Green & Blue Flowers Should Be Less Common in Nature
One would expect a relatively low frequency of green and
blue flowers because they would unnecessarily confuse insect and bird
pollinators. The problem of distinguishing flowers may be more acute for
insects than for birds, because the former have lower visual acuity (VA) [1].
The selection pressure should be greater against green flowers because they
could get missed in the midst of green leaves, and sometimes green trunks or
stalks. Blue flowers could get missed against the background of the blue sky,
at some angles and times of the day. However, the selection pressure against
blue flowers should be weaker because the sky is in the background and because
its color is much more variable due to clouds, rain, fog etc. (Nevertheless, in
spring, when a lot of pollination takes place, skies are largely blue.) So the
frequency of blue flowers should be higher than that of green flowers. This
argument assumes that the visual system of the pollinators could respond to
both these colors (and to other alternative colors), and that sight is the
dominant mechanism for finding the flowers.
No comprehensive database of flower colors:
According to Joanna Klein in the NYT [2], “Less than 10 percent of 400,000 floral species bear blue flowers. It’s
unclear why.” This article was written in the context of Japanese
researchers recently creating the first blue chrysanthemums by genetic
engineering [3].
“Less
than 10 percent of the 280,000 species of flowering plants produce blue
flowers,” Prof.David Lee [4] said. He added that true blue (as opposed to
structural blue [5]) is rare in nature, and explained the chemistry: ‘The key
ingredients for making blue flowers are the red anthocyanin pigments. “Plants
tweak, or modify, the red anthocyanin pigments to make blue flowers,” Lee said.
“They do this through a variety of modifications involving pH shifts and mixing
of pigments, molecules and ions.”’ A different argument for the rareness of
blue flowers is that most plants rely on chlorophyll which strongly absorbs
blue, which is useful since it is a high energy (and high photon flux) part of
the solar spectrum [6]. Plants that
have preferred chlorophyll find it difficult to come up with the chemistry for
blue.
There
is agreement on the proportion of blue flowers: less than 10% of the total -
whatever that is. Probably the total number of ‘floral species’ is 400,000,
because it includes 120,000 flowering trees as well. I Googled the internet but could not find a
corresponding number for green flowers. Nor could I find any number in David
Lee’s book [7]. But then I just glanced though the chapter on flowers… Absent
this critical bit of information, the argument about green flowers cannot
really go much further.
Another
expert [8] has this to say:
“No one has surveyed all of the world's flowers. All of the
world's plant species haven't even been discovered and named yet. Further,
flower color statistics have not compiled anywhere for the majority of the
earth's plant species. We know of no central repository of flower color
information… Green may actually be the most common flower color… If we had
to rank the four colors you asked about in order from most common to least, we
would guess -- and we emphasize guess,
here -- that they would line up like this: 1. white, 2. yellow, 3. blue, 4.
Red. ”
Note that this expert states that green may be the most
common color – but, oddly, when rank ordering the flowers does not put
green in the first four.
Pollinators
and their preferences:
One
might add that bees respond to ultraviolet light, so what is a white flower to
us, has clear ‘nectar guides (like landing strips)’ on it for the bee [1]. That
is not really relevant, but what is apposite is a study mentioned by Joanna
Klein [2]: Australian researchers, Adrian Dyer et al [9], found that bees
native to Australia (T.Carbonaria) prefer blue flowers.
Dyer et
al [9] state that: “insect-pollinated flowering plants often generate spectral
signals that suit the color capabilities of important bees, or other potential
pollinators in an environment”. Many studies show that: “bumblebees show a
preference for blue stimuli across a wide geographic range”.
“T. carbonaria bees showed a significant
preference for stimuli from the blue and
blue–green regions of the colour hexagon, consistent with findings that
honeybees and bumblebees
tend to prefer flowers
with such spectral characteristics. While it remains to be definitively shown
whether bee innate colour preferences may drive flower evolution, there is
evidence that such preferences are linked to flowers of these hues having
higher nectar rewards.” (emphasis added by me). That is, although it would be expected that ‘innate
color preference’ of bees drives flower color evolution, it has not yet been
definitively proven.
