There is a traveling exhibit about Darwin's botanical studies and his gardens at Down House that is ongoing near LA through January 5, 2009. The exhibition started at the New York Botanical Gardens, where it was described by the New York Times as "stunning."
The website for the New York version of the exhibition describes it as follows:
Charles Darwin is best known for his theory of evolution and other natural history achievements, but little is known about his enduring and insightful work with plants and the important role they played in formulating his ideas. Yet from cradle to grave, botany played a pivotal role in Darwin's life. From counting peonies and playing under the apple trees in his father's garden as a boy to collecting "all the plants in flower" on his famous voyage to the Galápagos as a young man and testing the sex and sensitivity of plants at his home, Down House, in his later years, plants were a lifelong preoccupation for Darwin.
Darwin's Garden: An Evolutionary Adventure explores the untold story of Darwin's botanical influences, his research, and his contribution to our understanding of plants, and ultimately, of life in general. The exhibition ... includes an "evolutionary tour" of living plants that demonstrate key points on the Tree of Life, which links all living beings through a common ancestry. [Also included is a] re-creation of Darwin’s garden at Down House, where he lived in the English countryside for the last 40 years of his life.
There is a catalog of the exhibition available for purchase at http://www.nybgshop.org/Darwins-Garden-An-Evolutionary-Adventure-p-18751.html
There is also a fascinating essay on the occasion of the exhibition in the latest New York Review of Books (the 45th Anniversary Issue of that august journal) here: http://www.nybooks.com/articles/22075
The essay is by Oliver Sacks. I say "on the occasion" because it is less a review of the exhibition itself and more an inquiry into the centrality of botany to Darwin's achievements. It is well worth a read.
If you are interested in going to the LA exhibit, here is a link to the website for the exhibit: http://www.huntington.org/Information/darwin.htm