in Thoreau’s Journal:

I think that the change to some higher color in a leaf is an evidence that it has arrived at a perfect and final maturity, answering to the maturity of fruits, and not to that of green leaves, etc., etc., which merely serve a purpose. The word “ripe” is thought by some to be derived from the verb “to reap,” according to which that is ripe which is ready to be reaped. The fall of the leaf is preceded by a ripe old age.