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Why Are Plants Green? To Reduce the Noise in Photosynthesis (quantamagazine.org)
247 points by theafh on July 31, 2020 | hide | past | favorite | 100 comments

Some trees and plants that are still rebellious about this:


Of course those also have chlorophyll, but the color is overridden by other color particles.

Sadly we have no blue trees. We have blue flowers though and it would probably be possible to create one. That would be so awesome.

A blue spruce is a fairly blue tree:


Blue spruce is actually pretty darn green, but its needs have a waxy coating that alters our perception of the colour of the underlying tissue. New needles, and needles you scrape with your fingernails, reveal their true green colour.

A waxy coating is a strong selector for survival in an environment where preventing evaporation is a must, such as locations in which the ambient temperature drops below the freezing point of water for much of the year.

Isn't that a little like saying that a shiny red sports car is actually metallic gray, but it has a coating that alters our perception of it so that it appears red to our eyes?

Color is what we perceive, nothing else. If it looks blue, it is blue.

Well, if you look the world through blue-tinted windows it doesn't make the world suddenly bluer though. I think the same kind-of applies here.

I think in this case the wax lining on the pines would be moreso a property of the object rather than the apparatus through which we see the object.

Is not the category of it Photosynthesis? Internally the environment of Blue spruce is the evolutionary standard of Green, not Blue. The blue perception would also reflect a Blue perception of photosynthesis standard.

It's more like saying that the steel your shiny red sports car is made of is actually metallic grey even though it has a veneer of red enamel on it.

Do black spruce have a waxy coating? They certainly don't appear blue, and they thrive in locations with much colder winters than the blue spruce.

As far as I know all coniferous (Pinophyta) have waxy coatings, black spruces are a species of Pinophyta so I'd assume they do.

I might be wrong, I'm no botanist.

The only blue flowers we have are dyed.

We do not know of any “true blue” plant pigments. There are violets and many colors that might look close..

The sky doesn't contain any blue pigment, either. And a blue-eyed person has no blue in their irises.

You're referring to pigment; there is no blue plant pigment, true, but that seems a little pedantic, even if it's interesting. Plenty of flowers reflect blue light, which is what people mean when they say an object is "blue".

What? I have a bunch of hydrangeas in my yard right now that range from light to dark blue. They are definitely not violet or any other close blue color. They are as blue as blue can be. And they haven’t been artificially tampered with. We planted them 15 years ago and have done nothing for them since. Not even fertilizer.

Those plants that do appear blue are in fact often using a red pigment known as anthocyanin. Through pH shifts and a mixing of pigments, combined with the reflection of natural light, the plants are able to generate the appearance of a naturally occurring, blue color. That's the reason why plants such as bluebells, hydrangeas and morning glories appear various shades of blue, when in fact, as Lee explains, "There is no true blue pigment in plants."

> through [...] mixing of pigments, combined with the reflection of natural light, the plants are able to generate [...] blue color

We have a way to describe objects which “generate a blue color when light shines on them”. We say they are “blue” objects.

Likewise, bird feathers or butterfly wings which absorb/reflect light based on their small-scale structure and appear blue are still called “blue”, even though they might be colorless when pulverized.

There are plenty of “blue flowers” out there.

Right, but botanically speaking, we have not found a “true blue” plant.

What does it matter whether they have blue pigment or not? If nearly every person looks at it with their own eyes and say, "That is definitely blue" then it is blue. I don't care why or how it is blue. Anything else is just pedantry.

Some of the "blues" generated in nature rely on specific physical structure or other tricks to appear blue. This means they reflect predominantly blue light, but may not retain that "blueness" when ground/processed into a pigment. That's one reason there are few natural organic blue pigments, even the ancient Egyptians relied on chemical solutions for "Egyptian Blue".


Interestingly, the one dye listed in the link above that came from a plant is indigo, which is extracted from the green leaves of the plant https://en.wikipedia.org/wiki/Indigofera_tinctoria . There's no animal or plant I know of that both appears blue and could be used to create even a poor blue pigment, though I'd definitely be interested in any exceptions I haven't heard of.


Thanks, this is really interesting stuff!

When studying flora, things like color or inflorescence, for example, are examined with excessive concern with minor details, though.

Do you ever manipulate your hydrangeas’ color by changing soil pH? One easy way is adding used coffee grinds.

