Q.1 Why are plants green?
There is currently no complete answer available for this question, however, here follow the basics of what is
known. Plants are coloured because they contain pigments that absorb certain colours of light more than
others. The picture below shows the spectrum of visible light:
|Above: the spectrum of visible light. Borrowed from:
Visible light is that part of the electromagnetic (EM) spectrum which humans can see with the naked eye.
However, the EM spectrum extends far to the left and right of the visible light spectrum, for example, moving to
the right of violet we encounter ultraviolet radiation, then X-rays and then gamma-rays. Moving to the left we
encounter infrared radiation, then radio waves and then microwaves. All these forms of electromagnetic
radiation, from microwaves to visible light to gamma-rays are waves of electrical and magnetic energy. These
waves travel at the tremendous speed of light across a vacuum (such as outer space) which is about 3 00 000
kilometres per second! (No massive object and no signal can travel faster than light under normal
circumstances). The term 'light' strictly applies to that EM radiation that can be seen by the naked eye, but
often scientists refer to the whole spectrum of radiation as light and that part that can be seen as 'visible light'.
Some animals can see infrared and/or ultraviolet light also, goldfish fore example can see visible light, infrared
and ultraviolet. Many insects can see ultraviolet, but cannot see red light at all well. Infrared light is radiated by
warm objects and so at night objects are still clearly detectable in infrared light as they cool off. This is why the
military use infrared night scopes and why moths can see infrared.
When you look at an object, you see light that is either emitted by the object, if it is hot and luminous, or light
that is reflected or transmitted by it. For example, a hot coal emits orange-red light, but red glass transmits (lets
through) red light whilst a green apple reflects green light (whilst absorbing the rest). The leaves of plants look
green because their pigments absorb red and blue light and transmit and reflect green light.
This explains what makes plants look green, but does not tell us why they are green?
Plants absorb light and use the energy of the light to make complex organic chemicals - fuels like sugar and
building blocks like proteins, from carbon dioxide and water and minerals like nitrogen. The sunlight they
absorb provide them with the 'solar power' they need to make these chemicals and build their bodies. This
process of using energy from sunlight to build bodies is called photosynthesis.
The graph below shows the relative amount of each colour in sunlight and the relative amount of each colour
used by a typical green plant in photosynthesis. The colour of visible light is shown in the spectrum at the top,
but is also given on the horizontal axis as wavelength in nanometres (with visible light extending from about 350
nm to 750 nanometres; one nanometre, 1 nm, is one billionth of a metre). Light is made up of particles called
photons. Plants work by using their pigments to absorb these photons and capture their energy. Thus it is the
number of photons absorbed that matter to the plant. Hence the vertical scale is given as the relative photon
flux density. Photon density simply means the number of photons striking each unit of area (say each square
inch) of leaf surface. The flux part tells us we are measuring the number of photons per second, so we are now
measuring the number of photons hitting each square inch of leaf surface per second. 'Relative' means that we
are not bothered by the actual numbers at present (only the shape of the graph) and so have rescaled the
vertical axis, giving the highest value 1 and the lowest zero. Thus, this graph shows us the relative number of
photons striking each square inch of leaf surface per second for each colour (wavelength) of light - this is the
'sunlight' curve. The 'photosynthesis' curve is the proportion of these photons hitting the leaf that are absorbed
and used in photosynthesis (1 means they are all absorbed, zero means none are absorbed) for each colour
(wavelength) of light.
Notice that sunlight peaks at about 600 nm, which is yellow light - there is more yellow light in sunlight than any
other colour because the Sun is a yellow star! Much of the Sun's light below about 300 nm (ultraviolet) is
absorbed by the ozone layer, which is just as well, since this light is damaging to living things, so plants don't
use this energy! Wavelengths above 750 nm correspond to infrared light and beyond. Green plants do not
utilise this infrared light directly for photosynthesis, but purple bacteria do. The two peak colours absorbed by
plants for photosynthesis, are around 450 nm or blue light and 670 nm or red light. Notice that green and
yellow light are not absorbed as well and so the graph here dips.
Can you see why plants are green?
Plants are green because they absorb almost all of the blue and red light and transmit and reflect almost half of
the green light and the yellow light and so their leaves appear yellow-green. Again this tells us how the green
colour comes about, but it doesn't really tell us why.
Here is the paradox:
Why don't plants absorb all the green and yellow light (which would make them appear more or less black)
since then they will extract more energy from the sunlight?
First, you need to understand that photons with a longer wavelength, that is toward the right or red-end of the
graph have less energy than photons toward the left or blue-end of the graph - blue light photons are more
energetic than red-light photons. This might explain why plants don't bother with the infrared light - it is low in
energy. Absorbing the blue light makes sense since this is high in energy. However, although a blue photon
with a wavelength of 450 nm has about 20% more energy than a yellow-green photon with a wavelength of 550
nm, there are about 30% more yellow-green photons available than blue ones (at the given wavelengths).
Scientists have tried to explain the lack of green absorption by energy arguments, but these arguments thus
appear to be invalid! So why don't plants use the green or yellow light?
Is there really a paradox?
