Free resources at Google Books - but not easy to find in its search engine.
Kew gardens; or, A popular guide to the Royal botanic gardens of Kew
Catalogue of the Victorian Exhibition 1861 [Melbourne]
London Journal of Botany 1 - 7, 1842-1848
Hooker's Journal of Botany and Kew Garden Miscellany 1 - 9, 1849-1857.
Official Descriptive and Illustrated Catalogue of the Great Exhibition of 1851