Sight is an interesting word. It can be defined as what we see or how we see. Sight can also be about perception of what we see as in “insight.” Suspending our perceptions, going beyond “sight” as an eyeball thing, isn’t too difficult. We know that humans who are blinded use touch, smell, and sound to put together a picture within their minds. Animals with poor vision may use sonar to “see” where they are going. We are familiar with this. In 1907 Francis Darwin (son of famed Charles Darwin) hypothesized that plants have organs on their leaves that act both as a lens and a photoreceptor allowing plants to “see”. He was right.
Plants are able to detect light into the ultraviolet and far red spectrums. Phototropins in the leaves are sensitive to blue light. Blue light helps the plant through the photosynthesis process. When the phototropins detect blue light, signals are sent to the cells to elongate, thereby bending towards the sun. Plants also have phytochromes, a light receptor switch that can turn off or on depending on the need for light. When the phytochromes detect red light, they are on alert to detect far red light. When the phytochromes detect far red light, they switch to detect red light. Think about a light activated night light. During the day, as the sun hits it, the night light is primed to detect dark but stays off. When light diminishes to the point that the receptor on the night light detects dark shadow, it switches on and is now primed to detect light. What all of this means for the phytochromes is that when it detects too much shade (far red), there is a reaction that lets the plant know it isn’t receiving enough sunlight (as far red is in shadow and red light is in sun), so the plant will increase its growth towards the sun. At dusk, plants see that there is a lot of far red light and will “shut off,” there by sleeping. At dawn, as red light increases and far red decreases, the phytochromes switch “on” and the plant awakens.
There are studies being done on heliotropic plants that show they can reset themselves at night to face the sun first thing in the morning (even when covered). Wow!
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