What does "close back up" mean?

[optimistic pessimist]

Hi everyone!

I'm reading an article about the biological clock.

"In the early eighteenth century, the French astronomer Jean-Jacques de Mairan was struck by the way the mimosa plants on his windowsill raised their leaves toward the sun at the same time every day. An effect of the light? Mairan placed the mimosas in a dark cellar, but the leaves kept right on unfuring every morning and closing back up every evening....

I'd like to ask about the words in bold font. Why is it not just "closing" or "closing back"? What does "up" mean here?

Thank you

OP

 

[Barb_D]

They return to a state of being closed.

I opened my bag. I closed it. I opened it back up.
The leaves were closed. They opened. The closed back up.

It's idiomatic, but perhaps not logical.