Experimentation in the nineteenth century was not confined to research laboratories. It often took place in domestic settings. Darwin used his house and garden as a site of experiment, often studying phenomena that had received little or no scientific attention before, such as the co-adaptive mechanisms of flowers and insects that assured cross-pollination, or the climbing habits of plants. One of Darwin’s most important correspondents was the German naturalist and émigré to Brazil, Fritz Müller. Their letters often discussed the details and difficulties of botanical experimentation.
Letter 4895 — Darwin, C. R. to Müller, J. F. T., 20 Sept 
Darwin thanks Müller for interesting letter on climbing plants. In what follows are details of experiments and observations, including Müller’s view on Anelasma which he thinks seems probable.
Letter 5173 — Müller, J. F. T. to Darwin, C. R., 2 Aug 1866
Müller provides some observations on orchids and on some plants which seem to be dichogamous.
Letter 5429 — Müller, J. F. T. to Darwin, C. R., 4 Mar 1867
Müller reports observations on experiments on fertility of orchids he has self-pollinated and crossed with pollen of other species.
Letter 5480 — Müller, J. F. T. to Darwin, C. R., 1 Apr 1867
Müller cites cases of difference in coloration between the sexes of some species of Crustacea, annelids, and spiders. He discusses dimorphic plants and self-sterility, outlines some experiments involving the crossing of different species of orchids, and encloses extract from Carl Claus, Die freilebenden Copepoden .
Letter 5551 — Darwin, C. R. to Müller, J. F. T., 26 May 
Darwin thanks Müller for information on sexual differences. He talks about orchids; self-sterility and difficulty of getting seeds to germinate.