Rooting around in the Rutaceae

   Let’s see, it’s been almost two full weeks since I last delved into botany. That must mean that it’s time to meet another plant family! So, what’s on our plates today? I swear, I’ll eventually get to a largely non-edible plant family, but that day is not today. Instead, we’re going to talk about the Rutaceae, also called the rue family. You might be familiar with one of the more familiar genera, the Citrus.

    Yes, as is so often the case, and entire family of some 148 genera is best known by the fruits of just one genus. Before we get to the joys of that one genus, let’s look a little more at the whole family. The Rutaceae are one of those “commercially important” families and not just because of the fruit. Rutaceae are widely distributed worldwide - mostly in the tropics and subtropics, but outliers like the Bush Rue and Mairehau are naturally found in temperate climates. The Rutaceae family is also a popular source of ornamental plants (another part of the commercial importance).

    So, what knits together this family of some 1,400 species? The Rutaceae are mostly woody plants - plants with permanent, durable stems and root systems, which contain sturdy cellulose and lignin as structural elements. Woody plants are most often associated with trees, but can be as small as bushes and shrubs, as long as they contain that permanent, woody core. In the Rutaceae, that permanent, woody core is such that some tropical species are highly valued for timber. Moving out from the stem, Rutaceae leaves, though variously shaped, are studded with oil-producing glands.

                 Compare the leaf shapes. That’s not what these two plants have in common.

    It’s the oil that is the real distinctive feature of the Rutaceae. Along with one other plant family, the Rutaceae are the only natural producers of a class of chemicals called limonoids. Limonoids are just one of a number of oils produced by the Rutaceae (another element of that commercial importance is the use of essential oils from the plant family in perfume manufacture. But it’s limonoids that are the really distinctive ones.

Limonoids appear to function mainly within the plant as an herbivore deterrent - they are responsible for the bitter taste of citrus peels, and, when consumed, act as a growth inhibitor, and even as a toxin to insects. Some limonoid extracts could even be used as a natural insecticide. Limonoids also have an array of potential health benefits to humans. Research is ongoing, but certain limonoid compounds appear to have anti-cancer properties. See? Citrus is good for you, and not just when it comes to vitamin C.

    Let’s talk a little about the vitamin C. Long before we started investigating the anti-cancer properties of orange peels, we worked out that the stuff inside the peels could prevent scurvy (even before it was understood that scurvy is simply a vitamin deficiency). The fruits of the Rutaceae, and of the citrus genus in particular, contain high concentrations of ascorbic acid (aka vitamin C). But wait, there’s more! The fruit of the citrus genus also contains high concentrations of folate and potassium. Beyond nutrients, citrus fruits are a veritable treasure trove of chemical ecology. Citrus fruits contain, besides limonoids, flavonoids, carotenoids, and hydroxycinnamic acid. Intake of flavonoids is correlated to a lower risk for heart disease, carotenoids are a source of vitamin A and may have anti-cancer properties, and hydroxycinnamic acid is an antioxidant with potential anti-cancer properties.

Now that I’ve all but told you to go eat some, let’s look at the common citrus fruits in somewhat greater detail. There are a number of citrus fruits out there, and seemingly more entries in the -elo, -ine, and -quat categories every year. Thing is, all of these fruits are cultivars and crosses of a fairly small number of species. Citrus sinesis, the sweet orange, is one of the prime ones - native to southeast Asia, it has been spread all over the world.

Sweet oranges, blood oranges, jaffa oranges, valencia oranges, navel oranges - all varieties of Citrus sinesis. Then there is Citrus maxima, the pomelo. These deceptively thick-skinned fruits are native to Malaysia, and have brought on buyer’s remorse among many a produce aisle adventurer drawn to the huge size of the fruit, and unaware of how much of that was rind.

Cross Citrus sinesis with Citrus maxima and you get the grapefruit, and all of its cultivars. Tangerines are not some dwarfed species of sweet orange, but are, in fact, a separate species, Citrus reticulata. Tangerines (or mandarins) were first cultivated in China and Japan, and have a huge amount of varieties, to the extent they were once considered a separate species. Cross a tangerine with a grapefruit, and you begin to realize what a genetic mishmash the tangelo is. Kumquats, Citrus japonica, are fun to say, but underutilized in the United States.  

