“As the brain of man is the speck of dust in the universe that thinks, so the leaves—the fern and the needled pine and the latticed frond and the seaweed ribbon—perceive the light in a fundamental and constructive sense. … Their leaves see the light, as my eyes can never do. … They impound its stellar energy, and with that force they make life out of the elements.” — Donald C. Peattie
I like thinking about the kinds of perception plants and other animals have that differ from human perceptions. And I wonder, what did Peattie, a botanist and naturalist, mean when he said that plants’ leaves (and, it’s understood now, stems) “perceive the light in a fundamental and constructive sense”?
We know that most plants have photoreceptors on their stems and leaves, which let them differentiate between red and blue light wavelengths, and to perceive wavelengths that humans can’t, those in the far red and ultraviolet parts of the spectrum. Plants need to detect light sources or they’ll go hungry [and so will we], because photosynthesis uses light energy to turn carbon dioxide and water into sugar, their food.
Plants detect light using several kinds of light receptors, including phototropins, which are sensitive to blue light, and phytochromes, a sort of on-off switch activated by red light and far-red light.
“… Tree, take a photograph of her thought, you can do it
with photosynthesis: silhouettes of seals appear, a swarmed
planet and its satellites, a
celestial atlas that breaks when tapped (it’s glass)
Some giraffes, some elephants, a lion scatter
in the clearing; in the clearing
the leaves of the world turn toward the light as do the letters of
the words are beautiful not for their accuracy but for their
dream … ” — from “Survey: Phototropes” by Eleni Sikelianos
When plants sense blue light (roughly 400-499 nm wavelengths) with their phototripins, “they initiate a cascade of signals that ends up modulating the activity of the hormone auxin. This causes cells on the shaded side of the stem to elongate, bending the plant towards the light.”
Blue light is instrumental in actuating phototropism, directional bending of a plant toward or, sometimes, away from a light source. It’s also critical in controlling other processes that optimize photosynthesis, such as the movement of chloroplasts, whose position within cells affects energy production, the positioning and expansion of leaves (which operate like little solar panels), and the opening of leaves’ stomata pores, which regulate gas exchange, allowing carbon dioxide uptake for photosynthesis.
When, using its phytochromes, the plant sees red light (wavelengths of around 667 nm), it becomes active in terms of physiological activity — growth, flowering, seed germination, e.g. — and the sensor is flipped so it will detect far-red light (around 730 nm in wavelength) when it next shines; “and when irradiated by far red it changes back to the form that is sensitive to red light. This has two key functions. It allows plants to ‘turn off’ at the end of the day –- because far-red light predominates at sunset -– and wake up again next day when the sun is high enough in the sky for red light to switch their phytochromes back on. It also allows them to sense when they are in the shade.”
The blue-light receptor, phototropin, and the red-light receptor, phytochrome, are completely different from the photoreceptors found in animals’ eyes, although all consist of a protein connected to a chemical dye that absorbs the light.
But “there is one type of photoreceptor … that we share. During daylight hours, cryptochromes within cells detect blue and UV light, using this signal to set an organism’s internal clock or circadian rhythms. In plants, this clock regulates many processes, including leaf movements and photosynthesis. So sight even helps plants tell the time,” just as it does for humans and other animals.
“What did the tree learn from the earth
to be able to talk with the sky?” — Pablo Neruda
Recently, there has been some evidence that plants may have rudimentary eyespots — ocelli — that can see colour and shape. In 2016 it was found that single-celled cyanobacteria, Synechocystis cyanobacteria, acted as ocelli, using their “entire cell body as a lens to focus an image of the light source at the cell membrane, as in the retina of an animal eye.” There are also studies showing “that some plants, such as the cabbage and mustard relative Arabidopsis, make proteins that are involved in the development and functioning of eyespots …. These proteins specifically show up in structures called plastoglobuli, which are famed for giving autumn leaves their red and orange hues,” and the suggestion is that plastoglobuli in plants may act as eyespots. Additionally, in 2014 it was reported (in Current Biology) that “the climbing wood vine Boquila trifoliolata can modify its leaves to mimic the colors and shapes of its host plant.” (This last observation takes one step further a theory I have long held about invasive and weedy plants that grow among non-weedy plants they strongly resemble, making it almost impossible to easily remove them.)
So — in a botanical, chemical, and perhaps mystical sense, plants (and algae and cyanobacteria) may actually see not only light but also colour and shape, and they certainly gather, impound, and transform the light energy of a star — our Sun, between 91 million and 94.5 million miles away — to make their own food and to keep animals alive on our planet. I’d call that very constructive, fundamental … and wondrous.
“The wonder is that we can see these trees and not wonder more.” – Ralph Waldo Emerson (supposedly said to John Muir when Emerson first saw sequoia trees)
“How Plants Perceive Light,” at In Defense of Plants, 26 July 2017.
Extremely detailed and scientific account of receptors, hormones, etc. in a paper titled “Phototropism: Growing towards an Understanding of Plant Movement,” in The Plant Cell, January 2014, by Liscum, Askinosie, Leuchtman, Morrow, Willenburg, & Coats.
“Blue Light Sensing in Plants: All You Need is a Little LOV,” by John M. Christie, Feb. 2008. Again, it gets scientific fast.
“Veggies with Vision: Do Plants See the World around Them?,” by Marta Zaraska, Scientific American, 1 Jan. 2017.
Experiment: How Do Plants ‘See’ Light, Untamed Science.