Plants utilize light not only for photosynthesis but also as an environmental signal for many developmental processes. Plants are very sensitive to seasonal, daily, and moment-to-moment changes in solar radiation. They are thus capable of perceiving wavelength, intensity, direction, duration, and other attributes of light to bring about appropriate physiological and developmental changes. These light-triggered growth and developmental responses are known as photomorphogenic responses (Box 13.1). Thus, photomorphogenesis may be defined as the developmental response of an organism to the information in light, which may be its quantity, quality (i.e., the wavelength), and direction or relative length of day and night (photoperiod). The importance of light in plant development can be most dramatically illustrated in case of early seedling growth. A dark-grown seedling is said to be etiolated. In etiolation, embryonic stem (hypocotyl in dicots and epicotyl in monocots) of seedlings exhibits very rapid and extensive elongation of internodes. There is no cotyledon/leaf expansion, and seedlings appear pale as there is no chloroplast development. So, etiolated plants are pale yellow, and the hypocotyl remains “hooked” at the apex. Curving of the hypocotyl is thought to protect the apical meristem from damage during seedling growth through the soil. On exposure to light, hypocotyl elongation slows down; cotyledons and leaves expand and become green. The apical hook straightens. In monocots, the etiolated coleoptile exhibits extended growth which gets decelerated (slows down) in light and developing leaves pierce its tip (Fig. 13.1). In contrast with scotomorphogenesis, seedlings grown in light exhibit photomorphogenesis and exhibit relatively embryonic stem, lose apical hook, and develop expanded green cotyledons, and there is rapid initiation of leaf development at the shoot meristem. Upon sensing light, a seedling emerging from the soil switches from scotomorphogenesis to photomorphogenesis. This process is known as de-etiolation, and it involves the coaction of red/far-red light-absorbing phytochrome and blue light-sensitive cryptochrome.