Lemons and limes are interesting, as rather than one species having many names, here many species have two names. Lemon refers to both the fruit of the Citrus limon, but also to a number of fruits of lesser-known citrus species, and hybrids of those species. Similarly, lime refers to the fruit of the Citrus aurantiifolia, and to the fruits of a number of related trees and bushes. The less said about the inevitable lemonime the better.And then there are the citrons, the papedas, the yuzus, and the bergamots (the stuff they use to flavor Earl Grey tea), to name a few of the more obscure citrus fruits. There are all kinds of interesting branches to climb around in the Rutaceae, and you never quite know what you’re going to find there. Plant families are fun that way.

Putting the ‘I’ in “HIGHLY REACTIVE”

Now there’s a heading that’s really funny to those familiar with roman numerals and the periodic table. Nothing like an incredibly obscure joke to get us started on an in-depth exploration of the periodic table! Don’t worry, we’re starting off with a bang today.

Yes, today we’re going to look at Group I of the periodic table, also known as the alkali metals. Long beloved by chemistry students and irresponsible chemistry instructors for their highly reactive properties, one of the more distinctive features of this group is that all the elements explode when they come in contact with water. The explosions grow stronger as you move down the group from lithium (atomic number 3) to cesium (atomic number 55). For those of you who never got to play with sodium and water, or have mused about what would happen if you introduced cesium to water, this video is both entertaining and informative. And for the record, that “francium bomb” video floating around the internet is a hoax. If francium were actually available, I’m sure someone would try to weaponize the stuff, but as it happens, francium is both extremely rare and extremely unstable .

    To get into why all the Group I metals are so reactive, we’re going to backtrack a little. Group I refers to the elements in the first column of the periodic table - lithium, sodium, potassium, cesium, rubidium and francium. As they’re all in the same group, we can assume that all these elements undergo chemical reactions in the same fashion - they are more likely to undergo some kinds of reactions, and less likely to undergo others. Hydrogen is placed on top of lithium in most table arrangements, but it isn’t actually part of the group. Group 1 is distinguished at a fundamental level by consisting of elements whose non-charged atoms have only one atom in the outermost valence shell. Remember those from last week? But wait, hydrogen only has one electron in its outermost shell. What makes hydrogen so different? Well, in the case of hydrogen, the outermost shell of electrons is the only shell of electrons. Unlike any other element with an unfilled outermost shell, there is no shell of electrons behind that partly empty shell. This leads the positively charged nucleus to have a much stronger draw on the lone electron - hydrogen might still lose its outermost shell to become a hydrogen ion, but it might also share that lone electron with another atom in a covalent bond (remember those from last week?). Also, the first shell to fill in any atom only holds two electrons - hydrogen could go either way. For comparison, the next shell to fill holds eight electrons.The alkali metals have a full shell of electrons behind the lone electron, and can deal with that unfilled shell by either losing one electron or gaining seven. Losing one electron is easier, so in a situation where there are atoms which attract electrons, an alkali metal is going to shed that one electron. And that shedding of an electron is the cause of alkali metal reactivity.

Ok, but how does that start a fire in water? Time for a quick tangent about the chemistry of water. Water is a bizarre substance. By virtue of its atomic weight alone, it should only exist at room temperature as a gas, not as a liquid - none of the other elements in the surrounding periods exist as liquids at room temperature. Some elements, like carbon or aluminum, bond many atoms together, to form solids. Other elements form relatively small molecules with other elements, and exist as gases. However, water experiences something called hydrogen bonding. Last week, when I mentioned covalent bonds, I described it as when atoms essentially merge their outermost shells and combine electrons. Well, it’s not a perfectly even merge. Frequently, one of the atoms involved is going to exert a stronger pull on the electrons, creating a molecule that is more positive at one end, and more negative at the other end. In the case of water molecules, the effect is particularly strong - the end of the molecule with the two hydrogen atoms has a relatively strong positive charge, the end with the oxygen atom has a relatively strong negative charge (water molecules have a sort of boomerang shape to them). Neither charge is anywhere near as strong as a true ionic charge (i.e., hydrogen with zero electrons instead of one), but the charge is strong enough to attract the positive end of one water molecule to the negative end of another, and on and on. The bonds are weak, but they allow water to remain liquid at room temperature.