Those ‘higher nectar
rewards’ for blue flowers: it looks like sheer bribery to me! I would speculate
that flowering plants added a nectar bonus to these blue flowers to compensate for
the slight disadvantage of a blue sky. However, I would have to mention that
Dyer et al nowhere mention a ‘blue sky’ problem, and they have replicated the
natural environment in their lab for testing.
Dyer also gave an
interview with ABC [10] in which he pointed out that there are two different
types of birds in terms of their visual systems: some birds see violet, green,
blue & red and these are called ‘violet-sensitive’, whereas there are
others that also see ultraviolet (and these are called ‘UV-sensitive).
Bees can see UV, blue and green – but not red. Pollinating flies can see four
different types of color i.e. they are tetrachromats (but they prefer yellow).
Dark
adaptation and visual acuity of pollinators:
Moths, which operate at
night, do not see color at all, and the plants they visit are white. Oops! The
hawkmoth, it seems, does see in color at night… [11a]. It seems that the
compensation for the lack of photons at night occurs in one of three ways [11b]:
a a)
Decreased focal length (distance between the aperture and the
light-sensitive cells), f
b b)
Increased diameter of the ommatidium or eye, D
c c)
Special light-trapping structures that cause a ‘double pass’ of
the photons, increasing the quantum efficiency h(probability
of detecting the photons).
So, the number of photons detected is increased since it is
proportional to h(D/f)2.
In Kelber’s words: “A
closer inspection of the geckos' eyes revealed that, with no rods to fall back
on, the cones in their eyes had evolved to become more rod-like, longer and
more sensitive. Like the hawkmoths, they also had large lenses and a shorter
focal distance to cut down how far the light had to travel through the
eye.” Kelber also points out that many
nocturnal eyes have [11c]: “a mirror-like structure at the base of the
eye, which reflects the light across the photoreceptors for a second time.”
With this adaptation, Kelber et al showed that nocturnal hawkmoths have color
vision even under dim starlight (10-4 cd/m2). At such
light levels, humans are blind - not just color-blind!
Some insects are crepuscular i.e. they are active at dawn and
dusk. They have a special adaptation, called a neural superposition compound
eye (as opposed to a simple apposition compound eye), in which the photons from
7 adjacent ommatidia are summed up in the neural layer, so that the number of
photons detected is increased by 7X – without sacrificing spatial resolution.
This type of neural superposition gives them an advantage during twilight over
predators and competitors who have apposition eyes and allows them to detect
small objects [12].
And we have not even
taken up bats, butterflies, midges, mosquitoes and wasps yet! Ok, forget the
bats…? Well, you cannot: they are dichromats. So if coevolution of
flowers and their pollinators is occurring, then it must be specific to the
species involved. In addition, Dyer adds that bees cannot see very well (their
bad VA was mentioned earlier [1]), and they can only see flowers when they get
quite close: “maybe 50-60 cms”, and probably use the scent of flowers to find
them from afar. This argument might imply that flower color should not really
be important for small pollinators! Dyer also states that primeval plants were
probably dull, pale yellow or green, until about 100 million years ago, when
they evolved the more vibrant hues that we see today.
Considering that bees do
not see very well, the smallest object that they can resolve with an ommatidium
is about 6.7 mm (at a distance of 50 cms, corresponding to an angular
resolution Dq of 13.3
mrad for a 30 micron diameter ommatidium d and
400 nm light wavelength l) since
a) diffraction constrains the angular resolution, and b) the diameter of the
ommatidium d is related to the radius
of curvature r of the bee’s head (about 3 mm) [1]:
Dq = l/d = d/r
Thus, the optimum
diameter of the ommatidium is given by: dopt = (lr)1/2, which is fixed by the wavelength of light and
the radius of the bee’s head [1].
Feynman [1] also states
that bees have 30X worse spatial resolution than humans: this would suggest
that they can resolve down to about 1.5- 3.0 mm (assuming the human eye can
resolve between 50 - 100 microns). In that case, the scent of the flower would
have to guide them to its vicinity, and then sight would do the rest in the
final (terminal), homing-in on the target.
The
blog [12a] gives the ommatidium diameter of A.Bilineata as about 4 microns. As
pointed out by Feynman [1] and by Ref.14, diffraction limits the diameter of
the ommatidium to above this limit. A ommatidium of 2 micron diameter would be
of no use because diffraction would blur any image beyond usefulness. For
smaller insects, such as flies, the ommatidium diameter may be somewhat smaller
(but it has to be > 4 microns anyway to avoid diffraction), so a significant
fraction of the surface of the head is covered by ommatidia!
