A 120-acre portion of James Arnold's estate was transferred to Harvard College in 1872 “for the establishment … of an arboretum.” Charles Sprague Sargent, its first director, together with Frederick Law Olmsted, Sr., penned a thousand-year lease agreement in 1882 between Boston and Harvard in which the land became part of the Emerald Necklace while the Arboretum retained control of the living collections. The city was to provide maintenance in exchange for public access. The overall design was intended to be naturalistic, fitting with existing topography and vegetation. Balancing the scientific with the Picturesque, the main road presented the collections sequentially according to genus. Entirely surrounded by a stone wall, the various entry gates were largely designed by the Olmsted firm, though the South Street entrance is attributed to architect H.H. Richardson. Other structures include three simple boulder culverts serving as bridges, all designed by John Charles Olmsted. In 1895, additional lands were purchased and designed by the Olmsted firm. In the 1940s, Beatrix Farrand added the Meadow Road azalea border. In 1974 Shurcliff, Merrill, & Footit redesigned the summit of Bussey Hill, adding seating. The Arboretum now covers 265 acres. In 1966 it was listed in the National Register of Historic Places.