The year is established by the reference to the 17 November 1874 issue of the Sitzungsberichte der Gesellschaft naturforschender Freunde zu Berlin (see n. 2, below).

There is an annotated copy of the 17 November 1874 issue of the Sitzungsberichte der Gesellschaft naturforschender Freunde zu Berlin in the Darwin Pamphlet Collection–CUL. At the meeting, Paul Wilhelm Magnus exhibited graft hybrids of two different potato varieties that had been grown by Adolf Reuter, a correspondent of CD’s. He also discussed a number of other graft-hybrid potatoes and compared the mixed coloration of graft hybrids with that of hybrids grown from sexual crosses. See Sitzungsberichte der Gesellschaft naturforschender Freunde zu Berlin, 17 November 1874, pp. 104–8. CD mentioned Magnus’s discussion in Variation 2d ed., 1: 422.