This
blog [12a], which gives a very detailed description of the insect eye, both day-
and night-adapted, mentions that the visual acuity of the compound eye of the
insect is about 100X worse than that of humans( compared to 30X quoted in [1]).
Rigosi et al [13] studied the honeybee
A.Mellifera. Previous studies had shown that: “bees could not discriminate
a dark object smaller than 3 deg”. Rigosi et al measure better angular resolution when the bees
are light-adapted, a value that is 30% lower than the above value. They also
found that: “in both frontal and lateral regions, responses saturate for
large objects that fill the receptive field but
decrease linearly as the
object area falls below 1 deg2 . As features fall below the size of
the receptive field they are increasingly blurred to a lower effective contrast
until the response is indistinguishable from noise”. This amounts to an area of
about 9 x 9 mm2 at a distance of 50 cms.
A standard definition of
a point target is when it occupies one pixel. The distance at which it occupies
on pixel depends upon its width (let us assume width w = 5 cms, arbitrarily) and
the minimum angular resolution (MAR) (or Dq), taken here as 1 deg. Then the distance d above which it is
seen as a point target by the pollinator is:
d = w/(MAR) = 5/0.017 =
287 cms.
The minimum requirement
for the target to be resolved is that it is covered by 6 x 6 = 36 pixels, which
will occur at a distance of about d/6 = 48 cms, which roughly agrees with the
number mentioned by Dyer [9,10]. This criterion of needing 3 line-pairs across
the target (or 6 pixels) was first proposed by J.Johnson of the Army Night
Vision Lab [14]. However, the size of the target (the flower) will vary
depending upon the angle of approach. The terminal homing-in phase mentioned
above refers to distances less than d, where the target is rsolved.
An interesting question
is whether at distances at which the flower is a point target, could the
pollinator use its color (apart from its scent) to detect it? Something like
the fact that we can see the color of a distant star.
Color
response, and the ambiguity in determining the color of flowers:
Since the spatial
resolution is in the range of about 2 - 7 mm, the color of the flower may
actually become more important to the bee than its shape (for identification, not
landing). This problem would become even more acute for butterflies which are
smaller than bees, or for smaller size bees, that also have smaller ommatidia.
Interestingly, one butterfly, G.Sarpedon, has been found that responds to 15
colors (i.e. it has more than fifteen types of photoreceptors) – whereas we can
only see three (trichromats) [15]. Note
that this is in the same league as the other record-holder, the mantis shrimp
(H.Trispinosa), which has 12-16 distinct photoreceptors [16].
According
to Virginia Morell [17]: “Butterflies need only four receptor classes for color
vision, including spectra in the UV region. So why did this species (G.Sarpedon)
evolve 11 more? The scientists suspect that some of the receptors must be tuned
to perceive specific things of great ecological importance to these iridescent
butterflies—such as sex. For instance, with eyes alert to the slightest
variation in the blue-green spectrum, male bluebottles can spot and chase their
rivals, even when they’re flying against a blue
sky.” (emphasis added). Well, the much-awaited blue sky finally showed up
somewhere (even if it leaves much to be desired)!
Feynman [1] mentions that
bees cannot see red (like bulls!), so they do not visit ‘true red’ flowers
(that do not have any other tinge of color to which bees might be sensitive) –
but these flowers are visited by hummingbirds which do see red. That sounds
suspiciously like coevolution to me – or is it just rank avian opportunism? –
or is it both?
Arnold et al [18]
hypothesized that there may be seasonal variation in flower color to better
attract pollinator insects in a ‘market’. Bees generally prefer blue, green and
UV, while butterflies prefer pink/purple and hoverflies prefer yellow and
white. “The pollination syndrome hypothesis might lead us to expect that if
particular pollinator guilds constitute a larger proportion of the total
pollinators at certain times of year, then those plant species blooming at that
time should be more likely to possess the flower colors associated with those
pollinators.” However, pollinators do not exhibit strong brand loyalty and are
quite capable of learning new behavior, “able to associate almost any color
with reward. “ In the presence of such perfidy, coevolution
sounds impossible!
