The year is established by the relationship between this letter and the letter from Robert Caspary, 25 February 1866.

Caspary delivered the paper on Cytisus adami (now +Laburnocytisus adamii) and other hybrids that he believed were produced by grafting at the International Congress of Botany and Horticulture held at Amsterdam from 7 to 12 April 1865 (Caspary 1865a). CD had read a notice of the paper in the Gardeners’ Chronicle, 29 April 1865, p. 386 (see Correspondence vol. 13, letter to J. D. Hooker, [1 May 1865], and letter from M. T. Masters, 12 July 1865). The paper appeared in Bulletin du Congrès International de Botanique et d’Horticulture réuni à Amsterdam (1865): 65–80; CD’s offprint of the paper, bound with an annotated handwritten English translation, is in the Darwin Pamphlet Collection–CUL.

CD had long been interested in Cytisus adami (now +Laburnocytisus adamii), a hybrid produced by grafting a scion of Cytisus purpureus (a synonym of Chamaecytisus purpureus, purple broom) onto a stock of Laburnum anagyroides (the common yellow laburnum). See Correspondence vols. 4–6, 10, and 11. Unlike most grafts, which preserve the floral character of the scion, +L. adamii has branches of purplish-yellow flowers, which regularly revert to those of both parent species (Bean 1970–88, 2: 510–11). CD had tried to propagate the hybrid by seed, and to produce the hybrid by crossing, but without success (see Correspondence vol. 6, letter to J. D. Hooker, 8 September [1856], and Correspondence vol. 10, letter to Daniel Oliver, 24 July [1862]). CD discussed the plant in Variation 1: 387–97, 405–11, and 2: 37, 364–5, noting the ‘extraordinary fact’ that two distinct species could ‘unite by their cellular tissue, and subsequently produce a plant bearing leaves and sterile flowers intermediate in character between the scion and stock, and producing buds liable to reversion; in short, resembling in every important respect a hybrid formed in the ordinary way by seminal reproduction’ (Variation 1: 390). He argued that the plant was a graft-hybrid, and not a product of sexual union, citing Caspary 1865a and other information obtained from Caspary’s letters (Variation 1: 388–9). Because it displayed both fusion and reversion of characters, +L. adamii served to illustrate the similarity of asexual and sexual modes of reproduction, an important aspect of CD’s theory of pangenesis (see Variation 1: 411, 2: 364–5, and Olby 1985, pp. 76–80).