Flowering is a cascade reaction consisting of several steps. In the photoperiodic flowering, the photoperiodic signal is first received by phytochrome in the leaf, and the signal from phytochrome starts the biological clock. After the biological clock measures a certain period of time (inductive photoperiod), the production of the flowering stimulus in the leaf starts. The flowering stimulus is transmitted from the leaf to shoot apex. Upon arrival of the flowering stimulus, the growth mode of the meristem is changed from vegetative to reproductive. The shoot apical meristem produces primordia of floral organs; sepals, petals, stamens and carpels, thus generating a flower bud. All of the steps from the photoperception by phytochrome to the generation of a flower bud tend to be included in the concept of flowering. However, the perception of the photoperiodic signal is common to all processes of photomorphogenesis such as seed germination, timing and direction of cell division, and stem elongation. The process of time measurement by a biological clock is also common to photoperiodic responses as seen in the dormancy of seed and bud, leaf abscission, bulb formation and others. These two processes are not steps specific to flowering itself. Only the production of the flowering stimulus and the response of the stem apical meristem to it are steps unique to flowering. The process between these two steps is flowering in a narrow sense.
　　The step of flowering at which the flowering stimulus is generated is called floral induction. The step at which the shoot apical meristem has received the flowering stimulus, and is irreversibly committed to form flower bud primordia is called floral evocation. The formation of flower bud primordia after floral evocation is called flower morphogenesis. The identity of each floral organ is determined by three classes of genes according to the ABC model for flower morphogenesis. The triple mutant lacking all three classes of ABC genes forms a flower-like structure where leaves are formed at the position of each floral organ, instead of the flower. This shows that the process of flower morphogenesis is a different phenomenon which occurs after the flowering process is completed.
　　After the processes of flowering and flower morphogenes are completed, flower buds develop and finally petals open. This is the process of flower opening or anthesis which allows pollination and fertilization. Flower opening is also confused with flowering.
　　Photoperiodic flowering is not only a photoperiodic phenomenon but also a photomorphogenetic one. The studies on photoperiodism and photomorphogenesis now use more simple experimental systems such as ferns, single-celled algae and cyanobacteria. Therefore, flowering physiologists have concentrated their studies on flowering in the narrow sense, that is, floral induction and floral evocation. The main theme in the studies of flowering has become the study on the flowering stimulus.

　　Flowering physiology focuses on the transition from vegetative to reproductive growth. In Pharbitis nil, the first morphologically detectable sign of the transition from vegetative growth to reproductive growth is the appearance of flower bud primordia. Therefore, flowering is almost simultaneous with the formation of flower buds in P. nil.
　　In contrast to P. nil, flowering is not simultaneous with the formation of flower buds in rosette-forming long-day plants such as Arabidopsis thaliana. When they were exposed to a flower-inductive photoperiod, the first sign of flowering would be the formation of cauline leaves and elongation of stem. Furthermore, the shoot apical meristem produces bracts and inflorescence axils before the flower buds are formed on the inflorescence axils. In these plants, the formation of flower buds starts after the transition from vegetative growth to reproductive growth, which is flowering. The pin-formed (pin) mutant of A. thaliana has an elongated stem but does not form flower buds. The cauliflower (cal) mutant repeats inflorescence divergence and therefore cannot form floral buds. Flowering occurs but flower buds are not formed in these mutants.

　　In physiological experiments, plants are exposed to a flower-inductive photoperiodic cycle and the response is determined by detection of flower buds. This response is the result of the whole process from the perception of photoperiodic stimulus to flower morphogenesis. The genes with expression specifically induced under the flower-inductive condition could be identified in these flower-induced plants. Among those genes, however, there might be genes that are not involved in flowering itself, such as genes relating to other photomorphogenetic or photoperiodic phenomena or flower morphogenesis. On the other hand, some mutants have been isolated as mutants enhancing or suppressing flowering. Some of these mutants, however, might not be mutants of flowering genes. If the genes relating to the photoreceptor, biological clock or any other factors such as plant hormones which influence the flowering response were mutated, the flowering response would be influenced. Accordingly, the studies on flowering genes must be done carefully. The definition of the term flowering must be clarified, and "flowering" should be used in a narrow sense.。