The potential for in vitro floral photoinduction of epidermal and subepidermal thin cell layers (TCLs) taken from the short-day plant Nicotiana tabacum L. ‘Maryland Mammoth’, the long-day plant Nicotiana sylvestris L., and the day-neutral plant Nicotiana tabacum L. ‘Samsun’, in both the floral and vegetative states, was examined. Whether cultured under long days (16 hr) or short (8 hr), only TCLs from the flowering day-neutral species flowered in vitro. These were termed responsive TCLs. TCLs from photoperiodic plants yielded only vegetative buds; these were termed nonresponsive. Vegetative bud formation in nonflowering TCLs generally was greater than in flowering TCLs but did not approach the number of flower buds on flowering TCLs. In vitro grafts of responsive TCLs to nonresponsive TCLs resulted in flowering only in the responsive portion, regardless of the position of the graft. Just as the nonresponsive TCLs were not induced to flower by some graft-transmissible substance, responsive TCLS were not inhibited from flowering when grafted to nonresponsive TCLs.