Ex-2.28

(define (fringe tree)
  (cond
   ((null? tree) '())
   ((pair? tree)
    (append (fringe (car tree)) (fringe (cdr tree))))
   (else (list tree)))) 

Used wishful thinking here. I thought like this, If we knew the fringes of left and right subtree then the answer would be to just append them and this does the trick :).