What's the mechanism of Rhododendron's leaves pulling back when it's very very cold?

I always use how much the rhodi's leaves are drawn back as an indication of how cold it is outside, but would like to know how it happens

What's the mechanism of Rhododendron's leaves pulling back when it's very very cold?
Rhododendron (from the Greek: rhodos, "rose", and dendron, "tree") is a genus of flowering plants in the family Ericaceae. It is a large genus with over 800 species. and most have showy flower displays. It includes the plants known to gardeners as azalea.





The highest species diversity is found in the Sino-Himalayan mountains from central Nepal and Sikkim east to Yunnan and Sichuan, with other significant areas of diversity in the mountains of Indo-China, Korea, Japan and Taiwan. This is one indication that Rhododendrons evolved in, and are superbly adapted to mountainous, cold climates.





When the temperature drops below 35F, rhododendron leaves begin to cup and curl at the edges. At 25F, the leaves have curled so tight that half the leaf surface has disappeared and the leaves droop. When temperatures hit the teens, leaves shrivel even tighter, turn brownish-green and dangle like stiff string beans. This response to temperature changes is a rhododendron's method of preventing loss of moisture through the leaves.





The upper side of a rhododendron leaf is leathery. The bottom side is dappled with tiny air valves that control the flow of air in and out of a leaf. Cold air contains less moisture than warm air. So when low temperatures and high winds arrive, the leaf valves close. By looking out a window on a winter day, one can determine roughly how cold it is by the degree the rhododendron leaves have curled and drooped. When temperatures rise, the leaves open again.





The rhododendron makes the same response in summer when temperatures become excessive; only the leaves curl upward to prevent undue moisture loss.