Question

Leaves of many grasses are capable of folding and unfolding because

Answer 1

potassium and chloride ions get pumped in and out of different parts of these organs. Then, 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.