It should be noted that plants do in fact utilise around 50% of the green and yellow light, so they are not wasting
it. Furthermore, the green and yellow light which is not used and passes through the leaf may well be absorbed
by a leaf lower down in the canopy - trees tend to have layers of leaves arranged to absorb as much of the light
as is economical. In other words, plants don't waste the green and yellow light at all, they use it! However, why
don't they absorb it as efficiently as blue and red light? Why do they rely on layers of leaves to do what a good
green-light absorbing pigment will do in fewer leaf layers?
Relic of evolution?
Green plants evolved from green algae that lived in the sea. Algae are not plants, but belong to the
protoctistans, but they are close to the ancestral form of plants. Not all algae are green. Water is blue in part
because red light is readily absorbed by water, such that very little red light penetrates to depths greater than
20 metres, so water transmits blue light better than red light (the sea also reflects blue light from the blue sky
overhead, adding to its blue colour). Algae living at greater depths tend to look red in sunlight, since they lack
pigments that absorb red light and so they reflect and transmit red light well. It would be a waste of their
resources making such pigments when they have no red light to absorb! Green algae live in shallower water
where there is plenty of red light to be used, and so they absorb this light. They also absorb the ample blue
light that is available. However, organisms that float in the water, such as microscopic algae and photosynthetic
bacteria, absorb much of the light before it reaches the green algae. It has been suggested that these bacteria
absorb much of the green light, but not the blue, leaving more blue light and less green light available to the
green algae. If this is so, then the green algae will have evolved to utilise the wavelengths of light remaining -
principally blue and red. There is some truth in this, since cyanobacteria are among the most abundant
photosynthetic bacteria in plankton, and they are blue-green, so they don't appear to use the blue light as well
as green algae. However, then one has to explain why the cyanobacteria did not evolve to use all the blue light!
If red algae had colonised land, would they have turned green? It was always more likely that the greens would
make it since they were already adapted to shallower water.
It seems to me, that evolving suitable pigments may be difficult. The cyanobacteria evolved first, and used what
pigments they had. Other organisms then had to make use of what colours of light were left. Green plants
would then be green more by accident than design - it is perhaps a relic of their evolution. So, why didn't they
change colour once they got on land - maybe because it is difficult to evolve suitable pigments, or perhaps
because they don't to, maybe using a multi layered canopy works just fine! On the other hand, what is often
overlooked, is that many probably have!
Not all 'green' plants are green
A recent research article attempted to explain why plants on Earth are green and to predict the colour of plants
on other planets with different colour suns. They concluded that blue vegetation is unlikely, because blue light
photons are energetic and tapping that energy is useful. However, cyanobacteria that dominated the Earth for
aeons are blue-green and many plants on Earth are blue!! Do an image search on Google for 'blue plant' and
see what you find! The copper beach is predominantly reddish in colour as it contains many red pigments
(which are photosynthetically active).
Should plants be black?
Some plants are almost black, which shows that they do a good job of absorbing all colours of light. Accessory
pigments absorb green and yellow light and pass the energy on to the chlorophyll. The fact is, that some colour
will be absorbed less well than the others, by however little, and this will colour the plant. Thus it seems to me,
that the question of why plants are green has no answer explainable by functional mechanisms - it is simply an
evolutionary relic that is probably of little consequence to the plant and is probably largely unavoidable, since
plants must be some colour if not black! Maybe reflecting some light stops the plants from overheating - black
objects get warmer in the sunlight! Plants go through considerable lengths to stop their 'solar panels;
overheating. This is the best answer I have so far obtained!
Although it is true that green is in the 'middle' of the spectrum, and so best placed to absorb the spectrum
either side, this is still not the whole story. ('Middle' meaning that part of the Solar spectrum that easily passes
through the atmosphere. Also 'middle' in the sense that if the energy is too far to the red-end then it has less
energy per quantum and if too far towards the ultraviolet actually starts to damage molecules since it contains
too much energy per quantum). First of all, plants still fail to utilise some photosynthetically useful light, which
can be used by other organisms. Perhaps the question shouldn't be 'Why are plants green?' but 'Why aren't
plants black?' Perhaps being black would require too many pigments to simultaneously evolve, or perhaps
utilising all these pigments would be too costly and the plant would encounter diminishing returns. Copper
beech trees are red, sometimes almost black, because they have additional pigments that absorb the useful
blue light for photosynthesis. Why haven't the majority of plants evolved to do this? True all have red pigments
that absorb some of the useful blue light, but in most the green pigment still dominates - why? What additional
costs does the copper beech incur by absorbing blue light so well? Perhaps if plants were black they would
absorb too much light and overheat? The fact that some land plants are blue, some red, some almost black
and most green tells us that there are additional factors at play.
Evidently green is the best bet, on Earth, if one colour had to be chosen, so most terrestrial plants are green,
but I still don't know why so few plants are black. Another question that comes to mind is 'How evenly must a
plant absorb the visible wavelengths to appear black to the human eye?' Certainly the amount of useful light
that plants let slip appears quite significant in many cases. In the end it probably comes down to economic
issues - plants have to balance costs and benefits, being 'optimal' may actually be sub-optimal if the plant
incurs too many costs, for example in pigment production - perhaps being black is just too expensive for what
I am not certain that a satisfactory answer has been found to the question, ;'Why are plants green?' and I am
open to input. (Email Bot at email@example.com or feel free to leave a comment on our blog).
Cronodon would like to thank Mike McNally of NASA for useful discussions on this topic.
|Plant FAQs (Frequently asked questions)