or coleoptile (monocot) No leaf growth No chlorophyll Rapid stem elongation Limited radial expansion of stem Limited root elongation Limited production of lateral roots De-etiolated characteristics: Apical hook opens or coleoptile splits open Leaf growth promoted Chlorophyll produced Stem elongation suppressed Radial expansion of stem Root elongation promoted Lateral root development accelerated The developmental changes characteristic of photomorphogenesis shown by de-etiolated seedlings, are induced by light. Photoperiodism Some plants rely on light signals to determine when to switch from the vegetative to the flowering stage of plant development. This type of photomorphogenesis is known as photoperiodism and involves using red photoreceptors (phytochromes) to determine the daylength. As a result, photoperiodic plants only start making flowers when the days have reached a "critical daylength," allowing these plants to initiate their flowering period according to the time of year. For example, "long day" plants need long days to start flowering, and "short day" plants need to experience short days before they will start making flowers. Photoperiodism also has an effect on vegetative growth, including on bud dormancy in perennial plants, though this is not as well-documented as the effect of photoperiodism on the switch to the flowering stage. Light receptors for photomorphogenesis Typically, plants are responsive to wavelengths of light in the blue, red and far-red regions of the spectrum through the action of several different photosensory systems. The photoreceptors for red and far-red wavelengths are known as phytochromes. There are at least 5 members of the phytochrome family of photoreceptors. There are several blue light photoreceptors known as cryptochromes. The combination of phytochromes and cryptochromes mediate growth and the flowering of plants in response to red light, far-red light, and blue light. Red/far-red light Plants use phytochrome to detect and respond to red and far-red wavelengths. Phytochromes are signaling proteins that promote photomorphogenesis in response to red light and far-red light. Phytochrome is the only known photoreceptor that absorbs light in the red/far red spectrum of light (600-750 nm) specifically and only for photosensory purposes. Phytochromes are proteins with a light absorbing pigment attached called a chromophore. The chromophore is a linear tetrapyrrole called phytochromobilin. There are two forms of phytochromes: red light absorbing, Pr, and far-red light absorbing, Pfr. Pfr, which is the active form of phytochromes, can be reverted to Pr, which is the inactive form, slowly by inducing darkness or more rapidly by irradiation by far-red light. The