Flower formation in the dayneutral tobacco (Nicotiana tabacum L.) cultivar “Trapezond” was accelerated by graft union with the short-day tobacco “Maryland Mammoth” when the grafts were kept on short days and by graft union with the long-day plant N. silvestris L. when they were kept on long days. When Maryland Mammoth/Trapezond grafts were kept on long days, flower formation in Trapezond was not, or only slightly, delayed compared to Trapezond/Trapezond controls; but when N. silvestris/Trapezond grafts were kept on short days, flower formation in Trapezond was inhibited and its growth changed to dwarf-like habit. The former results indicate transmission of flower-promoting material(s) (“florigen”) from photoperiodic plants maintained under flower-promoting daylength conditions to a dayneutral graft partner; the latter indicate the presence in the long-day plant N. silvestris under short-day conditions of potent flower-inhibiting and growth-regulating material(s) that can also be transmitted to a day-neutral partner. Analogous flower-inhibitory materials seem not to be present, or to be present to a much lesser extent, in the short-day plant Maryland Mammoth under long-day conditions.