HistoryAs Joy Morton began planning The Morton Arboretum in the early 1920s, he relied heavily on the advice of Charles Sprague Sargent of the Arnold Arboretum in Boston. Based on Sargent's recommendation, the Arboretum was to consist of living plant collections, an herbarium, and a library. In 1923 Morton added a library wing to his Thornhill residence, making it the first major structure built specifically for Arboretum use. Designed by John Root, the library addition had a stucco finish and a steep roof with red tiles. The interior exhibited English oak paneling, a carved stone mantelpiece, and a decorative plaster ceiling with chandeliers. The library's bay windows displayed stained-glass medallions depicting famous historic trees, such as the Linnaeus linden. The library held the books the Sargent had selected for the Arboretum as well as the Morton family library.
In 1941 the Morton residence was razed, except for the library wing. The Thornhill Building was constructed in its place and connected to the library. At this time, the library's steep roof was replaced with a flat roof, the fireplace facade was resurfaced with travertine marble, and a new parquet floor was installed. When the Sterling Morton Library was established in 1963, the Thornhill library, now known as the Founder's Room, became a space to memorialize The Morton Arboretum's historic roots. Housing portraits of Joy Morton and his brother Paul, the room now serves as a space for displays and events ranging from wedding ceremonies to yoga classes.