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John Cheeseman j-cheeseman at uiuc.edu
Sat May 28 13:45:44 EST 1994

In article <2s7re0$k2l at vixen.cso.uiuc.edu>, csogren at prairienet.org (Carolyn
S. Ogren) wrote:

> The absorption spectrum for plant chlorophyll shows that there is
> little absorption of green light. I understand that is why plants
> are green. But why don't plants absorb, use, these wavelengths?
> Isn't this a "waste of a valuable resource?
> -- 
> q
> ?

I don't know if it is fair for me to address your questions, but I am so
happy to see an Ogren on the net that I will anyhow!  ;-)

1.  If chlorophyll were the only pigment in plants, then they would not
absorb and use these wavelengths *very efficiently*.  But, other pigments
are also present in the chloroplast, and they can transfer energy to the
chlorophyll antennae so that in many plants, the overall efficiency of
green light use is only slightly less than other wavelengths.  (This could
well start up a conversation that was in progress earlier this spring, in
which case I will have to look up actual references.)

2. Though chlorophyll absorbs *less* in the green, the absorption is not
nil.  Thus, as light bounces around inside leaves and off other leaves in a
canopy, it becomes greener, and eventually, the green gets absorbed too.

3.  Your eye is about 100x more sensitive to green than to red, so even if
the reflection or transmission of the two wavebands was equal, you would
see the green.  If you don't believe this, would it help to know that many
of the green filters on theater spotlights actually transmit as much or
more red than green?  This is also why the newer fire engines aren't red,
but international green... it is more visible at night.
John Cheeseman                                 "We haven't the money,
Department of Plant Biology                     so we've got to think."
University of Illinois                          -- Lord Rutherford (1962)
Urbana IL 61801 USA

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