    Normally at room temperature, water bubbles along in its happy liquid state. Changes in temperature may either negate (by heating) or strengthen (by freezing) the hydrogen bonds, leading to changes in the phase state of water, but the molecules are stable, with each oxygen strongly attached to its two hydrogens. Alkali metals, however, are so reactive that they are capable of breaking water molecules. An oxygen atom is more strongly attracted to an alkali metal atom than it is to a hydrogen atom - if the oxygen atom is already bonded to hydrogen atoms (giving it a full outermost shell), it will break the bond with one of those hydrogen atoms, and make a new bond with one of the alkali metal atoms. Breaking any chemical bond releases energy in the form of heat. The hydrogen atoms released from the water molecule combine into hydrogen gas, which is highly flammable. The bigger the alkali metal atom, the stronger the reaction. I think you can see where this is going.

                                           Ok, impossible to test, but a sound theory.

    Cool as the reaction is, it's unusual (outside of chemistry classes). Normally, alkali metals exist as positively charged ions. They commonly combine with negatively charged ions - one such combination you might be familiar with is NaCl, aka table salt. Some of the alkali metals, in stable ionic form (i.e. Na+), play a role in biology - lithium functions as a neurotransmitter (and is administered in the treatment of certain forms of depression), while potassium and sodium are both key to proper cellular functioning. There aren’t as many non-explosive applications for applications for cesium and rubidium, but they are found in compounds in certain technical applications.

    The one place you don’t generally find the alkali metals is in a form that most would recognize as a metal. Oh, it is possible to isolate alkali metals into a solid form - the result is a soft, silvery grey substance.

   
    However, that substance reacts spontaneously with water, and in some cases, air. Chunks of solid alkali metals must be stored in a non-water liquid, like kerosene. Hence, you’re not going to see it much out of the lab. Alkali metal compounds and ions, on the other hand, are all around you, and even in you. When they aren’t exploding, Group I is a pretty useful collection of elements.

At fault

So, who remembers that earthquake back in December? You know, the one in Ohio? The fact that it happened in Ohio is what makes this earthquake so memorable. A magnitude 4.0 on the Richter Scale, while definitely noticeable, is not the thing that widespread news coverage is commonly made of. A 4.0 on the Richter Scale in a place where one does not expect there to be an earthquake, however, does tend to be newsworthy. And Ohio is really one of those places you don’t expect earthquakes. In one of his many books on geology, writer John McPhee once quipped that in the United States, “politics and plate tectonics tend to be at their most stable in the Mid-West”. He does have a point. Ohio is in the middle of the North American craton, a solid chunk of tectonic plate that has been cohesive for a little over 1 billion years. 

    For those of you with a limited geology background, that solidity is an important part of the story, and underlies the surprising element of an earthquake in Ohio. Technically, an earthquake is a vibration of the Earth’s surface following an event that releases energy into the crust. So, the shockwave that follows a bomb going off at ground level, or the shaking surrounding a volcanic eruption are, by definition, earthquakes. Usually, however, an earthquake is caused by movement of the tectonic plates at fault lines. Plates can move in several ways, with each form of movement causing earthquakes in its own special way. In each case, plate movement somehow puts strain on the crust, strain that is released in the form of an earthquake.

