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As a comet's orbit becomes circular (due to tail drag), it stops moving through the differently charged areas around the Sun, remaining in one region of electrical equipotential, thus losing its visibly charged tail. In essence, it evolves into an asteroid or potential moon in a stable orbit, its fiery early years over.In other words, asteroids are just "dead" comets.
The drooping effect is probably caused by loss of internal water pressure within plant cells, a phenomenon called turgor pressure. "It means branches and leaf stems are less rigid, and more prone to drooping under their own weight," says Zlinszky.
The trees may also be "resting" their branches. During the day, branches and leaves are angled higher, allowing leaves to catch more sunlight, because there's less self-shading from leaves above. But this is energy-intensive and serves no purpose at night, when there's no light.
So is the drooping deliberate, dictated by an active sleep-night cycle, or passive, dictated by differences in the availability of water and light? "This remains to be decided," says Zlinszky.
Comment: See also: Confirmed: The Eye Emits Actual Light (Biophotons)