The date is conjectured by the relationship of this letter to a note in DAR 157.1: 70, dated [circa 31 March 1864], in which it was first recorded that pieces of thread, weighed in grains, were placed on specimens of Clematis. Emma Darwin recorded in her diary (DAR 242) that the ‘Boys’, presumably including George, went back to school on 5 April 1864; they were at home for the Easter holidays (see letter from H. E. Darwin to W. E. Darwin, [16 March 1864], n. 3). In 1864, Easter Sunday was 27 March. See letter to J. D. Hooker, 5 April [1864] and n. 17, and n. 3, below. The letter was found in the folio containing CD’s notes on climbing plants.

CD wrote this phrase in pencil, probably as an afterthought. On the back of the letter is a note in ink: ‘I’m very sorry I forgot to Send it before | G. HD’; George may have forgotten to return this letter with his letter of [after 6 April 1864?]. CD occasionally made use of George’s mathematical skills (see, for example, Correspondence vol. 11, letter from G. H. Darwin, [before 11 May 1863]).

In March or April 1864, CD began weighing in grains the inches of thread that he tied to the petioles of some of the leaf-climbing Clematis species to test their sensitivity (see ‘Climbing plants’, pp. 27–8, 29, 31, 33–4; see also letter to J. D. Hooker, 5 April [1864] and n. 17). CD’s notes on these experiments are in DAR 157.1: 62–3, 70–1, 73–5, and 77. CD had been told the number of English grains in a gram several years earlier, when he was researching substances that he was applying to the insectivorous sundew, Drosera (see Correspondence vol. 8, letter from Edward Cresy, 10 November 1860).