The clue came from a photo of Dr. Woodworth in later life, published in A History of the City of Chicago: Its Men and Institutions. Biographical Sketches of Leading Citizens (p. 419) and available digitally at Google Books.
Take a close look at the two photos. Notice the hairline, the shape and set of the eyes, the nose, the ears. I'm comfortable with saying that the unlabeled photo is definitely Plumer Morton Woodworth. This conclusion also fits with the photo's placement in the album. (It follows individual photos of Jacob Woodworth and Ellen D. Bird Woodworth.)
Plumer has something else in common with his father: an impressive beard! The resemblance is really remarkable, beards included.