The date is conjectured from the fact that CD had evidently requested materials for his work on Drosera rotundifolia, the sundew. CD had worked on Drosera earlier (see Correspondence vol. 10, letter to J. D. Hooker, 26 September [1862] and n. 1), but he commenced another series of experiments on 23 August 1872, after finishing correcting the proofs of Expression (CD’s ‘Journal’ (Appendix II)). This letter is in the Darwin Archive–CUL among CD’s notes on Drosera; notes immediately before and after it are written on the back of corrected proof-sheets of Expression. CD’s research on Drosera was published in 1875 in Insectivorous plants.

The two essential oils were probably oil of cloves and oil of caraway (Insectivorous plants, pp. 211–12). CD repeated Baxter’s information about their solubility in water in ibid., p. 211. ‘Sparingly soluble’ refers to solubilities of 0.5 grams per litre or lower.

CD found in 1862 that opium placed on the glands of Drosera effectively put the plant to sleep; see Correspondence vol. 10, letter to Daniel Oliver, [17 September 1862]. In Insectivorous plants, he reported only on the results he achieved with acetate of morphia, a mixture of morphia (an extract of opium) and acetic acid (Insectivorous plants, p. 205).