Florigen (or flowering hormone) is the hypothesized hormone-like molecule responsible for controlling and/or triggering flowering in plants. Florigen is produced in the leaves, and acts in the shoot apical meristem of buds and growing tips. It is known to be graft-transmissible, and even functions between species. Florigen has been found identical to the transcription factor FLOWERING LOCUS T (FT). Mechanism For a plant to begin flowering, it must undergo changes in its shoot apical meristem (SAM). However, there are multiple environmental factors affecting the plant even before it begins this process — in particular, light. It is through "the evolution of both internal and external control systems that enables plants to precisely regulate flowering so that it occurs at the optimal time for reproductive success." The way the plant determines this optimal time is through day-night periods through the use of photoperiodism. Although it was originally thought that the accumulation of photosynthetic products controlled the flowering of plants, two men by the names of Wightman Garner and Henry Allard proved it was not. They instead found that it was a matter of day length rather than the accumulation of the products within the plants that affected their flowering abilities. Flowering plants fall into two main photoperiodic response categories: "Short-day plants (SDPs) flower only in short days (qualitative SDPs), or their flowering is accelerated by short days (quantitative SDPs)" "Long-day plants (LDPs) flower only in long days (qualitative LDPs), or their flowering is accelerated by long days (quantitative LDPs)" These types of flowering plants are differentiated by the whether the day has exceeded some duration - usually calculated by 24-hour cycles - known as the critical day length. It is also important to note that there is no absolute value for the minimum day length as it varies greatly amid species. Until the correct amount of day length is reached, the plants ensure no flowering results. They do so through adaptations like preventing immature plants from responding to inadequate day lengths. Plants also have the ability to prevent the response of the photoperiodic stimulus until a certain temperature is reached. Species like winter wheat that rely on just that. The wheat require a cold period before being able to respond to the photoperiod. This is known as vernalization or overwintering. This ebb-and-flow of flowering in plants is essentially controlled by an internal clock known as the endogenous oscillator. It is thought