It consists of four P's: photosynthesis, in which water and carbon dioxide are brought together under light to form carbohydrates; photoperiodism, the relationship between day and night lengths; phototropism, the reaction of plants to the light source; and, phytochrome response, a blue pigment called phytochrome occurring in two forms, one producing optimum growth response, the other nullifying it. A66-910+; A72-243+