Hooker was preparing The student’s flora of the British Islands (J. D. Hooker 1870). See letter from J. D. Hooker, 11 March 1869 and n. 6.The ‘supplementary’ Hooker refers to is probably new material added to the French edition of Orchids (Rérolle trans. 1870) and separately published in English (‘Fertilization of orchids’).
Thanks CD for supplementaries ["Fertilization of orchids", Collected papers 2: 138–56] which he will quote in the British flora [The student’s flora of the British Islands (1870)].