressed them together; other plants fold them downwards.

We already talked today a bit about light and how it affects organisms; these plants also respond to light. They have a circadian rhythm - a body rhythm or an internal clock, if you like - and this does seem to have an effect on how they hold themselves. 

The kind of nuts and bolts of how it happens is a little thing - a joint-like organ at the base of the leaves that's called the pulvini - they're basically little blobs of cells - these can change shape based on the pumping of ions. So, potassium and chloride ions get pumped in and out of different parts of these organs.

Because of osmosis, water shunts backwards and forwards and either pops these leaves up or squashes them back down again. That movement of those ions is affected by blue light in the daytime and by red light, which happens at more kind of dusk time and into the night.

We think it happens probably to protect leaves from getting cold; in other cases, there are plants in the tropics called sensitive shy grass. If you touch it, it collapses instantly. It's the same organ that's doing this: the pulvini. But they respond to touch rather than to light.

So, if you ever find them, you flick them and they just all collapse down. It's believed that this happens so they don't get eaten by herbivores that come along.