I have seen many blue flowers personally but since this is a trustworthy internet forum I believe you

Fwiw, I hold a degree in Horticulture. A good source is David Lee, author of Nature's Palette: The Science of Plant Color and a retired professor in the Department of Biological Sciences at Florida International University in Miami.

Maybe there's some kind of definition that make this make sense? On the face of it it seems very weird to claim that "this is actually a red pigment, it just looks blue". Why not say that it's a blue pigment which looks red in certain pH conditions?

I think our disagreement is over blue pigment vs blue appearance. Lots of things in nature use structural coloration https://en.wikipedia.org/wiki/Structural_coloration

Microstructure interference can create blue, but perhaps not the flower pigments themselves

The article seems nonsensical in the same way, though.

> For plants, blue is achieved by mixing naturally occurring pigments, very much as an artist would mix colours. The most commonly used are the red pigments, called anthocyanins, and whose appearance can be changed by varying acidity.

It's just saying the same thing, "a red pigment appearing blue", without any explanation. (Also, as artists know, blue and red are primary colors, you can't get blue by mixing red paints.)

For color variety, depending on yard location, I have a variation of Loropetalum, one Lilac, and a Japanese Maple. Had a pair of plum trees but the ornamental variety you can buy tends to not last as long.

For low ground stuff Nandina can offer greens, reds, and yellows, in good variety.

Interestingly, when organisms retain this kind of diversity it is because of the evolutionary advantage that it confers. In this case that would be the ability of plants in general to evolve towards other colors. For this to be an advantage the driver for that would have to be present in our long term environment. Possibly some sort of moderator appears in the atmosphere? Or does the spectra of the sun change due to some process?

I think trying to identify evolutionary motivation is generally risky and subject to all kinds of human biases. Any hypothesis is unfalsifiable (so, it's speculation, not science).

Having said that, "Green = provides a convenient way for animals to identify a great source of energy" is likely to be one driver here.

Perhaps the sun isn't the reason. I believe some plants hide from certain kind of predators by changing their colors or maybe they could draw certain insects or birds for additional pollination. Perhaps that was a more significant advantage.

Isn't that a cultivar though, artificially maintained in horticulture. Does it occur in significant numbers in wild populations?

Why wouldn't fluctuations of sunlight that cause instability in the green spectrum also cause instabilities in the red and blue spectra?

They do. The paper's point is that plants want to choose two wavelengths with different amounts of average power. This gives the plant a window to tune within to get the desired amount of output power. If the total illumination drops, the plant compensates by tuning more towards the high power wavelength, and vice versa.

But the paper makes the point that the tuning mechanism isn't perfect due to internal noise. If the power level difference between the two input wavelengths is too great, then the random fluctuation of the tuning mechanism will itself create a lot of noise in the output. So basically there's an optimal value for the difference of average power between the two wavelengths.

Now, why the plant doesn't absorb the peak green wavelength and the closest wavelength whose power is exactly the optimal difference less was unclear to me. I think the idea was that the plant also wants to minimize the wavelength difference, and a given power delta can be achieved with less difference in wavelength in the steeper sloping sections of spectrum power graph.

I think the point is that since the power available at these wavelengths vary sharply with frequency in these regions of the solar spectrum, the plant can easily compensate for brightness fluctuations in these portions of the spectrum with minor tweaks to the target wavelength of the relevant photosystems

I'd this is the case, it's a shame the article didn't mention it.

It makes sense the changing environmental conditions for a molecule could affect the wavelengths of light it absorbs (for example, pH, temperature, etc), but I'm not sure this has been demonstrated for plant pigments?

I'm confused though -- how can the plant possibly vary this?

In principle plants have several different variants of the light absorbing molecules with different absorption spectra but I don't know how this would actually be regulated. But it is certainly imaginable that plants vary the relative abundance of the different light absorbing molecules or their efficiencies and this could be more or less automatic because of varying conditions like pH or whatnot that affects efficiency or lifetime of the molecules with a simple feedback mechanism or it could be actively regulated by a more complex feedback loop.

> It might be highly efficient to specialize in collecting just the peak energy in green light, but that would be detrimental for plants because, when the sunlight flickered, the noise from the input signal would fluctuate too wildly for the complex to regulate the energy flow.

