Although plants have no eyes, light defines their relative movement and absolute growth. Such a dependency on light that most animals and plants possess is called photoperiodism. It affects their orientation, physiological activity and responses.
Adaptation towards the different amounts of light available in the surrounding environment was the universal goal of almost every life form we have encountered so far. When the ancestors of modern day plants began utilising sunlight as a stable source of energy, photosynthesis turned into a process that shaped life on Earth as we currently know it. By converting the Sun’s energy radiation into sugars, plants began to provide most of the Earth’s species with food.
But how is this energy transformation achieved exactly? It is all thanks to the molecule named Chlorophyll, which traps the passing through it sunlight. Another way to describe this molecule is as a photoreceptor. Interestingly enough, the name chlorophyll originates from the greek word chloros, which means yellowish-green.
When chlorophyll gathers passing sun rays, the process of photosynthesis occurs where they are transformed into energy. Or in other words, carbon dioxide and water are converted into carbohydrates and oxygen. Plants are one of the first biological organisms on this planet to have directly made use of solar energy as means to grow, which on the other hand also provided food for many animals.
Before plants reached this evolutionary step, however, they had a major problem to deal with. The frequent fluctuations in light intensity were extremely dangerous for plants, so they had to be selective of what light to absorb.
Most of the radiation reaching Earth is located in the midrange “green” section of the spectrum and an of overdose from these waves can easily destroy molecule bonds, thus damaging the cells. In order to protect themselves, plants evolved to reflect the majority of this “green” light.
On the other hand, the highly energetic ultraviolet and blue light waves produced enough energy for growth, whereas red lightwaves are believed to have been used for orientation.
There are two types of chlorophyll molecules. Chlorophyll a and chlorophyll b. Both can absorb light from the ultraviolet, blue and red part of the spectrum, but with different specialisations. Chlorophyll a primarily absorbs the waves with length from 600 - 700 nm like blue, whereas chlorophyll b specialises in absorbing the waves with length of below 500 nm like red. The middle, 500 - 600 nm, green wavelengths are reflected away and can then be caught by the human eye, which gives us the perception of plants being green.
Humans can only distinguish three basic wavelengths, when compared to birds, for example, with their ability to see four basic wavelengths. This allows them to observe a much greater variety of hues and tints and can much more easily distinguish between different types of plants, be it for food or nesting purposes. If you find it hard to imagine what their world looks like, just try and comprehend the world of some sea creatures with up to twelve observable wavelengths. Perception of the surroundings is only what evolution has allowed it to be.
Plants do not see or use light as we do and the fact of humans perceiving them as green is an outcome of their evolved metabolism and our subjective perception. It is important you understand that what our eyes see, does not mean it is in fact the very true nature of the object.