Five Fact Friday: Plants and Sight

1. Like us, plants can 'see' meaning that they can detect electromagnetic waves and respond to them. This is very important as light equals energy to a plant - so the response to light stimulus can be when to grow or how fast to grow.
2. Charles Darwin, and his son Francis, performed many experiments on plants during their careers. One particular experiment found that light hitting the tips of plant stems make the plant respond by bending towards the light: Phototropism. 
3. Plants also respond to the changing day lengths through the year. Known as Photoperiodism, the plant can detect the continuous length of darkness each night and reacts. This reaction to darkness is used on many plants, for instance the humble chrysanthemum. By switching on a red light just for a few seconds each night throughout the autumn and winter (the usual time for flowering), the chrysanthemum is prevented from flowering. Then two weeks before Mother's Day the darkness is not interrupted by switching on the light - this length on continuous darkness makes the plant respond by flowering - just in time to be shipped to supermarkets!
4. Plants are also regulated by the Circadian clock, just as we are. This developed well before there was a split that developed into the Plant Kingdom and the Animal Kingdom. It would seem that these clocks may have developed to prevent damage from high UV radiation. Seen in humans as sun burn.
5. Blue light is used by the plant for phototropism and the Circadian clock. Red light is used by the plant to know when to flower. Far red light, the red seen for a short period as the sun goes down is used by the plant to shut down for the night. And genetics is finding more and more photoreceptors showing the complexities of plants.

References
Chamovitz, D. (2012) What a Plant Knows, Oxford, Oneworld Book.