> Instead, for a safe, steady energy output, the pigments of the photosystem had to be very finely tuned in a certain way. The pigments needed to absorb light at similar wavelengths to reduce the internal noise. But they also needed to absorb light at different rates to buffer against the external noise caused by swings in light intensity. The best light for the pigments to absorb, then, was in the steepest parts of the intensity curve for the solar spectrum — the red and blue parts of the spectrum.

I admit there's a bit of a jump there (gotta read the actual article, not the journalist retelling I suppose), but I assume the gist of the math is something like this:

Lets say direct green light delivers a maximum 100 "units of photo-energy" -- gonna play loose with the physics to demonstrate the math.

When a cloud passes over it, lets say the intensity drops to only 75%. Lets also say for now we always convert the energy at 100% efficiency.

So with green light, your 100 units of energy drops by 25 units with each passing cloud.

Now lets say blue light delivers only 80 units of energy. When that same cloud passes over, 80 * 75% = 60 units of energy, or a drop of 20 units.

So, if your process is sensitive to changes in absolute energy, you would rather have a swing of 20 units for every passing cloud than a swing of 25 units. Yeah, you might get less absolute energy (60 units rather than 75), but if the cost of energy swings in your process was very high, the tradeoff might be worth it.

You could also play with the efficiency-of-conversion (ie have higher conversion efficiency at the lower-swing points) for some fun second-order effects.

This is just a toy example of what the underlying dynamics could be. Gotta read the actual paper to understand the actual model they developed. https://arxiv.org/pdf/1912.12281.pdf

> Now lets say blue light delivers only 80 units of energy. When that same cloud passes over, 80 * 75% = 60 units of energy, or a drop of 20 units. So, if your process is sensitive to changes in absolute energy, you would rather have a swing of 20 units for every passing cloud than a swing of 25 units.

That's not exactly it. The idea is that the blue light absorption can be tuned (presumably by shifting the absorbed wavelengths by a few nanometer) to compensate for external changes. So if the overall light intensity drops 1%, the system responds by shifting the absorbed spectrum to get 1% more power.

This only works in parts of the solar spectrum where power varies sufficiently versus wavelength, hence the preference for blue and red.

Ahh I get it. Maybe the leaves even actually change colour when shaded, but our eyes just aren't sensitive enough?

I'm confused though -- how can the plant possibly shift this?

Biological light harvesting complexes[1] act as an antenna network. I guess the shape and other properties of such a complex change depending on conditions.

The paper doesn't really try to explain the biological tuning mechanism. Instead, they created a model of an antenna network capable of tuning. As in, this model had a bunch of parameters describing how it captured light and tuned. Then if they optimized those parameters for stability given the light spectra that different plants are exposed to, they found that their model would reproduce the actual absorption spectra of those plants. This strongly suggest that the plants have done the same optimization by evolution.

[1] https://en.wikipedia.org/wiki/Light-harvesting_complexes_of_...

And on what timescales does this timing occur? Are we talking seconds? Or days, i.e. accomplished by growing different structures?

FWIW, hardening seedlings to sunlight from a greenhouse is all about switching their spectrum, and it takes 3 to 7 days depending on what plant you're talking about. Gardener's rule of thumb. 4/6/8hr exposure etc.

I wonder if anyone has used RGB LEDs and tormented plants with short cycle color changes yet? There's much anecdata and debate over spectrum vs efficiency in indoor gardening, but it's faded as power density became more obviously dominant, i think.

Yeah, on the orders of days is what I'd expect. So if that's right, then what kind of meaningful tuning are talking about for plants? On the order of days you can simple grow more or less of the green chlorophyll; is that not tuning enough?

Near instantaneous. In the paper it is described as "noise-cancelling".

(waves hands) proteins (or something)

Interesting but why would it not apply the same shifting within the green light spectrum which seems to be quite stabile?


That's kind of the point. The green section is flat - if you vary the frequency by a couple of nm, you do not see a meaningful change in power input.

But the slopes in the red and blue sections allow you to adjust the input power significantly by tweaking slightly the wavelength of the incoming light.

The color of plants and the statistics of light is fascinating.