Areas where new crust is forming and pushing two plates apart (spreading centers) are prone to small, shallow quakes - the newly formed crust is so weak that it can tolerate very little strain. Areas where one plate is being dragged under the other are prone to earthquakes at all depths in the crust. The process of one plate being dragged beneath another is neither smooth nor linear - periodically the plate going down with get jammed, and eventually break loose to sink further, accompanied by an earthquake. Areas where mountains are forming are also earthquake prone, unsurprisingly. When two plates are colliding and deforming to push huge amounts of crust into the air, there are going to be times when the collision process jams, strain builds up, and eventually the jam breaks, the mountains shoot up (metaphorically), and a huge amount of energy is expended in earthquakes. The catastrophic earthquakes of Pakistan, Iran and Turkey all owed their origins to this mountain building process. Then there’s the quake-generator most familiar to Americans, two plates sliding past each other. Again, “sliding” isn’t quite the right word. More like “two plates trying to move past each other, getting jammed, breaking free with an earthquake energy release, and getting jammed again”. The famous (infamous?) San Andreas fault system of California is such a place. Ok, so that’s earthquakes 101. Ohio, however, is quite clearly none of those things. It’s not even near any of those things.

Compare this map of earthquake locations worldwide to the following map of the tectonic plates. Now, try to find Ohio.

 

This is where we get into the exciting and ill-understood world of intraplate earthquakes. These are rare events, comprising maybe 0.5 percent of all earthquakes in a given year. Still, they can be dangerous. An intraplate earthquake in Gujarat, India in 2001 killed nearly 20,000 people, while a series of intraplate earthquakes in New Madrid, MO rank as some of the strongest earthquakes in the recorded history of the U.S. Intraplate quakes tend to be more destructive than the more common plate-boundary quakes, precisely because of their position in the midst of a plate. Cracked, broken rock as you would expect to find in an area prone to earthquakes does not transmit seismic waves very well - an earthquake occurring along a fault line, while potentially strong and dangerous, is unlikely to shake more than the immediate area. The solid, unbroken rock characteristic of the middle of a plate, however, is an excellent conductor of seismic energy. An 1886 earthquake epicentered in Charleston, SC rang church bells 900 miles away in Boston- at 7.6 on the Richter scale, the quake was strong, but a comparable earthquake in a fault zone in Mexico was felt only weakly 600 miles away. The recent magnitude 5.8 earthquake in the Washington, DC area was weak enough to merit laughs from more earthquake-prone regions of the United States, but was felt over 500 miles away in Maine. That would be like the recent magnitude 5.6 earthquake in northern California being felt as far away as Los Angeles. As it happened, the quake was barely felt 200 miles away in Sacramento. Intraplate quakes are hard to miss, but also hard to understand.

At this point, there are several theories out there to explain intraplate quakes. All three have one principal in common - while the centers of plates are certainly stable, they are neither uniform, nor one hundred percent solid. Pressures at the edges of plates can cause warping and deformation hundreds of miles away from the active faults, while a geologic phenomena called hot spots can cause volcanism and even rifting deep into a plate. We might get into hot spots in the future, as they’re very cool. And responsible for some really wacky geology out West. Anyway, crustal plates can get all bent out of shape far from a plate boundary. In fact, plates periodically rift apart, or begin to rift apart, but stop. The resulting areas of weakness from a so-called failed rift, hot spot volcanism, or past deformation from an episode of mountain building can all react to stress emanating from a plate boundary. While the North American craton is generally stable, it is crisscrossed with zones of weakness, and actively colliding with/sliding past other plates. The stress from those plate interactions is periodically relieved by earthquakes in the lower Mississippi Valley (site of a failed rift), the South Carolina coast (deformed by rifting 100+ million years ago), or a host of other places (including, worryingly, a rift running right across Manhattan Island).

So that’s what happened in Ohio, right? Not quite. Remember back when I said that the rare human activity can cause an earthquake? Well, it looks like that’s what happened in Ohio, but it wasn’t a bomb this time. No, for this we can credit disposal of waste fluid from a natural gas hydraulic fracturing (aka fracking) operation. The timing, location and depth of the quake was closely correlated with fluid disposal via injection. Additionally, research out of Oklahoma indicates that injecting fluid into a faulted or otherwise weakened area of rock that is under stress (i.e. any potential intraplate earthquake zone) can cause an earthquake by lubricating the rock, and thus reducing the friction keeping the rock stable. Earthquakes happen when the strain on an existing zone of rock weakness becomes stronger than the forces keeping the zone immobile (typically friction). By reducing the stabilizing force, introduction of fluid to an earthquake zone can cause an earthquake. Fluid injection doesn’t increase strain, but it does diminish friction, which is kind of the same thing.

