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Why is Grass Green?

    You recall that yesterday a question arose about grass and its color (J.A.: "Why is the grass so green after it rains? ... is it just an optical illusion because the overcast skies at dusk soften the blues and reds and drives our eyes toward the middle (green area) of the spectrum?"). A member of the Commentariat, iowabiologist, offered an explanation:

"One of the reasons that grass is greener after rain is that the light reactions of photosynthesis require water. The oxygen that plants produce through photosynth. comes from the splitting of H2O. In the absence of water, photosynth. can not proceed, reactive ions build up and
damage the photosynthetic aparatus (including chlorophyll) which then get broken down and plants yellow. Rain immediately restores the ability to
perform photosynthesis, the plants produce chlorophyll to accomodate, and they turn bright green again."

     This was helpful, but lurking in this discussion was the mystery of chlorophyll's color. Why green? So iowabiologist explained that, too:

    "Plants have pigments that absorb, for the part, all wavelengths of light in
the visible spectrum except around 500-560 nanometers, which is green. Your brain perceives the plant as green when the unabsorbed wavelengths reflect off of the plant and hit the retinas of your beautiful blue/brown/hazel/other eyes."

     Excellent. But! But! As someone who once made a living slinging Why questions, I still have to puzzle over the conscious decision of plants to use pigments that don't absord the "green" wavelengths. (Green is in quotes there for complex philosophical reasons; I can't honestly argue that electromagnetic radiation in certain wavelengths is truly green. It just LOOKS green, to us.) Over the years I've posed this "why are plants green?" question to a number of scientists, but it's kind of trapped between physics and biology. My assumption is that plants play to extremes: Red and blue light, on opposite ends of the visible spectrum, have distinct properties (blue, for example, is highly energetic, a socko punch of radiation), and chlorophyll would rather cover two bases than play just one position in the middle. It's like politics these days: No one wants to get trapped in the center. Moderation is death. Red and blue are all that matter anymore.

By Joel Achenbach  |  June 24, 2005; 8:50 AM ET
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Um, wouldn't that make all the centrists that matter purple?


Posted by: bc | June 24, 2005 9:25 AM | Report abuse

Yeah! That's what I meant when I asked, "Why green?" Pretty much all visible light is transmitted equally through the atmosphere, so why the big gap right where green is? If evolution was able to produce a chemical that produces energy from all colors in the spectrum besides green, why not one that absorbs green as well? Granted, that would make for some really weird (invisible?) leaves, but would it be a Really Bad Thing, like crossing the streams?

Posted by: jw | June 24, 2005 9:30 AM | Report abuse

I mean, all the dead centrists.

On a related note, doesn't "Land of the Dead" open this week at the local Goolgleplex Cinema?


Posted by: bc | June 24, 2005 9:32 AM | Report abuse

bc: it would. I think the media has actually started that trend. Newsweek had a cover story a while back that described Barack Obama as the "purple" candidate.

Posted by: jw | June 24, 2005 9:33 AM | Report abuse

There you go again, people, how does the color of plants lead to a discussion of Obama's politics? It's weird, but it is one of the things I like about this blog. I guess it's the Washington Post influence, the inside-the-beltway worldview.

Anyway, if we all changed our terminology from left-right to blue-red, I think it would be very interesting. It just leaves me feeling a little disoriented. I am a liberal, and I am comfortable being "far-left" but in the past the color that was associated with that was red (for Communism) or pink (for the American version of Communism, which also implies sympathy for gay rights.) I don't really want to be "blue," although that did describe how I felt after the past two elections.

Posted by: kbertocci | June 24, 2005 9:55 AM | Report abuse

Yay! I've learned something today, so it's not a totally wasted day.

And this reminds me that I wanted to try and dig up a couple of articles on color cross-culturally, because I seem to recall reading some interesting stuff on that subject back in my days as an anthropologist. ::is nostalgic::

I vaguely recall that there's a minimum set of colors that appear in every language (black, white, and red?), but I can't remember which colors get introduced next, and what the explanation is.