Tangentially related, I was developing a CV app for farmers in an early stage weed startup right before legalization in California so they could monitor analytics on the health of their sprouts as well as identify strains. The camera conditions were all over the place and data preprocessing stage was an absolute nightmare. We tried everything from filters to background removal, augmentation techniques through transformations, noise induction for generalization - nothing really improved the baseline models because the photos honestly sucked. Farmers ignored our guidelines on lighting and framing and format and kept sending in inconsistent garbage.

I had a eureka moment. What if we sent each of the farmers a little piece of square cardboard painted magenta? The idea was that the increase in contrast would allow us to process the leaf contour a little bit better. The fact that the card was square meant some farmers even took the time to take the plant indoors so they could frame it better within the edges. Data quality improved dramatically. It worked.

Unfortunately legalization did not work out as we planned, farmers disappeared and the market was monopolized by corporations, there wasn't any interest in helping develop strains locally.

Related, one of my favourite videos: Richard Feynmann explains fire:


My understanding of the evolution of Earth flora is that prior to plants being green, the dominant plant life was red (think of red algae blooms) and that the current dominant green plant life likely evolved to use different photons along the EM spectrum where there was less competition.

Funny enough as I understand visible light and the EMR spectrum there is no "green" (color/wave length/energy) rather the color green is a construct originating not in the light spectrum but in the mind of the observer.

There is a range of wavelengths of light that humans perceive as green. The same is true for every color that is part of the rainbow. In contrast, the "pure purples" do not appear in the rainbow, and there is no single wavelength of light that humans perceive as purple (it requires red light plus blue light).

The perception of color is a pretty wild area of science. Colors seem to be culturally dependent. In that, people literally cannot see the difference between blue and green if their language does not have words to distinguish them. Even when big rewards are given for the 'correct' answer. Colors also follow certain patterns, with colors like blue being the last to be named in a culture.


Fun fact, the color blue does not appear at all in the Iliad, Homer describes the ocean as wine, despite the stunning blue colors of grecian seas.

Color is still there, just like there are frequencies between A4 440 Hz and A#4 466.16 Hz. Most of the people can't name "color" of pure sound. Yet they feel difference.

Purple is a chord.

Its the other way around. A classic study from the 1960s found that color words and the correlated perception were pretty similar among a large number of languages. That suggest something physiological about color perception.

What is curious is how color words evolve. Most languages have between two and eight basic color words (and color concepts). Those with two colors is almost always the same two colors- light and dark. The third color is usually red-brown. And fourth usually blue-green.

>In that, people literally cannot see the difference between blue and green if their language does not have words to distinguish them.

People literally can see the difference between blue and green even if their language doesn't differentiate between the two, I don't know if you're misremembering a claim and a negation sneaked in so please don't take this post too harshly if that's the case, but the idea that say Japanese people can't tell the difference between blue and green is patently false and should be addressed, in fact the video you linked almost does so at 2:34 -

>Some researchers took this and other ancient writings to wrongly speculate that earlier societies were colour blind.

That Homer described the ocean as wine in colour is not an issue of perception but one of language in trying to describe a colour that is not differentiated from other colours, the same is true for other 'perception' issues in the ancient world like green coloured honey. To be clear visual acuity tests have been done on modern populations and tribes which don't differentiate between such colours or overall define less colour categories and it should be no surprise to learn that they can see the difference between those colours just fine.

The whole idea that it's a difference in perception is fraught with issues, like what happens when a language naturally develops words for new categories of colours or new colours? Does a generation undergo the collective experience of literally being able to see/differentiate a new colour? If so why isn't this written about more, is it something that only happens in kids? What would be the reason for this sudden shift in perspective, because it certainly isn't a physiological change that occurs.

What happens when an adult learns a second language which differentiates between more colours? The classic romanticised view here is that learning a new language literally let's you see the world in a different perspective, but then why is it that enhanced perspective rarely more than a curiosity (language x has two words for this colour)? The Russian language has separate words for a dark blue (siniy) and a light blue (goluboy) but English doesn't differentiate between them, do the Russians see an extra colour? What does the science say? Well the science is somewhat interesting here, Russians are able to differentiate between dark blues and lighter blues ever so slightly faster (124ms), but this is worlds apart from the claim that some languages are literally capable of seeing more colours.