The good news is that hydrofracking induced quakes tend to be shallow and weak, but they are still earthquakes. Intraplate quakes are messy and hard enough to understand or predict, without introducing further complications. As in all things in science, more research is required, but we might want to take a closer look at the geologic maps for a given area before doing, well, anything with it.

Red sky at morning

    This is a landmark day in the history of this blog. I'm pleased to be showcasing our first ever guest entry, courtesy of Virginia (my favorite atmospheric chemist). Enjoy!

“Red sky at morning, sailors take warning.

 Red sky at night, sailor's delight.”

     It's easy to dismiss as folklore, but weather prediction has always been a matter of life and death to sailors. In the days before storm advisories and instruments like barometers, the best they could do was to watch the sky—and they found they could learn a lot that way about the next day's weather. So while folk predictions for the upcoming growing season are no better than guesses, nautical weather lore usually has some basis in truth. The venerable “red sky” rhyme is one of these. It works because light scatters differently through haze and cloud droplets than it does through clean, dry air (see Mie scattering for the optical physics). It also assumes that storm systems travel from west to east, hence sunset and sunrise in the rhyme. How good is that assumption? It depends on your latitude.

    The trade winds these early sailors relied on make up what is known as the general circulation of the atmosphere: the global-scale, semi-permanent pattern in which all other weather moves and develops. It begins at the Equator—or more accurately at the thermal Equator, the intertropical convergence zone (ITCZ). I called the circulation “semi-permanent” before. This is because the ITCZ moves with the seasons, closer to us in the summer and farther away in the winter. You've heard that warm air rises and cold air sinks. The ITCZ is the line on the earth's surface where, thanks to the heat of direct tropical sunlight, the rising motion of warm air is stronger than it is anywhere else on the planet. Here it is now over Africa, courtesy of NOAA:

     When air rises at the ITCZ, it creates an area of low pressure that cooler air rushes in to fill. And the rising air eventually spreads out in the upper atmosphere and flows away from the ITCZ, eventually sinking back to the surface somewhere in the subtropics. This completes a circle known as the Hadley cell. The Northern Hemisphere and the Southern Hemisphere each have one. Since rising air is associated with precipitation and sinking air with dry weather, it's easy to pick out the vertical parts of the Hadley cell on a map of biomes: notice how many of the world's deserts come in bands near 30° N and S, and how tropical rainforest is always situated close to the Equator. Less easy to spot are the Ferrel cells, which cover the planet roughly between 30° and 60° N and S, and the self-explanatory polar cells. Still, the rising motion near 60° N on the boundary between the Ferrel and polar cells is responsible for the storminess of the North Atlantic and the seas around the Aleutian Islands, and its counterpart in the Southern Hemisphere is what made the Southern Ocean so dangerous to sailing ships.

 

    So far I've talked about the vertical and north-south components of the general circulation, but these do not explain the east-west trade winds unless you take the planet's rotation into account. Think of a parcel of air moving from the North Pole toward the Equator as part of the polar cell. It is close to the surface, but not of a piece with the solid Earth, and it does not have to rotate with the planet.

    Near the axis of the Earth's rotation, a point on the surface does not have to move very fast to cover 360° in a day; a point on the Equator moves the entire circumference of Earth in the same time. So it follows that as the air parcel moves straight south, it would also have to go faster and faster eastward to keep up with the rotation of the planet. Since it does not, its position relative to the surface moves westward, and the point on the surface feels the passage of the air parcel as an easterly (i.e. out of the east) wind. More generally, this is the Coriolis effect: movement on the rotating Earth is deflected to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. (It has nothing to do with the flow down a household drain, however; the effect is so weak that it is only visible at large scales).

