Phytochrome is a blue-green pigment that acts as a photoreceptor for the detection of night and day in plants and is universal in vascular plants. It has two forms Pr (inactive) and Pfr (active). Pr is readily converted to Pfr under natural light. Pfr converts back to Pr in the dark more slowly. Pfr predominates in daylight. The plant measures night length by the amount of phytochrome in each form. Phytochrome interacts with the ‘clock genes’ that maintain the plant’s biological clock. The hormone is transported to the apical meristem where it causes a change in a gene expression that leads to flowering. Pfr promotes flowering in long-day plants and inhibits flowering in short-day plants. Short day plants flower in the summer e.g. potatoes. Long day plants flower in the winter e.g. lettuce.