It's all becoming clear

The phenomenon of transparency is way more interesting than it appears at first.

I remember thinking about the concept when I was a kid, the first time I watched the classic horror/science fiction film The Invisible Man.  Coincidentally, I was in high school and was in the middle of taking biology, and we'd recently learned how the human eye works, and Claude Rains's predicament took on an added layer of difficulty when it occurred to me that if he was invisible -- including his retina -- not only would we not be able to see him, he wouldn't be able to see anything, because the light rays striking his eye would pass right through it.  Since it's light being absorbed by the retina that stimulates the optic nerve, and Rains's retinas weren't absorbing any light (or we'd have seen them floating in the air, which is kind of a gross mental image), he'd have been blind.

So an invisibility potion isn't nearly as fun an idea as it sounds at first.

It wasn't until I took physics that I learned why some objects are transparent, and why (for example) it's harder to see a glass marble underwater than it is in the air.  Transparency results from a molecular structure that neither appreciably absorbs nor scatters light; more specifically, when the substance in question has electron orbitals spaced so that they can't absorb light in the visible region of the spectrum.  (If not, the light passes right through it.)  Note that substances can be transparent in some frequency ranges and not others; water, for example, is largely transparent in visible light, but is opaque in the microwave region -- which is why water heats up so quickly when you put it in a microwave oven.

The second bit, though, is where it really gets interesting.  Why are some transparent objects still clearly visible, and others are nearly invisible?  Consider my example of glass in air as compared to glass under water.  You can see through both, but it's much harder to discern the outlines of the glass underwater than it is in air.  Even more strikingly -- submerge a glass object in a colorless oil, and it seems to vanish entirely.

The reason is something called the index of refraction -- how much a beam of light is bent when it passes from one transparent medium to another.  A vacuum has, by definition, an index of refraction of exactly 1.  Air is slightly higher -- 1.000293, give or take -- while pure water is about 1.333.  The key here is that the more different the two indices are, the more light bends when crossing from one to the other (and the more the light tends to reflect from the surface rather than refract).  This is why the boundary between air and water is pretty obvious (and why those amazing photographs of crystal-clear lakes, where you can see all the way to the bottom and boats appear to be floating, are always taken from directly overhead, looking straight down; even at a slight angle from perpendicular, you'd see the reflected portion of the light and the water's surface would be clearly visible).

Likewise, the more similar the indices of refraction are, the less light bends (and reflects) at the boundary, and the harder it is to see the interface.  Glass, depending on the type, has an index of refraction of about 1.5; olive oil has an index of 1.47.  Submerge a colorless glass marble in a bottle of olive oil, and it seems to disappear,

The reason all this comes up has to do with the evolution of transparency in nature -- as camouflage.  It's a pretty clever idea, that, and is used by a good many oceanic organisms (jellyfish being the obvious example).  None of them are completely transparent, but some are good enough at index-of-refraction-matching that they're extremely hard to see.  It's much more difficult for terrestrial organisms, though, because air's lower index of refraction -- 1, for all intents and purposes -- is just about impossible to match in any conceivable form of living tissue.

Some of them come pretty close, though.  Consider the "skeleton flower," Diphylleia grayi, of Japan, which has white flowers that become glass-like when they're wet:

The transparency of the flower petals is likely to be a fluke, as it's hard to imagine how it would benefit the plant to evolve a camouflage that only works when the plant is wet.  An even cooler example was the subject of a paper in the journal eLife last week, and looked at a group of butterflies called (for obvious reasons) "glasswing butterflies."  These are a tropical group with clear windows in their wings -- but, it turns out, they're not all closely related to each other.

In other words, we're looking at an example of convergent evolution and mimicry.

The study found that some of the clear-wings are toxic, and those lack an anti-glare coating on the "windows."  This makes the light more likely to reflect from the surface, rather than pass through; think about the glare from a puddle in the road on a sunny day.  Those flashes of light act as a warning coloration -- an advertisement to predators that the animal is toxic, distasteful, or dangerous.

The glasswing butterfly Greta oto of Central and South America [Image is licensed under the Creative Commons David Tiller, Greta oto, CC BY-SA 3.0]

The coolest part of last week's paper was in looking at the mimics; the species that had the transparent windows but weren't themselves toxic.  Unlike the toxic varieties, those species had evolved anti-glare coatings on the windows, so the mimicry was obvious in bright light -- but in shadow, the lack of glare made them seem to disappear completely.  In other words, the clear parts act as a warning coloration in sunshine, and as pure camouflage in the shade!

Even more amazing is that a number of only distantly-related species have stumbled on the same mimicry -- so this particular vanishing act has apparently evolved independently more than once.  A good idea, apparently, shouldn't just be wasted on one species.

So that's today's cool natural phenomenon, which I hope I've clarified sufficiently.  There seems truly to be no end to the way living things can take advantage of physical phenomena for their own survival -- as Darwin put it, to generate "endless forms most beautiful and most wonderful."

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It's kind of sad that there are so many math-phobes in the world, because at its basis, there is something compelling and fascinating about the world of numbers.  Humans have been driven to quantify things for millennia -- probably beginning with the understandable desire to count goods and belongings -- but it very quickly became a source of curiosity to find out why numbers work as they do.

The history of mathematics and its impact on humanity is the subject of the brilliant book The Art of More: How Mathematics Created Civilization by Michael Brooks.  In it he looks at how our ancestors' discovery of how to measure and enumerate the world grew into a field of study that unlocked hidden realms of science -- leading Galileo to comment, with some awe, that "Mathematics is the language with which God wrote the universe."  Brooks's deft handling of this difficult and intimidating subject makes it uniquely accessible to the layperson -- so don't let your past experiences in math class dissuade you from reading this wonderful and eye-opening book.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

A botanical chameleon

One of the things I love most about science is its capacity to astonish us.

