Looking Over Clover

To any person who has spent enough time sitting on a patch of lawn to get a little bored, the leaf in the picture below will be instantly familiar. 

One clover leaf.

It's a leaf of the white clover plant, and there is plenty of it at out outdoor classroom right now.  One clover leaf has three parts, which is reflected in the Latin name of the plant: Trifolium repens.  Tri- means three, and folium means leaf.  Repens means reclining, which this plant does well, as it spends its entire life within about 3 inches of the ground.  Clover leaves usually have faint white lines in them, and they are never heart-shaped.  Another common lawn plant called oxalis has leaves divided into three heart shapes, and sometimes people get confused about it.

A patch of clover.  See any lucky ones?

White clover has several claims to fame.  First, they are tough little plants, and they survive well on lawns even under heavy foot traffic, so they grow everywhere there is a lawn.  People who are sticklers for uniform-looking lawns consider the plant a weed, but many people value the plant for its ability to grow in harsh conditions where grass can't grow.  Second, clovers are known and loved for their sweet-smelling flowers, which are white or pinkish clusters on stems.  I bet you have made a flower chain from clovers before.  Bees love the flowers even more than humans do, and they make great honey from it.  The sight of clover flowers on a lawn should serve as a warning to wear shoes, since bees are likely to be on the flowers, and stepping on a bee will get you stung.  Clover's third well-known benefit is its value as a food source for animals.  Clover seed is often included in the mix of seeds farmers plant for growing cattle forage (the plants cattle eat).

A small clover plant with leaves, stems and roots.

Clover also has a secret.  Most people don't know much about the hidden power that makes clover so important in nature and explains some of its better-known characteristics.  If you dig up a small clover plant and look at the roots, you see something that is not usually present on plant roots: tiny lumps.  Those lumps are called nodules, and they are actually little areas where clover keeps its own pet bacteria.  The clover provides food and housing (and maybe even affection) to the bacteria in return for the services the bacteria provides to the clover.  The bacteria produce an otherwise almost unobtainable nutrient called nitrogen.  Nitrogen is a nutrient used to make protein - the microscopic parts of organisms that provide much of their actual structure as well as much of the machinery to conduct life's processes.  Other plants can only get nitrogen by absorbing leftover nitrogen from dead clover-type plants or from decomposing animals or animal manure, but clover has its own constant supply.

Root nodules on a clover - where nitrogen is fixed.

Clover's source of nitrogen means it can grow on poor soil where grass can't.  It also means it has a high nitrogen content, making it more nutritious than grass for animals to eat.  When clovers die, they leave behind richer soil with more nitrogen, where other plants can now grow.

Clover is related to bean-type plants (pinto beans, Limas, black beans, lentils, peas), and all types of bean plants have the same pet bacteria for making nitrogen (actually called fixing nitrogen).  This type of interaction between two organisms that live in close contact and help each other is called mutualism.   Can you think of other examples of mutualisms?

Have you ever found a four-leaf clover?  Sometimes the plant makes an error when growing its leaves, resulting in our four-leaf symbol of good luck.  If you look at a patch of clover for long enough, you will probably find a four-leaf clover.  If you do, press it flat between pages of a book for a week or so, then you can glue it to paper or press it between clear tape to preserve it.

Weeds: The Superhero Gang of the Plant World

The lawn in our outdoor classroom is lush, thick and inviting.  It looks like a perfect sea of even, green grass.  Just look at it!  If you stop and really look, though, you'll start to see weeds.  They are stealthy and hidden, but they are everywhere!

A clover plant thriving in a nutrient-depleted patch.

Weeds, by definition, are plants that humans consider to be growing in the wrong place.  They annoy us in our lawns, we spend time removing them from our gardens, and when they grow amongst our crop plants, they reduce the amount of food that is produced, so they cost us food, time and money.

Still, I rather admire weeds.  If you look at them from the plants' perspective, weeds are the ones that manage to survive even after people have done everything they can to get rid of them.  To make our outdoor classroom, humans removed all the vegetation and reseeded with very thick grass to completely out-compete the weeds for sunlight and nutrients, but the weeds found a way.

Spring cress, false-strawberry and a dandelion battling their way into our lawn.

