In regard to the sterility of hybrids in successive generations; though Gartner was enabled to rear some hybrids, carefully guarding them from a cross with either pure parent, for six or seven, and in one case for ten generations, yet he asserts positively that their fertility never increases, but generally decreases greatly and suddenly.
Hibridoek hurrengo belaunaldietan izaten duten antzutasunari dagokionez, nahiz eta Gärtenerrek zenbait hibrido hazi ahal izan zituen sei, zazpi eta, kasu batean, hamar belaunalditan zehar, guraso garbietako batekin ere gurutzatzea eragotziz, hala ere, garbi dio haien ugalkortasunak ez zuela gora egin, eskuarki behera baizik, asko eta berehala, egin ere.