Make cave ferns give some kind of fibre/string.

[Roguetinkerer]

I'm doing a cave only challenge character and have not been able to get string.
This would also add a use for cave ferns other than growing more cave ferns.

[Audiosmurf]

For a second I thought you meant dusk ferns and I was on a hard no, but sure, cave ferns having a use would be cute, and a bush that produces strings would likewise be cute