Weeds usually have some unique 'special power' (well, growing ability) that lets them grow in hostile habitats.  Some weeds, like the spring cress in the picture above, can grow when it's too cold for other plants, so they take off while the grass pauses for winter (plus they have exploding seed pods!).  Clover's super power is to produce a nutrient called nitrogen that other plants can't make, so it can grow in nutrient-depleted habitats.  Dandelions, are shape-shifters: generalists that can adapt to just about any condition (plus their seeds fly on the wind).   The spurge's power (seen below) is speed: the ability to grow and make seeds so fast they can live their lives before people notice them and kill them.

A spurge weed with milky sap - tear the stems and notice it oozes a white liquid.

Some conditions are too harsh even for weeds.  Notice the worn pathways in the grass where students walk.  There don't seem to be any grass plants or weed plants there.  Now we just need to find a weed whose special powers are to grow despite dozens of people walking on them every day!

Another reason I admire weeds is that they provide variety to the types of habitats available for other organisms.  The more types of plants that grow in an area, then the more types of insects and birds and mammals and other species you can have.  Variety of types of living organisms is called biodiversity.  A pure, uniform lawn is like a desert in terms of biodiversity, because it only has one type of organism.  Weeds increase the biodiversity of our outdoor classroom.

Do you think weeds are more likely to be found in the middle of the lawn or at the edges of it?  You can experiment to find the answer.  Use a small hula hoop as your measuring device.  Throw the hula hoop randomly onto the grass in the center of the lawn and count how many weeds are present in the circle.  Then randomly toss the hoop on the grass at the edge and count weeds again.  Do this a couple more times, and you should have your answer.  Now you just have to figure out an explanation for why you think weeds prefer one habitat over the other.

Parthenogenesis and the Dandelion

If you've been following along, you know that flowers are the sex organs of plants, specifically adapted to combine half of the DNA of a male plant (in pollen) with half the DNA of a female plant (in ovules) to produce a new plant, packaged in layers of a seed and wrapped in a fruit.  Sexual reproduction allows for each parent to pass on some of his/her genes to make an offspring that is combined from those genes.  It's why we all can see a little of each of our parents when we look in the mirror, and plants would too, if they used mirrors.  Unless they are dandelions.

Dandelions and Lake Michigan

Common dandelions, belonging to the species Taraxacum officinale, mostly do not reproduce sexually, though a few dandelions in Southern and Central Europe do.  The dandelions in North America are all clones of a few original European dandelions.  The method of cloning, or asexual reproduction, used by dandelions is called parthenogenesis.  In parthenogenesis, offspring are produced that look like normal offspring - starting out as embryos and growing to adults (versus asexual reproduction by fragmentation where the adult parent breaks in two, and viola, you have two new organisms).  For plants, parthenogenesis usually means producing ovules that have a complete copy of the mom plant's DNA, called apomixis.  The apomictically-produced ovules develop into seeds that are genetically identical to the mom plant, and there is no pollination involved.  That's female liberation (though there is a plant that does male apomixis!).

Dandelions decorating a lawn.

It takes a while to get used to the idea that plants have sex, but once you think about it, it makes perfect sense.  Sexual reproduction allows for continuity of traits being passed on from generation to generation along with genetic variation to survive in diverse habitats.  After you're used to 'normal' plant reproduction, parthenogenesis seems positively bizarre. Why would a plant want to give up on genetic diversity? Actually, lots of plants reproduce asexually by parthenogenesis (blackberries, onions, grasses and more), and there are some specific advantages.

The main advantage to asexual reproduction is the ability to capitalize on a successful genotype.  If humans could do this, we might have hundreds of little Steve Jobs growing up to make our future world a better place.  Instead, he left us a few offspring, but they each only have half his genes - and they may or may not have gotten the good ones.  For dandelions, their successful growth strategy can be reproduced in perpetuity because they are identical copies, with only random mutations providing genetic diversity.  Dandelions may have less need for genetic diversity, since the DNA they have allows them to grow differently depending on their growth conditions.  This is what makes them such successful and widespread weeds.

One last issue for today:  dandelions grow without being pollinated, but most dandelions do produce pollen (Did your childhood friends used to rub dandelions on your face?  Remember the yellow dust they left behind?).  Botanists scratch their heads a bit on this issue.  Dandelions that don't produce pollen can make more seeds, since they are not wasting their energy.  It may just be that because pollen production is genetically determined and dandelions don't evolve very quickly, the pollen production may just be an evolutionary leftover.  Also, there are some sexually-reproducing dandelions in the world, so if dandelions maintain the ability to pollinate potential dandelion mates, they could just happen upon a better combination of genes than the ones they already have.  I don't know that dandelions really need to become more successful at growing...though I love them, we have enough already!

