Ask me how I know just how cold it is on any morning, and I will walk you into my bathroom and point out the window at a wise, old rhododendron tucked into a dark corner behind the house.
Yes, we have plants that can tell the temperature. Leaves in a very tight curl, as in the first photo? Temperature in the teens.
Slightly more open? 20-25°F.
Almost flat? 30°F.
Wide open, flat leaves signal temperatures near 40°F.
As thermometers go, this one is pretty reliable. And when the weather tops 50°F day after day, some time in early May, those buds will open, and the view out my bathroom window will be filled with purple blossoms.