The date is established by the endorsement; the Monday preceding 2 May 1865 was 1 May.

The reference is to Robert Caspary’s paper on Cytisus adami (Caspary 1865), which was read at the Congrès International de Botanique et d’Horticulture held at Amsterdam from 7 to 12 April 1865 under the auspices of the Royal Horticultural Society of the Netherlands. In its report on the Congress, the Gardeners’ Chronicle, 29 April 1865, p. 386, noted that ‘Professor Caspary spoke at some length on … hybridisation produced by grafting, adducing in illustration … the Cytisus Adami.’

CD had long been interested in the hybrid laburnum, Cytisus adami (now known as +Laburnocytisus adamii), a graft hybrid of the common yellow laburnum C. laburnum (now Laburnum anagyroides) and C. purpureus, a dwarf purple broom (Bean 1970–88, 2: 510–11). See, for example, Correspondence vols. 4–6, Correspondence vol. 10, letter to Daniel Oliver, 24 July [1862] and n. 15, and letter to Thomas Rivers, 28 December [1862], and Correspondence vol. 11, letter to Thomas Rivers, 7 January [1863] and n. 12. Branches of +L. adamii have red, sterile flowers that regularly revert to both parent species in the colour of their flowers. CD collected articles on seed production printed in the Gardeners’ Chronicle over many years (CD’s annotated copies of the Gardeners’ Chronicle are in the Cory Library, Cambridge Botanic Garden). Cytisus adami is discussed in Variation 1: 387–91, 406–7, and 2: 37, 364–5. In appearing to display hybrid fusion and reversion, C. adami was an important case contributing to CD’s view that there was no essential difference between asexual and sexual reproduction; see Olby 1985, pp. 76–8.

There is a lightly annotated copy of Caspary 1865 in the Darwin Pamphlet Collection–CUL. The pamphlet is a reprint from the Bulletin du Congrès International de Botanique et d’Horticulture réuni à Amsterdam (1866): 65–80; bound with it is an annotated handwritten English translation of the pamphlet. CD asked Caspary for a copy in February 1866; see Correspondence vol. 14, letter to Robert Caspary, 21 February [1866], and letter from Robert Caspary, 25 February 1866. Caspary 1865 is cited in Variation 1: 388–9, but CD noted that he owed most of his information to Caspary’s letters to him.

A short report of the proceedings of the Congrès International de Botanique et d’Horticulture in Amsterdam appeared in the Gardeners’ Chronicle, 29 April 1865, pp. 385–6 (see letter from M. T. Masters, 12 July 1865); the report was written by the principal editor of the Gardeners’ Chronicle, Maxwell Tylden Masters. CD annotated this report in his copy of the Gardeners’ Chronicle (see n. 3, above).