(define sum-of-divisors
(lambda (n)
(define sum-from-plus
(lambda (low addend)
(if (> low n)
addend
(sum-from-plus (+ low 1) (if (divides? low n)
(+ addend low)
addend)))))
(sum-from-plus 1 0)))
Note: You don't have to check all integers
in the range 1..n, just those in 1..(sqrt n).
(See Exercise 3.6, p. 60.)