(Does it have anything to do with the fact that babies see black, white, and red first? Something to do with rods and cones and such?)

Hey, I can digress with the best of them.

Posted by: Mara | June 24, 2005 10:10 AM | Report abuse

kw: shows you how much attention I pay to Newsweek. Sorry, Was Post Co. employees. You made me chuckle with the "crossing the streams" reference, though.

Back to Joel's somewhat anthropomorphic question "I still have to puzzle over the conscious decision of plants to use pigments that don't absord the "green" wavelengths".

I don't think any of us get to choose what wavelenghts we absorb or refelct (with the possible exception of Mr. Jackson), which is why we should treat absorbers and refelctors of all wavelentgths equally under the sun.

BTW, I'm looking at the Japanese red maple in my back yard as I type this...


Posted by: bc | June 24, 2005 10:24 AM | Report abuse

Mara: here's a refresher course for you, an interesting article that starts with colors and goes on from there.

Posted by: kbertocci | June 24, 2005 10:24 AM | Report abuse

What if we could choose the colors that we absorb or reflect? Do you think we'd live in a rainbow colored society or do you think we'd all stick with our respective colors that we are now so as not to stand out too much? I'm pretty sure that had the Wicked Witch of the West had this option she wouldn't have been green. Kermit, too. I'd probably just change colors in private for my own amusement. Puce, now there's a color I'd like to finally see.

Posted by: Sara | June 24, 2005 10:42 AM | Report abuse

bc: too weird. I just finished watering my japanese maple before turning on my laptop this morning. After reading this, I can now appreciate this beautiful tree even more.

Posted by: Chris | June 24, 2005 10:53 AM | Report abuse

Whew. Joel's post on Rove has been completely invaded by crazies. What a Friday. I predict that it crashes the server, Joel loses the blog, and I'll have to do actual work again.

Posted by: jw | June 24, 2005 10:59 AM | Report abuse

jw, I saw that, too. Let me make another apology for referring to you as 'kw' earlier - just a bit of hasty keying.

Remember Joel threatened to quit earlier in the week when the blog hosting started getting flaky (remember the multiple posts & browser wierdness).

I'm sure we can avoid real work with or without this blog.


Posted by: bc | June 24, 2005 11:10 AM | Report abuse

Someone mentioned that a link to the blog is right on the top of Google's news site. We're suffering from overexposure again.

And no worries about the typo ;)

Posted by: jw | June 24, 2005 11:13 AM | Report abuse

Thanks, kbertocci! I glanced at the page and I see they're quoting Berlin and Kay, who I seem to recall were the big dogs in the language of color, so that looks like exactly what I wanted.

Posted by: Mara | June 24, 2005 11:13 AM | Report abuse

The Achenblog can't stop. I'm a secretary with a scatterbrained boss.

Posted by: Sara | June 24, 2005 11:17 AM | Report abuse

Wah! I want to ignore the Rove blog, but I'm strangely drawn to it. And every time I read a comment I can feel myself getting stupider :(

Posted by: jw | June 24, 2005 11:23 AM | Report abuse

And kw, Sara, I admire you sticking with the Rove comments.

Hey, Joel - the social particle acclerator's working GREAT today. Look at all those bizarre particles scatter! The 4th of July came early, hooray!

I'd look over there, but I'm afraid of getting detatched retinas from the g forces.


Posted by: bc | June 24, 2005 11:25 AM | Report abuse

For the 15:

In an attempt to return to the proper order (as I can't scroll fast enough to follow the Rove(r)s)....

Here is a little piece you might like to get Friday going in the right direction:

For some non-history.

Turn Sound down, Sara--> the boss

Posted by: Dolphin Michael | June 24, 2005 11:27 AM | Report abuse

right about now i'm appreciating joel's restraint normally in talking about politics and such. My eyes were starting to cross for awhile. but i do have a question for joel -

Is your porch open, or screened in? How do you feel about screened-in porches.

i think they're kinda sissy.....

