The Chicago Botanic Garden is a 385-acre (156 ha) living plant museum situated on nine islands featuring 26 display gardens and surrounded by four natural habitats: McDonald Woods, Dixon Prairie, Skokie River Corridor, and Lakes and Shores. It is owned by the Forest Preserve District of Cook County and managed by the Chicago Horticultural Society. It opened to the public in 1972 and is home to the Joseph Regenstein, Jr. School of the Chicago Botanic Garden, offering a number of classes and certificate programs. The Chicago Botanic Garden is accredited by the American Alliance of Museums and is a member of the American Public Gardens Association (APGA). The mission of the Chicago Botanic Garden is to promote the enjoyment, understanding and conservation of plants and the natural world. The Chicago Botanic Garden is one of the United States' most visited public gardens and a center for learning and scientific research. Nearly one million people visit the Garden annually. It has a membership of 50,000, the largest of any U.S. public garden, and is Chicago's 7th largest cultural institution and 12th largest tourist attraction. More than 2,000 volunteers assist with all aspects of the Garden’s mission, from planting and propagating natural areas, to teaching educational programs and staffing public programs and exhibitions. The Chicago Botanic Garden is only one of 17 public gardens accredited by the American Alliance of Museums, recognizing its living collection of 2.5 million plants.