When I want to know how cold it is outside, I just have to look at the rhododendron leaves on the bush outside the office window. The colder it is, the tighter the leaves are curled! At about 25 degrees and below, they are drooping and tightly curled.
I often wondered why this strange thing happens so I looked it up. According to one web site....
The leaves curl because they lose water, just as many other plants wilt. Severe droughts also cause rhododendron leaves to droop and curl. The pores through which the plant loses water and takes up air are located on the underside of the leaves. When the leaves curl, these pores are protected, and the evaporation of additional water is reduced. The curled leaves protect the rhododendron plant from more water loss.
So on warm days, the leaves are open and perpendicular to the plant stem. As it gets closer to freezing, they start to droop. Below freezing, the leaves droop and curl. Pretty cool plant! I have my own thermometer just outside the window in winter!