Arnold et al [18] conclude:
“…(although) data
collected appears to suggest that in some habitats, certain colors of flowers
bloom at particular times of year…we found no statistically significant
evidence that the colors of flowers change throughout the year.” Apparently, a
lot of hypotheses just do not pan out! They add: “During much of the year,
pollinators in woodland must forage under lower light levels, and also under
light that is spectrally different from normal daylight (with a spectral peak
around 550 nm owing to filtering through green leaves)”.
After all this argument about the supposed disadvantages
suffered by blue and green flowers, it is useful to add a caveat. Even a
disadvantageous situation may be exploited by some plants and animals because
it serves as an ecological niche in which there is lower competition. In fact,
one might well say that Nature abhors a vacant ecological niche! This would
imply that, even if the frequency of blue flowers is low, it will never be
negligibly small. Another problem is with the definition of ‘blue’. The
researchers who tweaked the color of the chrysanthemum were concerned that it
should be ‘true blue’, without any tinges of other colors. So even the
estimates of 10% for the fraction of blue flowers could be dependent on its
precise definition. The expert [8] adds: “Many flowers are multi-colored. Some
species feature flowers that change in color as they age. Other plants bear
flowers of different colors on the same plant.” No wonder there is no exhaustive database! And
the color seen depends not only on the eye of the beholder (the visual system),
but the type of illumination (solar, lunar, direct, reflected, polarized…).
Another authority [19] also stated:
“We simply have no idea
what the most common flower color is in the world but it's probably green. Big
or small, we like bright colors and we like weird colors. All the rest just get
glazed over. In reality, many plant species, especially trees, produce small,
nondescript green flowers.”
He adds:
“It is actually an easier
question to ask ‘what is the rarest flower color?’ To that, most botanists will
probably say black.”
I started with an idea
that green and blue flowers should be less common, but the literature does not
support any such easy generalization. In fact some experts claim that green may
be the most common flower color. The literature says that blue flowers are less
than 10%. Does that compare with 1/7 or 1/3 – if we assume all colors have
equal weight? But color is clearly important to pollinators, since their visual
systems have even adapted to provide color sensitivity at night.
To summarize:
a)
No complete database exists of flower colors – and green is
probably under-counted, since humans may just not notice them
b a)
Flower color itself is not clearly defined, since it depends on
many environmental variables such as lighting in the day, twilight or night,
angle, texture etc.
c b)
Different pollinators have very different visual systems, so
coevolution is specific to the flower-pollinator pair
d c)
Small pollinators have low visual acuity (e.g. distance to flower
should be less than about 50 cms for clear vision for bees), and they home in
on flowers from a distance using scent, but use color for the final phase, to
actually make contact. Even nocturnal moths do this! This highlights the
importance of color for the pollinator – even at night!
e d)
There is no conclusive proof for coevolution of bees (that are
the most well-studied pollinators) and flowers - although it is generally
accepted that coevolution happens.
f e)
Blue flowers are less than 10% of the total, maybe because
chlorophyll absorbs blue, and because anthocyanins are mostly red (but change
to blue by changing pH), The ‘blue sky’ argument that I have suggested is not
mentioned by anyone.
References:
1 1)
Feynman lectures Vol.1 Ch.36 (Addison &
Wesley, 1963)
3 3)
N.Noda et al., Sci. Adv.
2017;3: e1602785 26 July 2017
7 7)
David Lee “Nature’s palette: the science of
plant color” (Univ. of Chicago Press, 2007)
9 9)
A.G.Dyer et al J.Comp.Physiol.A (2016) DOI
10.1007/s00359-016-1101-4
b) A.Kelber Nature 419 (2002) 922
c) A.Kelber & L.S.V.Roth J.Expt.Biol.209
(2006) 781-88
b) Michael Land & Dan-Eric Nilsson “Animal
eyes” (Oxford University Press, 2012)
13)
E.Rigosi et al Sci.Rep. (2016) DOI:
10.1038/srep45972
1 15)
P.-J. Chen et al Frontiers in Ecology &
Evolution 4 (2016) doi: 10.3389/fevo.2016.00018
1 18)
S.E.J.Arnold et al Israel J.Plant Sciences 57
(2009) 211
