As pointed out by Lang, two basically different possibilities can be visualized in photoperiodic and cold-requiring plants: inductive conditions promote flower initiation; and noninductive conditions inhibit it. There is increasing experimental evidence favoring the existence of transmissible floral inhibitors, at least in some species. These inhibitors are apparently produced in leaves exposed to photoperiodic conditions unfavorable for flower initiation and presumably act at the shoot meristems. The basic evidence for the existence of florigen comes from numerous grafting experiments in which a "receptor" plant kept in noninductive conditions is induced to flower by union with a previously induced "donor" plant. The relationship between vernalin and the floral stimulus of photoperiodic species has been investigated using grafting techniques. Induced LDP or in one case an induced SDP are effective donors for nonthermoinduced cold-requiring plants. Early work with photoperiodic species indicates that the floral hormone moves only through living tissues.