Posted by: LP | June 24, 2005 11:29 AM | Report abuse

Call me sissy but I like pink. Pink and green match.

Posted by: fdg31 | June 24, 2005 11:31 AM | Report abuse

HA! I forgot about the particle accelerator. Maybe this is all a thoughtfully contrived experiment by Joel.

More likely, someone was like, "Um, Joel? You rally need to generate more than 15 hits on your blog. Stop posting about dorky science stuff and your porch. We want political fisticuffs!"

Posted by: jw | June 24, 2005 11:34 AM | Report abuse

I think I've given up on the Rove blog. I realized I didn't really care what any of them were saying. I'll probably still go back's Friday, I have no will power.

Posted by: Sara | June 24, 2005 11:34 AM | Report abuse

I just tried to post something and I got an error! Is the blog crashing???

Posted by: jw | June 24, 2005 11:36 AM | Report abuse

That happened to me earlier. Then it showed up several reloads later. I think we're safe for now.

Posted by: Sara | June 24, 2005 11:38 AM | Report abuse

I imagine Joel as a non-screened-in-porch kind of guy. I just don't see him sitting behind a screen enjoying the view. That's just my take on it, though.

Posted by: Sara | June 24, 2005 11:40 AM | Report abuse

I agree most likely he has a non-screened in porch. However, i live in the north east and lots of folks all of a sudden seem to be screening in porches to keep bugs away (west nile, anyone?) I imagine further south, where its probably buggier, perhaps screened in porches are also gaining popularity? hmmmm......

Posted by: LP | June 24, 2005 11:47 AM | Report abuse

I live in Minnesota and there are massive mosquitoes here. We could use a screened in porch. Instead, we just never go outside.

Posted by: Sara | June 24, 2005 11:50 AM | Report abuse

But if it's screened in, is it really a porch? Or just another room with really thin walls?

Posted by: LP | June 24, 2005 11:53 AM | Report abuse

Green isn't the color the plant chose for the proper wavelength.

Instead green is the color resulting from God's choice of the chemical chlorophyll to create the oxygen and energy the plant uses. The chemical chlorophyll is what certain plants use to produce their energy. The result? Plants look green because the physical makeup of the chlorophyll molecule causes the electrons to excite and rerelease the green light that is absorbed while other colors of light are absorbed and their energy carried by electrons for use in the photosynthesis process (kreb's cycle.)

On the other hand, this stupid blog isn't about physics or biology, just some stupid editorialist looking for new and dull ways to discuss politics. Dude I'd say get a life but, apparently yours is working quite well for you. So instead I'll give all these people reading this a big GET A LIFE!!!!1111!!! I went to this article expecting a scientific discussion.

Posted by: the answer | June 24, 2005 12:11 PM | Report abuse

hilarious - someone on a blog telling the other folks on the blog to get a life, cuz obviously his intellect is greater than ours. Obviously............

Posted by: LP | June 24, 2005 12:16 PM | Report abuse

LOL, that is hilarious "the answer"
Have you read the "About Achenblog" maybe you will get you scientific answers there.

Posted by: fdg31 | June 24, 2005 12:23 PM | Report abuse


You know it! The key to this all is to write without reading. BUT, what do you expect when someone runs off and gets a college degree.

Personally, if there is A lot of interest in a political discussion of sorts, I think that is a good thing. If m. answer doesn't want to read it, do what millions of eyes do each day: go elsewhere. Furthermore, the country will be better off with these discussion, goodness knows, we haven't shown an ability to think before we vote and the stilted postings on the Rove thread prove that we still have a ways to go.

