Light regulates plant growth and development through two main systems - photoregulation and phytochrome regulation. Photoregulation involves blue light receptors like cryptochromes and phototropins that control responses like phototropism and chlorophyll synthesis. Phytochrome regulation uses the red light receptor phytochrome, which exists in active and inactive forms, to regulate flowering and photoperiodism, the biological response of plants to changes in the ratio of light and dark periods. Phytochromes measure seasons to coordinate events like flowering. Plants are classified as long day, short day or day neutral based on their photoperiod requirements for flowering.