   So applying the Coriolis effect to the north-south motion of the Hadley cell near the surface, we find that the wind should be deflected westward, resulting in trade winds that blow more-or-less diagonally westward and toward the Equator. In the Ferrel cell, the winds blow away from the Equator and eastward. In the polar cell, it's toward the Equator and westward again.

 

    Though no one put the pieces together until the eighteenth century, this is in fact the pattern of trade winds that sailors have observed for hundreds of years. The “red sky” rhyme is valid only in the Ferrel cells, sure, because that's where there are westerly prevailing winds. But the Ferrel cells are also where the vast majority of humans have lived throughout our history, and where most of our shipping has taken place.

Thanks again, Virginia!

Tables can too be fun...

In many ways, college was a simpler time. Every part of the day and week had its allotted activity. Thursday nights were for frantically completing problem sets, Saturday mornings were for sleeping, Sunday afternoons were for doing all the work that hadn’t been done over the weekend. In this accounting, Tuesday night was the time to flunk a chemistry exam - correspondingly, Wednesday morning was for balefully eyeing your Chem professor. In honor of it being Wednesday, I’m going to dip into general chemistry. Don’t worry! The exam will have a very generous curve.

General chemistry gets a somewhat undeserved bad rap. It is a science defined in part by its many comprehensive tables. Tables lack the friendly approachability of species distribution maps, the mysterious elegance of quantum mechanics formulae, or the romance of the double helix. To worse matters, chemical tables cram a lot of information into not a lot of space by using certain shorthands and abbreviations, which can appear nigh on indecipherable to the uninitiated. And on some level, there is no table more incomprehensible or misunderstood than the periodic table.

    Just look at that thing! Here’s a more interactive version (http://www.ptable.com). So, what exactly does all of this mean? Time for a quick review. The periodic table lists all known elements, in order of how many protons are in the nucleus of each atom. Hydrogen, as element 1, has 1 proton, osmium, as element 76, has 76 protons, etc. The number of protons in the nucleus is what makes an element. Let’s take a closer look at one of those little boxes on the table.

    Sodium (to be referred to afterwards as Na) has eleven protons in its nucleus, and is correspondingly element 11. Hence the 11 in the top left corner. In most elements, the nucleus has a corresponding number of protons, giving each atom an atomic mass of (roughly) twice the atomic number. Naturally, not every atom wants to follow this rule, and there are always atoms kicking around with more neutrons than protons, or less neutrons than protons. These slightly different configurations are referred to as isotopes. Typically, the most common isotope of a given atom has an equal number of protons and neutrons (there are, of course, exceptions to this rule). You can do all kinds of fun and wacky things based on different naturally occurring isotopes, but that is a topic for another day. The atomic mass listed for each element on the periodic table is actually an average - while most Na isotopes have an atomic mass of 22, a certain percentage have an atomic mass of 23 or 21. Hence, the average atomic mass isn’t quite 22. By the way, that mass is not given in any commonly used unit  - these are atoms we’re talking about here. “Small” doesn’t quite do it justice.

    What about the other numbers on that image? Well, the 883 and 98.0 refer to the boiling and melting points of Na, respectively, the 1.0 has to do with a quality called electronegativity (a topic for another day), and the .971 is a measure of density. By the way, since Na has a density of less than 1, it floats. Stay tuned next week for an explanation of exactly why you should not test that quality. Not every version of the periodic table includes these particular bits of information about an element, useful though they may be. The element number, the mass, and that weird Ne thing at the bottom, however, will always be included in some form or another.

    Now, what’s the deal with the [Ne]3s? Here’s where it gets fun. This whole time we’ve been talking about protons as if they were the real stars of the atom. In reality, protons are a bit like male lions. They’re big, they’re flashy, everything is identified in relation to them, they get the starring roles in Disney movies, etc. But in truth, they mostly just sit there. It’s the lionesses (or in this case, electrons) who do all the real work. With the notable exception of radioactivity, almost all chemical behavior fundamentally comes down to electrons. Atoms bond one to another and form molecules by sharing electrons in various ways. Everything is composed, at its most basic level, of various configurations of shared electrons.