Maybe PBS can do a series on "missing the point"

Posted by: Dolphin Michael | June 24, 2005 12:25 PM | Report abuse

People expecting "scientific discussions" should perhaps check their facts first. (And not summon up unprovable hypothesis to explain nature.) Plants don't use oxygen, they use CO2 and give off oxygen, photosynthesis is not the same as the Krebs cycle, and finally chlorophyll does not release green light after excitation. That would be fluorescence or phosphorescence, entirely different than reflection and transmission of green wavelengths, which is what my trusty biochem textbook says is responsible for the green color. There are other pigments present in plants that absorb over the rest of the visible spectrum, although they are less abundant than chlorophyll, so we see mostly green leaves. (all from "Principles of Biochemistry", Nelson and Cox)

Posted by: Chemist | June 24, 2005 12:30 PM | Report abuse

I agree with the answer, somehow you idiots turned a valid question into your own boring pseudo-salon hangout where you can judge the coolness of others. No wonder people still believe in creationism, they are too stupid to try to understand the science

Posted by: wtf | June 24, 2005 12:31 PM | Report abuse

Oops, I meant hypotheses. =)

Posted by: Chemist | June 24, 2005 12:31 PM | Report abuse

Ha! Go chemist. Unfortunately "the answer" probably won't return to read the truth because he was disappointed in us all.

Posted by: Sara | June 24, 2005 12:32 PM | Report abuse

i think they already have that, and it's called "fox news"

Posted by: LP | June 24, 2005 12:34 PM | Report abuse

I thought this blog in particular had to do with taking the extremes just like the grass?

Posted by: fdg31 | June 24, 2005 12:44 PM | Report abuse

The grass is green because it knows where it's moral values should be. Go nader!

Posted by: LP | June 24, 2005 12:47 PM | Report abuse

The grass is green because it knows where it's moral values should be. Go nader!

Posted by: LP | June 24, 2005 12:48 PM | Report abuse

I'm trying to be optimistic and hope that The Answer was trying to be ironic.

Posted by: jw | June 24, 2005 1:02 PM | Report abuse

Hey Achenfan,

where are you?

Posted by: nottamember | June 24, 2005 1:16 PM | Report abuse

433...dear God.

Posted by: jw | June 24, 2005 1:22 PM | Report abuse

The blog item is a cool, shaded oasis of calm, protected from the sandstorm of the Rove item. FYI, of course it's an open porch. I have two open porches, front and back, and am mostly a back-porcher. I am not saying that other people shouldn't have screened in porches, just that...well, they just shouldn't participate in this blog. [Exception: Beach houses can have screened in porches.]

Posted by: Achenbach | June 24, 2005 1:23 PM | Report abuse

thanks, Joel! now i can sleep easy, justified that yes, all those screened in porch people are, indeed, sissies...

Posted by: LP | June 24, 2005 1:25 PM | Report abuse

And someone called me a war-monger! Hold me, Sara.

Posted by: jw | June 24, 2005 1:28 PM | Report abuse

Joel, did Google pick the item up independently, or did the Post feed it to them?

Posted by: kbertocci | June 24, 2005 1:33 PM | Report abuse

I noticed they asked you to enlist with Father Tom and someone else and I thought, "Why jw? He doesn't fit in that group." Kelly is being called out now, too. She's an absolute genius. If holding you to keep you safe from the angry anti-war mob is going to associate me with Father Tom and Kelly, I'm going to have to pass. But if you can assure me that it won't, I'm here for you. Even if you have too much faith in mankind if you're hoping that The Answer was being ironic.

And LP, I think a screened in porch could be a room or a porch, depending on the thickness of the screen. Most would still qualify as porches in my book, though. Sissy porches, of course.

Posted by: Sara | June 24, 2005 1:39 PM | Report abuse

oooh - cheetah cubs! cute AND fuzzy. i feel better now...

Posted by: LP | June 24, 2005 1:39 PM | Report abuse made no sense. Especially since I've been in the military since, oh, 1998.

Posted by: jw | June 24, 2005 1:41 PM | Report abuse

And i'm going to call them "rooms" from now on, as in "rather than hanging out and drinking beers in your back ROOM, let's hang out on my back PORCH." You just have to take a stand somewhere.

