Why do ferns curl up?
For grown-ups: the science
Fern leaves curl when they are new and growing, and some curl in dry air to save water.
Most fern fronds emerge as coiled fiddleheads, a growth pattern called circinate vernation. Differential growth causes the frond to unroll as it expands. In some species, leaves may also curl or roll during water stress, which can reduce exposed surface area and water loss.
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Transcript
Look at this new fern leaf. Why is it curled like a little spring?
A young fern leaf starts out rolled into a tight curl. The curl helps the tender leaf fit safely as it grows.
As the leaf gets bigger, its green parts grow and stretch. The curl slowly opens into a long, feathery leaf.
Some ferns also curl their leaves when the air is very dry. Curling can help them lose less water.
So ferns curl when new leaves are growing, and sometimes when they need to save water.