    Electron behavior is where we start to get into quantum mechanics, so I’ll try to keep this simple. In an atom, you have the nucleus, and then surrounding it, you have the electrons. If you thought the nucleus was small, the electrons are almost unimaginably smaller, but they’re there, and they’re negatively charged. Commonly, there will be an equal number of protons and electrons, giving the atom a neutral charge. Now, the electrons aren’t just randomly zinging around the nucleus. Well, they are, but there’s an order to it. Think of a nesting doll. The doll in the middle is the nucleus. Each of the dolls nested around it represents an area called a valence shell that can hold a certain number of electrons. Elements low in the periodic table, like helium and lithium, only have one or two possible shells, while elements high in the periodic table, like cesium or radon, have many possible shells. Shells fill in order - it’s only possible for the outermost shell to have less than its full complement of electrons. 

      When an atom has anything other than a full outermost shell, it’s going to react, alone or with other atoms, in an attempt to fill that shell. An atom with only one or two atoms in a shell than can hold eight atoms is likely to shed its outermost electrons, leaving it with a full shell (remember, the second-outermost shell is, by definition, full), while an atom with an outermost shell only missing one or two atoms is likely to acquire the necessary extra electrons. This leaves us with an atom with fewer electrons than protons (positive charge), and an atom with more electrons than protons (negative charge). These two atoms are then likely to bond as a result of charge attraction - an ionic bond. Covalent bonds form when atoms quite literally share electrons - the outermost shell of each atom merges together to form a new, full shell. Pretty cool, huh?

    The one thing missing is a nice, simple way to write how many atoms are in the outermost shell. You can’t just write the number, because as more shells accumulate, the shape of the outermost shell, and the number of electrons it can hold, changes. There are a few ways you can write it out. You could write out all of the shells individually, but that takes up a lot of space. All of the shells theoretically possible are named by a letter-number combination. So, you can name the outermost shell, and then list the number of electrons contained. This might looks something like “3d7". Looks weird, but all it means is that the 3d shell is the outermost, and it contains seven electrons. The 3d shell can contain up to 10 electrons, so an atom with that electron configuration is going to bond in a way that gives it those 3 remaining electrons. Still, just writing down the last few shells to fill looks messy, and it’s possible to make a mistake. At this point, we cleverly take advantage of the so-called “Noble Gases”. This refers to a group of elements luck enough to exist with a full outermost shell. Elements from this group are notably unreactive because, well, they don’t need to. One such element is neon (Ne).

    And that is where the Ne in [Ne]3s comes from. Na has only one proton and one electron more than Ne. By writing down “Ne”, this shorthand uses the known electron configuration of Ne (full) to indicate that behind that 3s shell in Na is a set of full shells. Oh, and by the way, if there’s only 1 electron in a shell, we don’t bother with the superscript letter. That one, I’ll admit, could be clearer. As you go from left to right across each row in the table, the number of electrons steadily increases by one each time, and the shells fill in order. Based on this order, simply looking at the place of an element in the table already tells you quite a bit about how that element is going to behave. Far to the left? Mostly empty outermost shell, likely to lose those one or two electrons and become a positively charged atom (or cation). Far to the right? Mostly full outermost shell, likely to gain one or two electrons and become a negatively charged atom (or anion). As far right as possible? Nobel gas, generally unreactive. Right smack in the middle of the table? Half-full outermost shell, likely to wind up sharing electrons. At the top of a column? Small atomic mass. At the bottom? Large atomic mass. To be even more helpful, groups of elements that tend to form the same kinds of bonds are shown via color coding. Now, I’ll let you in on a little secret. The rows are called groups. Each group is marked by having the same number of electrons in its outermost shell, so all the elements in a group share certain characteristics (i.e., going straight down from helium, a Nobel gas, you encounter neon, argon, krypton, xenon and radon, the rest of the Nobel gases). And those lovely straight-across rows that tell you how the shells fill with electrons? Those are the eponymous periods.

Where did you get that banana?