In general this line of thinking is known as linguistic relativity, or the view that language shapes perception and cognition, and is something that has generally been discredited among linguists as being discriminatory and harmful as well as being based on faulty reasoning or studies and occasionally fraudulent papers. For example, and I really don't mean to attribute any malice to your post, but if we're considering Homer as being unable to differentiate between an ocean blue and a dark red wine, what do we make of cultures and languages that don't differentiate between smoking, drinking, or eating? Do they not know the difference between those actions? What about the Pirahã people who only have two words (differentiated by tone) for 'small quantity' and 'large quantity' and no other words for numerals? This line of thinking is fairly harmless when applied to the way we perceive colours but can be actively harmful to people who perceive the world the exact same way we do but don't have as expressive language for these particular topics.

For anybody interested in more linguistic oddities and/or the damage linguistic relativism can do I recommend the book 'The Language Hoax' by John McWhorter, there's also an hour long talk on it available on Youtube [0]. The book deals with the more recent studies on how language affects the ways we think in a grounded way and shows how minor some of the best examples given can be like in the case of dark and light blue in Russian. The book is also in response to the general public's view and romanticism of linguistic relativity and in particular in response to a book by another linguist Guy Deutscher titled 'Through the Language Glass', where Guy feeds into the perception that language helps shape the way we think, and it is a good book but it still doesn't get close to saying that other languages see more colours.

[0] https://www.youtube.com/watch?v=yXBQrz_b-Ng

There is no need to travel to jungles. Subtle differences are all around us.

* plants - trees, grasses, flowers, native and garden species, once I knew maybe 400 names, now come to disuse and quickly slip away

* food - ingredients and prepared

* fonts - Comic Sans, Times New Roman, Helvetica and many more

* car models - a lot of people know them by heart

It would be a strange claim we do not perceive difference without a name. We do but we do not care. And when we care we want to communicate and names become handy.

I had heard of the similar Purple Earth Hypothesis[1] wherein organisms with photosynthesis based on retinal arose in the oceans early on. Chlorophyll-based life developed deeper and took advantage of the red and blue light that filtered through.

The hypothesis seems pretty speculative, but maybe it's compatible with this new research, which could explain why green plants came to dominate despite retinal being simpler.

[1] https://en.wikipedia.org/wiki/Purple_Earth_hypothesis

You're probably thinking of magenta [0]. Green light absolutely does exist, it has a wavelength of around 500-550 nm [1].

[0] http://www.biotele.com/magenta.html

[1] https://en.wikipedia.org/wiki/Light#/media/File:EM_spectrum....

I wonder how could one come up with such a "green" theory

* green in rainbow and prism makes it equal to other rainbow colors

* green in RGB requires pure color for wider gamut

* birds receptors are not screwed [1]

[1] https://upload.wikimedia.org/wikipedia/commons/e/e8/BirdVisu...

Land plants are newer than algae, but green algae is older than red algae (as evidenced by the endosymbiosis order). But maybe green algae got dominated by the others pretty early on.

edit actually that's brown algae that's the derived one. red and green it looks like both come from the original endosymbiosis.

I was thinking about the same hypothesis but IIRC it was highly speculative, with the main evidence for it simply being that modern plants evolved to use those other photosynthesizers' castoffs. We may not need that hypothesis anymore if we have a solid reason to avoid green anyway.

I was really looking forward to reading this but its a bit of a disappointment.

First, green does not have the most energy of the visual spectrum, Blue, or more specific Violet does; https://socratic.org/questions/5348556b02bf347bedff8fed

Second the noise difference from 10% of the green would be negligible compared to the energy we are talking about. Also how does a plant regulate this 'noise'? The only logical explanation would be expand into the green in dark and out of it in light, but they have shown no mechanism for a plant to do that.

Sorry to say that after that fairly long winded article i have come to the conclusion we still don't know exactly why plants aren't all black.

Edit; Thinking about this more, maybe Chlorophyll a and Chlorophyll b take inverted wavelengths of light to produce a rectification effect..? Its interesting, but even if that were true, it would not explain the gap at green.

2nd Edit; I stand corrected, considerably more green light make it through the atmosphere thank you for the information spacemark.

>First, green does not have the most energy of the visual spectrum, Blue, or more specific Violet does

The article is correct. Blue photons have more energy, you're right. But more green photons are emitted by the sun, and even more reach the ground through the atmosphere than blue, so the total energy from green photons is significantly greater. Google blackbody spectrum.