Posted by: LP | June 24, 2005 1:44 PM | Report abuse

Well, you couldn't exactly fault them for not knowing that. I know that I'm not in possession of a "Know jw" encyclopedia. What confused me is that you aren't crazy and I'd been questioning the mental capabilities of the others since they first unleashed their fingers on their keyboards.

Posted by: Sara | June 24, 2005 1:44 PM | Report abuse

Well, LP, we all have to take on our big battle at some point in life. Good luck in your stand against screened "porches" and know you're not alone.

Posted by: Sara | June 24, 2005 1:46 PM | Report abuse

and i bet rove's porch is screened in, too...

Posted by: LP | June 24, 2005 1:47 PM | Report abuse

NAd i'm going to call them "rooms" from now on, as in "rather than hanging out and drinking beers in your back ROOM, let's hang out on my back PORCH." You just have to take a stand somewhere.

Posted by: LP | June 24, 2005 1:47 PM | Report abuse

Anyone have exciting plans for the weekend? I'm going to the Nationals game tomorrow, and trying to evade all the interns that have invated my favorite bars.

Posted by: jw | June 24, 2005 1:54 PM | Report abuse

It is certainly a strange group in there. I think the problem is that it's bad enough when the blog is on the page, but now it's on the Google news page. At least most people who read the paper (or an online one) clear a certain bar. Google...apparently not so much.

Posted by: jw | June 24, 2005 1:58 PM | Report abuse

I was going to go to Phoenix and hit up the Diamondbacks game and basically just have a four day vacation. My plane took off 15 minutes ago, though. I'm not going anymore. So, no. Nothing exciting.

Posted by: Sara | June 24, 2005 1:59 PM | Report abuse

Roses are green,
Violets are red,
Grass is blue,
And I am too.

Posted by: Videlicet | June 24, 2005 2:01 PM | Report abuse

Oh no! That's horrible! You can always got to a Nationals game...only $7!

And they're better than the D-backs ;)

Posted by: jw | June 24, 2005 2:04 PM | Report abuse

Depends on your climate, guys.

A porch is not part of the house. In the winter you close the door and practically forget it's there--although your smokers may brave it, they will come in as soon as the nicotene craving is satisfied. Even in the summer, there is a door between the porch and the house, and most people keep the door closed and the air conditioner on.

Here in Florida, I have a "Florida room"--it's part of the house; we use it year round. It has a cement floor, like a porch, and it is screened, floor to ceiling. (My post-hurricane chores last year included rescreening the north side--three times). The door to this room is always open, and we do not use our air conditioner. We do use the Florida room like a porch, though, since it's cooler than the house and we can sit out there to watch the flora and fauna. (Fish pond with turtles, bird feeder, squirrels, etc.--yesterday there was an iguana in the yard.) The screening is necessary because of the insect situation, given that the door to the house is open.

Posted by: kbertocci | June 24, 2005 2:05 PM | Report abuse

Patriotism has to do with love of country,not leaders. There is no doubt that the Bush administration has led us down a path of war which at least they have not been candid about or at worst they lied to the people. Lots of circumstantial evidence is available to support this. Most of the people now understand this. The question is how long will the will of the people continue to support this war, how long will the volunteer army be able to sustain this war, Will it backfire? Publish an exit date, maybe not but he who believes that the people will continue to support a war which currently puts plenty of US soldiers in the cross hairs of terrorists is not living in reality. Why would a terrorist want to come to America to kill Americans? There are plenty in Iraq with easy access from Syria or Iran and they blend in better. They fought the Russians for many years in Afganistan and they'll fight the US military for many years in Iraq, even after the Iraqi miltary takes over of most of the operations. This current exit strategy still means many soldiers will die and the terrorists will not be banished. Anyone who doesn't see this is not unpatriotic, just blind.