Let me start with a joke. A traffic light can be considered the opposite of a banana. With a traffic light, green means go, yellow means wait, and red means stop. With a banana, green means wait, yellow means go, and red means - wait, where did you get that banana? This is funny because bananas aren’t red; some of you would argue that this isn’t funny at all, but that’s not the point. The point is that bananas aren’t red, right? Well...

A few weeks ago, I swore I’d stick with one botany post per week. This is a new week, and we’re going to delve into the wild world of the Musaceae. The Musaceae family is one of those big, tropical plant families that you don’t hear much about in the temperate climes, save a few overachieving members. In the case of this family, bananas and plantains are the overachievers. There are over one hundred Musaceae species, but the whole family gets called “banana”. Which isn’t really fair to the Manila hemp, now is it?

Still, if we’re going to judge an entire plant family by just a few of its constituent species, we should at least have a better appreciation for the sheer diversity of banana (and plantain) fruits. Readers from the tropics, a lot of this may be old hat to you, but we really don’t know much about bananas in the temperate regions. For most of us, our experience with these fruits (technically berries) is limited to one cultivar of banana, and maybe two cultivars of plantains. The banana you buy in the supermarket is almost guaranteed to be a Cavendish, the plantain is probably either a Bluggoe or a Pelipita. For the record, bananas are traditionally defined as sweet fruits from the Musa paradisiaca plant, while plantains are the starchier fruits of the Musa sapientum plant. Bananas can be eaten raw, plantains can be cooked. Of course, the distinction starts to break down, and there are other species of fruit banana out there, but that's the condensed version.

I could do this whole post on the story behind the dominance of those cultivars, but I’ll condense it to one paragraph. Others have written far more eloquently on the tale of the Cavendish banana, but let’s just say that the Cavendish banana grows quickly, is easy to ship, has a long shelf-life, and is resistant to a wide-spread banana disease. Prior to the 1950’s, the top banana was the Gros Michel cultivar, which was big, fast-growing, reasonably easy to ship, and really tasty. Unfortunately, it was not resistant to the aforementioned disease, the Panama fungus, and is now all-but-extinct. The Bluggoe and Pelipita plantain cultivars have a somewhat similar story - both grow quickly, ship well, and are resistant to Panama disease, as well as several other common banana diseases.

Over-exposure to just a handful of cultivars (and in the case of the Cavendish, not even particularly tasty cultivars) has left many of us blind to the incredible diversity of the banana family. Similarly, while plantains are increasingly familiar to Americans, many temperate regions still view the banana as a fruit crop, and not an overly important fruit crop - compare traditional American and European uses for the apple or the grape with uses for the banana. It’s different in the tropics, though. Bananas are a staple crop in many parts of the world, playing a role similar to that played by the potato in the Andes. In Uganda, for example, the average adult eats five pounds of bananas per day (and no, that does not include the skin). And, like most staple crops, there are varieties of bananas suited for different uses, growing cycles, and microclimates. Not to mention different taste preferences.

Not quite the local produce section, eh? There are bananas out there that are ripe when green, or orange, or maroon. There are striped bananas, and tiny bananas, and gigantic bananas. Bananas grown as staple crops in Ecuador are adopted to one microclimate and set of pests, while bananas grown as as staple crop in Ethiopia are adapted to completely different conditions.

                         

 All ripe! All of them!

    This diversity is the secret to the success of the banana, and the key to preserving its future. There are a lot of microbes out there with the potential to wreck havoc on commerical banana production (remember the fate of the Gros Michel?). Places like Africa and Oceania are hot spots of banana cultivar diversity, and by extension, genetic diversity. While we in cooler climes eat just a few genetically identical cultivars, the rest of the world is growing bananas that are resistant to just about every banana pest and pathogen out there. True, there is no one banana that is resistant to everything, but do we really need that? In a world full of pink and stripey bananas, do we really need to only eat one boring kind? We should all embrace the diversity of the banana family, and if we’re going to insist on calling an entire botanical group by the common name of a handful of species, let’s at least see what that handful of species really has to offer.