Ferns have a funny, replicated kind of structure. Look at the picture below. Each of the parts sticking out to the side looks a lot like the whole fern, but smaller and pointing out sideways. Or, put another way, a fern is just a branch with two rows of smaller ferns stuck alongside the branch. And, of course, eventually we get to some actual leaves.

  

Here's a pair of data definitions that capture that intuition.

A fern is either
  - (make-leaf number)
  - (make-branch number number list-of-fern list-of-fern)

A list-of-fern is either
  - empty
  - (cons fern list-of-fern)

(define-struct branch (length thickness left right))
(define-struct leaf (area))