After Charles Walker died, Mary Root Walker re-married.
I found this marriage record.
Marriage Date: 26 Dec 1911 Place: Battle Creek, Calhoun, Michigan Bride is Mary M. Root Walker, 54 years, b. 1857 in Michigan Bride's parents are E.B. Root and Martha Clark Groom is Millard B. Williams, 69 years, b. 1842 in New York Groom's parents are Wm. W. Williams and Thirza Flint