You can be really knowledgeable in a field, and then the natural world slings a curve ball at you and leaves you amazed.  Sometimes these unexpected twists lead to profound leaps in our understanding -- an example is the discovery of the parallel magnetic stripes in igneous rocks along the Mid-Atlantic Ridge leading to the theory of plate tectonics -- but sometimes it's just a fascinating bit of scientific trivia, one of those little things that makes you smile in a bemused sort of way and say, "Science is so cool."

I had a moment like that yesterday.  I taught biology for 32 years and have been interested in plants -- especially tropical plants -- a great deal longer than that.  I have a fine collection of tropical plants, currently jammed into my greenhouse so tightly that I can barely walk through it because the ones who spend the summer on my deck have to be tucked away in a warm place during our frigid winters.  I have bromeliads, cacti, three species of ginger, two different kinds of angel's trumpet (one of which got to be seven feet tall last summer, and sometimes had twenty giant, peach-colored flowers all blooming at once), a fig tree and a lime tree that produce every year, and two species of eucalyptus.

Among others.

While I wouldn't call myself an expert when it comes to tropical plants, I'm at least Better Than The Average Bear.  So I was startled to run, quite by accident, into an account of a species I had never even heard of -- and even more startled when I found out how truly bizarre and unique this plant is.

It's called the "chameleon vine," and its scientific name is Boquila trifoliolata.  It belongs to a small and rather obscure family of dicots called Lardizabalaceae, which contains forty species found in two places -- southeast Asia and western South America.  (How a group of plants with common ancestry ended up in such widely separated locales is a mystery in and of itself; populations like this are called peripheral isolates and are a perennial puzzle in evolutionary biology.)

Boquila is one of the South American ones, and lives in southern Chile and Argentina.  It's a woody vine whose leaves are composed of three leaflets (thus the plant's species name).  Here's a picture:

[Image licensed under the Creative Commons Inao, Boquila trifoliata [sic], CC BY-SA 2.0]

It's not really much to look at, and you non-botanical types are probably tapping your fingers and saying, "So what?"  But wait till you hear what this plant can do -- and why it merits its common name of "chameleon vine."

Boquila trifoliolata has an extraordinary ability called mimetic polymorphism.  It's capable of altering its leaf shape to mimic a variety of different (unrelated) plants -- including the ones it most commonly twines up as a support.  We're not talking about small differences, either.  It can be glossy or dull, have different petiole lengths, have different leaflet sizes and shapes, and even change whether or not it has serrations or spines along the edge!  

This ability, first described in a paper by botanists Ernesto Gianoli and Fernando Carrasco-Urra in Current Biology in 2014, was first attributed to genetic transfer from the host to the vine, a sort of genetic parasitism.  I'll admit that was the first explanation I thought of -- although how a plant could take up DNA from another species and only express the genes related to leaf morphology left me scratching my head a little.  But Gianoli and Carrasco-Urra were able to rule out this possibility, because Boquila can alter its leaf shape without touching the plant it's mimicking.

All it has to do is be nearby.  So it isn't a parasite at all.  The current guess is that Boquila is picking up volatile organic compounds emitted by the other plant, and those are altering gene expression, but those organic compounds have yet to be identified -- nor has any kind of specific mechanism by which that kind of alteration in phenotype could happen.

Even though we still have no idea how Boquila is managing this neat trick, the why is pretty clear.  If it's hiding amongst the foliage of another plant, herbivores can't single it out for a snack.  Gianoli and Carrasco-Urra found that when Boquila is climbing up a non-living support like a chain-link fence, herbivores actually seek it out for browsing.  But when it's camouflaged within another plant's leaves, it can avoid being seen and identified -- and, they found, browsing of its foliage dropped by as much as 50%.

Fascinating, isn't it?  And yet despite study, we haven't been able to figure out how the plant evolved this amazing (and apparently unique in the plant world) ability, nor what kind of information it's gleaning that might say, "Okay, time to change color and grow some spikes!"

So yet another example of how science is really freakin' cool.  It also illustrates how every new discovery opens up new avenues for investigation.  The crazy chameleon plant should make it absolutely clear that if you go into science, you'll never be done learning.

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Last week's Skeptophilia book recommendation was a fun book about math; this week's is a fun book about science.

In The Canon, New York Times and Pulitzer Prize-winning writer Natalie Angier takes on a huge problem in the United States (and, I suspect, elsewhere), and does it with her signature clarity and sparkling humor: science illiteracy.

Angier worked with scientists from a variety of different fields -- physics, geology, biology, chemistry, meteorology/climatology, and others -- to come up with a compendium of what informed people should, at minimum, know about science.  In each of the sections of her book she looks at the basics of a different field, and explains concepts using analogies and examples that will have you smiling -- and understanding.

This is one of those books that should be required reading in every high school science curriculum.  As Angier points out, part of the reason we're in the environmental mess we currently face is because people either didn't know enough science to make smart decisions, or else knew it and set it aside for political and financial short-term expediency.  Whatever the cause, though, she's right that only education can cure it, and if that's going to succeed we need to counter the rote, dull, vocabulary-intense way science is usually taught in public schools.  We need to recapture the excitement of science -- that understanding stuff is fun.  

Angier's book takes a long stride in that direction.  I recommend it to everyone, layperson and science geek alike.  It's a whirlwind that will leave you laughing, and also marveling at just how cool the universe is.