The "most energy" statement isn't about the frequency of light, it's about the power output of the sun, which peaks around green.

That power output really is the area under the curve, in which case most of the sun's energy is in the infrared, actually.

Sure. But the peak is around green, which is the point of the article and the answer to parent's question.

One theory is that yellow-green wavelengths don't pass through water as readily, hence ancestral sea-dwelling plants adapted to absorb red and blue wavelengths and that adaptation remains "good enough" for contemporary land plants.

Highly recommend the book "How the Earth Turned Green"


Not "good enough"! There are so many known instances where Evolution gets trapped in local maxima and can't escape it... Another local maxima that photosynthesis trapped itself is its efficiency being jumbled by higher O2 concentrations.

You do understand that this is a popular science article and that is reporting on this paper, right?


Did you conclude "we still don't know exactly why plants aren't all black" after reading the paper?

Thanks, looks like the full paper is for paid members only, but i will look into this further.

Well, nowadays there is a very convenient way of getting scientific papers: Sci-Hub. I guess everyone should decide for themselves if they want to use something like that or not.

I've skimmed the paper, its answered a lot of my questions. I think the article tried to dumb it down too much and skipped over some important information;

Still not sure why the absorption points couldn't be uniformly widened by nature, but i will go over the paper more thoroughly when time allows, its very interesting. thank you.

As a rule scientists are pretty smart and thorough and journalists, even if they understand the science, have to assume most people would be bored by too many details.

The same applies to software development; local optimization does not weigh up against stability across the entire supply chain.

I might just use this trick of nature to inform how I manage my staff at work. We spend so much time trying to maximize efficiently, but perhaps at the cost of stability.

This is the nerdiest sentence I’ve seen today. Dang!

But, why do plants need a regular amount of light energy? Can't a black plant just absorb everything available?

Why are some plants purple like tradescantia pallida?

I always just assumed there wasn't much green light, so plants optimised for the rest of the spectrum:

The sky is blue, the sun is orangey.

Where's all this green light I'm missing?

The sun is actually green (that is, is approximated by a black body emitter with a peak in the green region of the visual spectrum, see [1]). Its light appears white because that is how we are conditioned evolutionarily. ("True" white -- i.e. equal spectral energy -- appears blue-grey to us.)

The sun "looks" orange because when you can look at it (sunrise/sunset), its light is heavily filtered by the atmosphere.

[1] https://qph.fs.quoracdn.net/main-qimg-f9bc312e4d114e9e32a627...

Your green receptors overlap with red, and also a little with blue. There is no single wavelength of light that only stimulates your green photoreceptors without also setting off red or blue ones. https://en.wikipedia.org/wiki/Color#/media/File:Cones_SMJ2_E...

Look at a graph of the spectral power output of the sun, you'll see that it peaks around green.

Orange is red and green. The intensity we perceive is also not the absolute intensity of that part of the spectrum. Neither is the luminosity equivalent to the power (as in energy over time).

Finally a clickbait with explanation!

Quanta magazine typically does a great job backing up their titles with real science-based content.

Wanted to say the same.

Ppl who downvote comments like these are responsible for killing any joy and fun on the internet.

Your fun and joy definition might not meet the definition of others. I for example do not enjoy metoo comments. If you do, you are likely a minority.

I was referring to "Finally a clickbait with explanation!".

You started off well, but you ended with a logical fallacy.

Why? I shared my experience. Allmost no one I know enjoys these comments. Wheres the fallacy?

You started out sharing your experience, yes, but that was not your last sentence. The fallacy is fallacy of composition, one of the forms of illicit transference. Another way to phrase that is you formed a hypothesis that you assumed to be true.

That sounds a lot like talking around to me. Can you say concretely, where you believe my fallacy is?

> That sounds a lot like talking around to me. Can you say concretely, where you believe my fallacy is?

How is that talking around?

I literally told you where your fallacy resides in your words. Then, I labeled the exact fallacy, providing two common names for the fallacy. Finally, I provided a simplistic rephrasing of the fallacy to help with your identification. What else do you need?

I thought plants are green because sun is green. Edit: not sure about down votes, sun has peak in green spectrum. Our eyes are most sensitive to green, plants are green to utilize it as well.

If they would utilize the green light they would not reflect it - so should exactly not be seen as green by your eyes.

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