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How do leaves detect day length and signal a shoot apex to begin flowering?

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fern life cycle — Plant development The life cycle of the fern. (1) Clusters (sori) of sporangia (spore cases) grow on the undersurface of mature fern leaves. (2) Released from its spore case, the haploid spore is carried to the ground, where it germinates into a tiny, usually heart-shaped, gametophyte (gamete-producing structure), anchored to the ground by rhizoids (rootlike projections). (3) Under moist conditions, mature sperm are released from the antheridia and swim to the egg-producing archegonia that have formed on the gametophyte's lower surface. (4) When fertilization occurs, a zygote forms and develops into an embryo within the archegonium. (5) The embryo eventually grows larger than the gametophyte and becomes a sporophyte.

In the context of plant development, many plants utilize the length of the day—a phenomenon known as photoperiodism—to determine the appropriate time to transition from vegetative growth to flowering.1

The process typically begins in the leaves, which act as the primary organs for perceiving the day-length signal. While the exact mechanisms of this perception involve complex physiological reactions, the plant detects changes in the duration of light and dark periods. Once the plant perceives a critical day length, it generates a signaling agent that is transmitted to the shoot apical meristem.213

This mobile signaling agent is known as florigen, a hormone or hormonal agent that promotes the transition of the shoot apex from producing vegetative structures to forming reproductive buds. The induction of flowering is a highly regulated process, often involving shifts in the balance of various hormones and carbohydrates within the plant, which ultimately trigger the physiological changes necessary for the development of flowers.435