Mentioned by Lonely Planet Top Choices
Top things to do in London, England
"In 1759 botanists began rummaging around the world for specimens to plant in the 3-hectare Royal Botanic Gardens at Kew. They never stopped collecting, and the gardens, which have bloomed to 121 hectares, provide the most comprehensive botanical collection on earth (including the world's largest collection of orchids). A Unesco World Heritage Site, the gardens can easily devour a day's exploration; for those pressed for time, the Kew Explorer hop-on/hop-off road train takes in the main sights."