Quoting from Ipanema.com – An Insider’s Guide to Rio de Janeiro, Brazil (a terrific site for anyone planning a trip to Rio):
Jardim Botânico do Rio de Janeiro was founded on June 13, 1808 by Dom João VI, who then was prince regent of United Kingdom of Brazil and Portugal. It is one of the great tropical botanical gardens of the world.
The Botanical Garden was originally created to adapt to the Brazilian climate plants like nutmeg, pepper and cinnamon that were imported from the Western Indies. It was open for public visitation in 1822 during the reign of Dom Pedro I, Dom João VI's son. In 1937 it was listed as part of the Brazilian Historical Heritage, and in 1992 it was considered by UNESCO a biosphere reserve.
The 340 acre Gardens just celebrated its 200 year anniversary.
We spent the better part of three hours wandering the trails and walkways looking at the huge variety of plants, flowers, sculpture, historical buildings and even spotting a black beaked toucan. As a special treat we watched a small group of monkeys enjoy their lunch high in the trees.
There is an orchid house, a bromeliad exhibit and a little greenhouse filled with carnivorous plants. Lots of formal gardens and lots more just quiet green space.
I think my favorite spot was the lagoon with Amazon water-lilies more than a meter in diameter.
So many of the national treasures in Brazil suffer from the lack of resources to maintain and preserve them. Thankfully the Jardin Botânical has an apparently thriving "Friends of the Jardin Botânical" association and restoration work is either completed or in progress throughout the garden.