Posted by: Reuben | June 24, 2005 2:05 PM | Report abuse

I can see lots of green from my front porch, including Sugarloaf mountain.

Very calm, very relaxing.

Good place to crack a couple of beers and a lot more jokes.

I went back in to the Rove Matter/AntiMatter Warp Core a couple of minutes ago, my face is distorted like I was riding a centrifuge or a rocket sled.

Good day for the Was Post folks, they know what to put into the Achencellerator to get some nice displays...


Posted by: bc | June 24, 2005 2:08 PM | Report abuse

If I may animadvertize, I have neighbors with both front and back porches. You can tell which porch they prefer by looking for the comfortable chairs. Front porches should never be screened because you can't accost your neighbors as they trundle by. Also they become detritus havens.

Posted by: Videlicet | June 24, 2005 2:14 PM | Report abuse

I like the Mr. Science guy that apparently left. Even without the corrective tweaks inserted by chemist, his unprove-able premise of God originating things is a frontal challenge to the "can't prove anything to me" statement we all began with (i.e. not accepting green wavelengths as green because--why??) Some things are just the way they are, and either God did it or we can come up with our own pseudoscience versions of cause-and-effect which still must trace back to something at a beginning.

Posted by: zerb | June 24, 2005 2:15 PM | Report abuse

I'm personally not a big D-backs fan. My ex was and he bought the tickets and then we broke up this week and I didn't catch the plane because, well, that would have been weird. I'm not necessarily a Nats fan, either. I'm a Braves girl. Not that I have any attachment to Georgia. I like the Braves, the Suns and the Chiefs. Three different sports, three different teams, three different states. None of them my own. But for $7, I'll be any team's fan. Yes, I can be bought when it comes to baseball. I just like to wear a hat and eat a hot dog and hope to catch a ball.

Posted by: Sara | June 24, 2005 2:17 PM | Report abuse


all hail the maine-guys. we know how to do it right. AND it's a good place to hide when the proverbial crap hits the fan!

Posted by: LP | June 24, 2005 2:22 PM | Report abuse

Sorry about the ex thing, I've been in that sort of situation...keeping the plans is usually no fun. I like your reasons for liking baseball, I feel like it would be equally appropriate for talking about life!

And, I might be mistaken, but were we just invaded by the crazies? Reuben seems lost...

Posted by: jw | June 24, 2005 2:24 PM | Report abuse

If the Queen Elizabeth is the public face of England, can John Bull be patriotic and not want God to Save the (gracious) Queen?

Posted by: CodBlessAmerica | June 24, 2005 2:26 PM | Report abuse

Reuben, I think you posted comments to the wrong blog item.

jw, et. al. I'm going to Ohio to do some auto racing, and hang out with friends at the pool when we're not at the track.

Not that my racing matters a whit, with respect to why anything absorbs or reflects certian wavelengths of radition, or why species sense them in certain ways.

I bet something ate all the other color plants hundreds of millions of years ago, and all that were left were the green ones. Sorta like a bowl of M&Ms at a children's party.


Posted by: bc | June 24, 2005 2:27 PM | Report abuse

and to kbertocci - that's why god never meant for folks to live in florida. Screened in porches and sinkholes.

Posted by: LP | June 24, 2005 2:29 PM | Report abuse

I was just about to say "I think Reuben is in the wrong room" but just like the 100 comments post, you beat me to it. I was actually wondering if he was THE Reuben from American Idol. I never liked him.

It could be equally appropriate for talking about life, except I don't think I can be bought as easily for $7 in other aspects of life. Just baseball. And there's less hope of catching a ball in most other situations. A metaphorical ball, maybe. I once had a horoscope that said my happiness would depend on where and whether an enormous man caught a ball. It was The Onion, so they were probably referencing an actual ball, but I took it as a metaphor.

