The Jewel Box is a greenhouse
located in Forest Park
in St. Louis, Missouri
However, calling it a greenhouse is rather like calling the Queen Elizabeth II a boat. While technically correct, it fails to capture to magnificence and splendor of the subject.
Built in the 1930s (and officially known then as the St. Louis Floral Conservatory), its unconventional vertical glass walls were hailed as a masterpiece of Art Deco design, and throughout the decade it attracted over 400,000 visitors a year (more than the nearby Art Museum). The design is also practical: all horizontal surfaces are metal, effectively protecting the glass from most hail storms and other weather effects.
Times after the great war were tough on the Jewel Box; very few improvements were made, and the facility was just barely maintained. Though still receiving hundreds of visitors daily, park funds were inadequate to maintain the greenhouse displays at more than a basic level. Eventually, in 1970, an admission fee of 25 cents was added; today, that fee is just fifty cents.
Through the 1990s, a number of private donations have helped restore the Jewel Box proper, and its 17 acres of grounds, to their former glory: the fountains are running again, the statuary has been restored, and the annual Christmas poinsettia show is once again on display.
No description can hope to do justice to this place. Neither can pictures, but the ones at http://stlouis.missouri.org/citygov/parks/parks_div/jewelbox.html are a decent place to start.
Sources: The above-referenced URL, a nifty pamphlet I got on my last visit, and of course numerous visits.