A diffusible substance plays a role. This is clear since the leaves are the receptors, but the meristem is converted to reproductive stage. Rate of transport consistent with phloem movement. The diffusible substance is probably similar in most plants. There is little direct chemical evidence for its existence.
Most evidence is from physiological experiments as described above.
An extract of induced cocklebur has weak floral-inducing ability. Ethylene, IAA or cytokinin are associated with flowering in some species. One problem with many of these studies is that the concentration of hormone used was much larger than the amounts that are naturally-occurring and could stimulate abnormal processes. Other substances may be involved in flowering, too. This existence of this substance is dubious.