Posted by: Sara | June 24, 2005 2:31 PM | Report abuse

Sorry to disappoint you LP, but the Sugarloaf mountain I'm referring to is the closest mountain to Washington DC, IIRC. And I work in the city of DC. If the s*** hits the fan, there's a good chance I'm gonna be in it - I was here on 9/11.

I have friends in Maine, and visit when I can, it's beautiful up there.


Posted by: bc | June 24, 2005 2:32 PM | Report abuse

too bad, our sugarloaf is better.....

Posted by: LP | June 24, 2005 2:34 PM | Report abuse

kbertocci, I'll bet it was the dinosaurs.
I know I go for the blue and orange M&Ms myself. Complimentary colors.

Posted by: Sara | June 24, 2005 2:34 PM | Report abuse


Posted by: jw | June 24, 2005 2:45 PM | Report abuse

LP, I'm sure your SL is better than ours down here. Still, it's a nice view.

Sara, could be dinosaurs, though I think plant life was pretty well developed by the time they came along...

609. Damn.


Posted by: bc | June 24, 2005 2:56 PM | Report abuse

Had to be cockroaches then. They've been around since before time began. They'll outlive us all. Especially the Texan cockroaches.

Posted by: Sara | June 24, 2005 3:01 PM | Report abuse

Democrats ate all the plants.

Posted by: jw | June 24, 2005 3:03 PM | Report abuse

It's been about a gazillion years since I studied photosynthesis, but here are a few things I recall:

The first question is, why do plants stick to the visible spectrum? If you look at the whole range of electromagnetic radiation from the sun which makes it through the atmosphere, the majority of the energy is actually in the infrared. Why don't plants use this?

The answer to this one seems pretty clear. While there are a lot of photons, and hence total energy, in the infrared range, the individual photons don't have enough energy to do the necessary work* - briefly, create reduction pathways which can break water down to hydrogen ions and oxygen, using the former to reduce carbon dioxide to carbohydrates and releasing the latter. Light in the red range (around 700 nm wavelength) is the maximum wavelength (minimum energy) which can do that.

* Like everything else, there is a caveat here. There are bacteria which can make use of energy from infrared light, but do so through different mechanisms (e.g., starting with hydrogen sulfide instead of water).

In fact, the current assumption is that photosynthesis originally just consisted of a single reaction pathway, called Photosystem I, which makes use of 700 nm light. Later, a second system, called (obviously enough) Photosystem II, developed which could make use of 680 nm light. Photosystem II is the main oxygen production pathway - as an aside, the development of Photosystem II has been called "the greatest ecological disaster in world history," since the release of free oxygen it entailed is presumed to have killed off most of the (anaerobic) life that had existed until that point.

OK, that explains why plants don't use longer wavelength light, but why do they make such spotty use of shorter wavelengths? After all, the shorter the wavelength, the more energy available. Forget about green - ultraviolet photons have more than twice the amount of energy available per photon than those wimpy red ones. And although the atmosphere absorbs a lot of the ultraviolet radiation, significant hunks still make it to earth, or at least enough to burn me every time I go to the beach.

Actually, that last observation provides part of the answer. Photons with too much energy can be just as bad as those with too little - they can cause reactions to go out of control, knocking covalent bonds loose, kicking molecules asunder. Energy from shorter wavelength photons must be carefully husbanded and controlled, to avoid damaging the photosynthetic reaction centers (the areas where Photosystems I and II take place).

To handle shorter-wavelength photons, higher plants developed elaborate structures called "antennae" which consist of pigment molecules carefully arranged and held in place by chains of proteins. These antennae gather energy from the shorter wavelength (blue) photons and, through a series of transfers, step the energy levels down to something the photosynthetic reaction centers can handle. There are also mechanisms for bleeding off excess energy as heat, blocking the electron flow when it gets too high (photoinhibition), etc.

