Why aren't plants black?

This popped into my head just before I went to bed last night, so don’t expect it to make a whole lot of sense.

Essentially, what’s so special about non-green light? Wouldn’t it be more energy-efficient for a chloroplast to absorb all wavelengths of light (or even, all wavelengths in the visible spectrum)? Is it a heat thing? Would a black plant absorb so much energy that it bursts into flames, ruining my curtains?

I think nature just happened to hit on a formula that worked very well, maybe there isn’t anything more efficient that chlorophyll (using the existing chemistries) or maybe it just hasn’t happened yet.

BTW, some plants are black (there’s a grasslike plant )Xanthosma or something like that), but it still only has green chlorophyll and some other pigment that isn’t directly involved in the process of photosynthesis).

Maybe it would be more energy-efficient for plants to be black, but they aren’t green deliberately. Natural selection has failed to use a black-coloured pigment that has similar chemical properties as chlorophyll - presumably because there isn’t one, or at least because there isn’t a naturally occuring one.

I suppose you could even use the fact of plants being green in an anti-creationist argument.

I wouldn’t worry about black leaves bursting into flames. Plant leaves do already get pretty hot, but lets face it, how many black objects spontaneously combust under sunlight? But perhaps the increased temperature of a black leaf would interrupt its internal chemistry enough to cancel out the gains from increased light-gathering.

Because they prefer to be called ‘botanical-american’ :smiley:

Sorry I dont have an answer but the question did get me thinking…hrmm, black plants, seems that would cause Earth to be a very different place at least in terms of climate. I’m pretty sure the majority of the heat we enjoy is a result of reradiation. IIRC heat (infrared light) has a heck of a time traveling through a vacuum (space).

I don’t see why; green is clearly God’s favourite colour for vegetation.

the black plant I was thinking of was Ophiopogon; I have no idea where I got the other name from (I think I might have made this error before, hmmm).

There was a brief piece about this in the journal American Scientist a few years back. Green would seem to be a poor choice for plants, because they appearc to be colored green by reflecting the green wavelengths of light. There are plenty of green photons to be utilized, however. Green wavelengths penetrate our atmosphere very effectively (green is the peak wavelength response of the human eye, in fact). So why not purple plants, whgich accept mainly the bountiful green photon?

The suggested answer was that chlorophyll-bearing plants originally had to compete with purple bacteria, which used substances similar to the rhodopsin/visuasl purple that the eye uses. The early plants therefore developed chlorphyll, which efficiently used the available non-green radiation. Later on, when plants with chlorophyll took over the lans, they were committed to chlorophyll economies, so rhodopsin never got the chance to be big again (there are few U-turns allowed on the road of evolution, apparently). You work with what you got.

There’s no fundamental reason I’m aware of whty purple plants (or black plants) wouldn’t work, but the scheme of evolution that got us here didn’t go that way. Plants on some other planet could well be different colors. Given a sufficiently great catastrophe here on Earth that wiped out most plant life, purple could take over here.

isn’t that just because mammals have spent a lot of time needing to look at plants or living in a forest environment where much of the light is filtered/reflected by plants?

part of the problem is that red light is low in energy. Its hard to make it do much useful work, so might as well concentrate on the more useful blue-yellow part of the spectrum

No, the solar spectrum (sunlight) peaks at green. Our eyes have adapted to sense the brightest light available.

Green plants are able to make use of blue and green light because they also contain yellow-orange compounds called caroteniods. Carotenoids absorb photons in the more abundant green part of the spectrum and pass the acquired energy on to the chlorophyll. Carotenoids are normally masked by the chlorophyll, but become apparent in leaves in the fall when the chlorophyll breaks down.

Notwithstanding the fact that most plants are green, there are definitely some that can best be described as black, purple, etc.

Here’s a pic of some black moss from Isle Royale. This particular species is also commonly found on city sidewalks.

Also, check out the second picture in the fourth row here for a nice purple plant.

Presumably these have green in them, but the color is obscured by other more intense pigments.

bferullo, that’s a very good question. (mainly because I asked it before. :))
http://boards.straightdope.com/sdmb/showthread.php?threadid=79012

[very old joke alert]

[/very old joke alert] Always remember to recycle. :smiley:
Scientific American about that there was purple before green photosynthesis:
http://www.sciam.com/news/091200/4.html

I found this website explaining the purple/green theory: http://www.abc.net.au/science/k2/trek/4wd/may99_1.htm
Though I don’t buy the argument about temperature control. The same evolution preventing plants from becoming black because of overheating would have worked backwards in colder climates. If heat was a major evolution-affecting factor, there should be black algae under the polar ice caps and near-white desert plants, with pale purple trees in between.

Temperature didn’t prevent some plants from becoming black or purple, but it seems only for UV-protection. Does at least some of the non-green color contribute more to photosynthesis than the average caroteniods? The red of rhubarb, for instance, does not at all.

Heck, there have evolved iridescent plants: http://www.sigmaxi.org/amsci/amsci/articles/97articles/lee.html

Green plants may have evolved because of purple algae, but those aren’t stealing light from our trees anymore. Since then they created some number of accessory pigments utilizing parts of the light that the chlorophyll didn’t use. Why don’t they conquer back all of the profitable green light? They had time enough. They don’t even have to re-invent the purple pigment. Why weren’t both ancestors wiped out by a green+purple pigmented symbiotic plant? Wasn’t that the way the chloroplasts started in the first place?

“carotenoids” :rolleyes: I know how to spell that. Don’t type with other peoples posts still on the screen.
dqa, your first link didn’t work.

I was going to let this die, but I see it hasn’t been answered with great certitude.

My link above is to a pic in bright light(high contrast) from a bit of distance, so it’s no great loss. I was unable to see it in IE, and in Netscape only after clicking “View Image.” However, now (11 pm or so) it may load more readily.

Anyway, it is relevant to your point about cold weather. This dark moss is on an exposed clearing of rock, and so could be among the first plants to make use of sunlight when the snow melts. The dark color could be a great advantage. However, if this were the case one would expect lighter colored moss to be outcompeted in most cases like this, which I’m pretty sure is not true. Also, evergreen trees should be dark - or at least dark during winter. It has been my experience that when evergreen trees are dark, they are so because of the shadows of the needles, not the pigments.

So I’m still no help.

You saved me from making three posts in a row, is that nothing? :wink:

Not only when the snow melts, dark moss beneath it could even be the reason why the snow melts there! Color from additional passive pigments creates heat at least partly from energy that could be used for chemical reactions instead. If the advantage of heating is outweighed by the one of direct photosynthesis, the plants don’t get any darker.

Travelling through a vacuum photons of any wavelength like best. But once low-energy photons are caught in our atmosphere, it’s difficult to get the energy out again into space, so you’re right.
You may be thinking of the daisyworld model with a population of black and white daisies affecting both a planet’s temperature and thus each other’s growth because of the different albedo. It would explain different shades of gray or purple, but not why plants are similar shades of green all over the planet. Shifting the temperature equilibrium would not affect the evolutionary advantage of the more efficient photosynthesis of black plants.

However, plants don’t do that. Scroll about half way down on this great page about photosynthesis:
http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookPS.html The action spectrum for anacharis sp. shows it concentrates on blue, but rather ignores the green in favor of the red. Photon energy can’t be the major limiting factor.

We don’t know why plants aren’t black yet.
Since we ruled out all the simple reasons, it must be the stupid one you always overlook. :slight_smile: