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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.
What makes this an interesting venture is that the robots will learn from the humans - the way they move, talk and think will be learned by observing us.
Comment: James McCanney's electric model may account for this strangeness: In other words, asteroids are just "dead" comets.