As the name "antennae" indicates, these structures are fairly finely tuned to particular wavelengths of light themselves. My guess is that this then explains the limitations on the blue end of the spectrum. If you can only fit a limited number of antennae around each reaction center, and each antenna can only handle light within a limited wavelength range, then the best strategy overall is to concentrate on the shortest readily-available wavelength that can be absorbed without too great a risk of molecular damage. Ultraviolet is too short and energetic, and green doesn't offer the energy possibilities that blue does.

That's my totally unqualified theory, at any rate.

Posted by: Mahousu | June 24, 2005 3:04 PM | Report abuse

Well, I'm off for the weekend! Be safe everyone.

Posted by: jw | June 24, 2005 3:54 PM | Report abuse

Mahousu, that was great.

Thank you.


Posted by: bc | June 24, 2005 4:02 PM | Report abuse

Achenfan... hello !?!?!?

Posted by: nottamember | June 24, 2005 4:44 PM | Report abuse

So I did post it wrong, oh well...

Posted by: Reuben | June 24, 2005 4:49 PM | Report abuse

I cannot imagine with what confusion joel/ his editors scroll through the posted comments. What would happen to the Select Audience if he didn't blog for, say, a week? Is there a fallback blog?

By the way, I'm part of your regular audience; I just don't comment. Don't ignore the silent majority!

Posted by: nomes | June 25, 2005 5:03 AM | Report abuse

Oh wait.

Posted by: nomes | June 25, 2005 5:03 AM | Report abuse

Hello nottamember!!
Here I am! Thanks for noticing my absence; fdg31 also noticed. I'm touched, really. I took a few days off, and I'm currently in an Internet cafe, checking, among other things, on the Achenblog. (Some serious help needed here.) Catch ya later.

Posted by: Achenfan | June 25, 2005 7:33 PM | Report abuse


It was like a red tide, a mongol invasion, a nuclear chain reaction, an avalanche.

Anyway, the GOOGLERs totally overran the blog over there on the Rove post. Thank you, Joel, for following up with Why is Grass Green so we would have an alternative on Friday afternoon when none of us wanted to work.

Posted by: kbertocci | June 25, 2005 9:22 PM | Report abuse

It's great to have you back Achenfan!

Posted by: fdg31 | June 26, 2005 10:14 PM | Report abuse

Is it really true the grass isn't greener on the other side?

Posted by: Ryan | June 27, 2005 9:24 AM | Report abuse

Yes, that IS true. Once you actuallly get to the other side, you are able to see the dirt, etc., surrounding the grass, which you couldn't see when you were on your own side. The dirt makes the grass look less green. So now the original side looks greener than the "other side," which you can't wait to get back to. (A good metaphor for life, really: happiness depends on how well you are able to screen out the dirt and focus on the good stuff.)

Posted by: Achenfan | June 27, 2005 9:48 AM | Report abuse

Oops . . . I mean you can't wait to get back to your own side, not the other side. Although your own side has now become the other side . . . . Never mind.

Posted by: Achenfan | June 27, 2005 9:50 AM | Report abuse

Look at the spectral distribution of solar energy and you see a peak at approx 0.5 microns - green light. Plants are green due to the goldilocks principle, i.e., photosynthesis via chlorophyll is designed to absorb visible light, which is not absorbed by our atmosphere, but by reflecting (not absorbing) green light the plant is sure not to get too much of a good thing.

Posted by: mccormajp | June 27, 2005 1:58 PM | Report abuse

By Jove, I think he's got it!

Posted by: jw | June 27, 2005 4:21 PM | Report abuse


Posted by: g | March 31, 2006 10:13 AM | Report abuse

huh we need an aslyum

Posted by: p | March 31, 2006 10:14 AM | Report abuse

what the is that 1-800 # for life insurance

Posted by: dude | March 31, 2006 10:15 AM | Report abuse

plz tell

Posted by: dude | March 31, 2006 10:15 AM | Report abuse

i do not no

i am dude

Posted by: hey | March 31, 2006 10:19 AM | Report abuse

hole of a man

Posted by: Anonymous | March 31, 2006 10:20 AM | Report abuse

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