Because plants can't walk away from adverse conditions, the most successful plants are those with finely honed response mechanisms. For example, when humidity is low, plants lose water to the dry air through a system of leaf pores called stomata. When their moisture balance is becoming unfavorable, plants respond by closing their stomata.
All plants contain a light-reactive pigment called phytochrome.
The pigment is instrumental in sensing light quality, especially whether a plant or seed is shaded. With thanks to the National Science Foundation , we're Block and Byrd for Earth and Sky.