The year is conjectured from the reference to extract of Hyoscyamus (the genus of henbanes); CD used this in experiments on Drosera that he began on 23 August 1872 (Correspondence vol. 20, Appendix II). Drosera is the genus of sundews.
Baxter, a dispensing chemist in Bromley, Kent, often supplied CD with chemicals for his experiments. Although CD had experimented on Drosera with poisons in the early 1860s, he had used only tincture of henbane in his earlier work (see Correspondence vol. 9, letter to W. W. Baxter, 22 January 1861). The experiments described by CD in Insectivorous plants, pp. 84, 206, were done with a powdered extract of Hyoscyamus that he prepared in solution himself, thus ensuring greater accuracy.