Plant growth regulators control the growth and development of plants. In addition to PGRs, extrinsic factors such as light and temperature also influence the growth and development of plants. In fact, plants can perceive the duration of light and dark exposure and respond accordingly. Flowering in some plants is induced by their relative exposure to light and dark periods. This response of plants to the relative period of day or night is called photoperiodism. The site of perception of photoperiod is the leaves. Therefore, scientists hypothesised that these plants synthesise a flower-inducing hormone called florigen in their leaves. When these plants are subject to photoperiod, this hormone travels from the leaves to the shoot tips to induce flowering. Later, they found the PGRs to be the actual hormones that control flowering in plants. Based on their requirement of light, plants are categorised into long-day, short-day and day-neutral plants. Long-day plants like strawberry and soybean need to be exposed to more than 12 hours of light to induce flowering. Short-day plants like pea and carrot need less than 12 hours of light exposure to induce flowering while day-neutral plants such as sunflower and cucumber can produce flowering with more as well as less light exposure. Apart from light, temperature also influences flowering in some plants. Some plants produce more flowers when exposed to a specific period of low temperature. This kind of development of flowers due to the influence of low temperature is called vernalisation. Since these plants need to be exposed to low temperatures for a specific period, vernalisation provides the time for vegetative growth and hence prevents early flowering. Plants that exhibit vernalisation include winter annuals and winter biennials. Winter annuals such as rice, wheat and barley are planted during autumn. Their vegetative growth starts in winter when they undergo vernalisation which induces them to flower during spring. Finally, they are harvested in summer.On the other hand, cabbage and carrot are winter biennials. Their vegetative growth starts in winter when they undergo vernalisation which induces them to flower during their second year. Thus, vernalisation and photoperiod play a vital role in plant growth and development.