I C4...there I am!

Sorry for the bad pun!

It's the hottest and sunniest time of the year.  That means the C4 plants are about to lap the C3 plants in the race on the farm.  C4 plants are ones that have evolved an extra step to their photosynthesis process that makes them more efficient than other plants, especially during the heat of summer.  Most plants are C3 plants, but crabgrass, sugar cane, corn, sorghum and many other plants are C4 plants.  C4 plants comprise only about 3% of flowering plants, but they do 25% of the photosynthesis that occurs on land*.  Unfortunately, the only C4 plants on the farm are weeds, and the worst of the worst is crabgrass!!!!!

My nemesis.  Notice the star shape of the first few crabgrass stems.

Now brace yourself.  We're going to have to wade into some technicalities of photosynthesis.  Trust me...it's way cool, as the young kids say.  It is imperative that you understand the tricks that C4 plants can do, because if you ever have to spend an entire day weeding crabgrass, you'll want something to think about.

First though, C3.  Think back to 9th grade.  Remember photosynthesis?  It's that thing that plants do to make their food.  The idea is that plants capture the energy that the sun is giving off, and then they use that energy to knit carbons, hydrogens and oxygens together to make sugar.  As we all know, sugar is a basic food.  In plants, it stores the sun's energy in chemical form until the plant needs it.  Sugar is also the basic molecular building block for a lot of the structural molecules plants use to build themselves.  In fact, all the food on our entire planet originated from photosynthesis (even meat because cows get their energy from grass).  Also, all the oxygen in the air on Earth was made as a byproduct of photosynthesis.  So it's kind of important.

Now to the name C3.  The C refers to carbon, which is the most important atom involved in photosynthesis.  Plants collect carbons form the air in the form of carbon dioxide, which they use to make sugar.  The 3 refers to the size of the molecule that the plant uses to 'trap' the carbon dioxide.  In C3 plants, when a carbon dioxide molecule (which has one carbon atom) is caught by the plant, it joins with the plant's carbons to make a three-carbon molecule (called 3-phosphoglyceric acid, or PGA).  Eventually, when a plant has collected six carbon dioxide molecules, it has enough carbons to make one sugar molecule.  In a single teaspoon of plant sugar, there are approximately 1.7 x 10^22 molecules of sugar, so plants do a LOT of photosynthesis.

But, you can't understand the C4s' advantage until you understand the last piece of this photosynthesis puzzle: RUBISCO.  I'm not just typing in all caps because I'm excited about RUBISCO; it's also an acronym.  RUBISCO stands for ribulose 1,5 bisphosphate carboxylase oxygenase, which is a handy phrase to work into almost any conversation.  RUBISCO is the molecular machine, or enzyme, that allows the plant to 'grab' CO2 out of the air and bond it to make the three carbon molecule.  The carboxylase part of the name refers to its ability to bond carbon.  Unfortunately for plants, RUBISCO can also bond oxygen (that's the oxygenase part of its name).  It's unusual for enzymes to be able to bond two different substrates - usually they have one job only.  For RUBISCO, it's a problem to be able to bond two things, because every time the RUBISCO bonds to oxygen instead of carbon dioxide, it costs the plant energy instead of gaining energy like photosynthesis is supposed to do.  In the summer, when plants are photosynthesizing fastest (and producing oxygen, remember), there is even more oxygen around the plant.  So plants are not able to photosynthesize to their full potential because their RUBISCO is wasting more time with oxygen.  Scientists think the RUBISCO problem exists because RUBISCO evolved in plants' ancestors before there was any oxygen in the air to worry about.

It all used to look like the right side before I pulled out the crabgrass.

C4 plants have evolved more recently, and they have solved the RUBISCO paradox.  As you can see in the pictures above and below, the C4 crabgrass is kicking the butts of the C3 beets growing in identical conditions. 

Unweeded beets on the left, free beets on the right.

 Crabgrass, like all other C4 plants, bonds carbon dioxide to make a....drum roll please.....FOUR carbon molecule!!!  Crabgrass has a stand-in molecule that doesn't use RUBISCO to trap carbon dioxide from the air.  This C4 molecule exists near the surfaces of the plants that are exposed to air, where carbon dioxide (and oxygen) is.  It can ONLY trap carbon dioxide and not oxygen.  Once it traps a carbon dioxide, the four-carbon molecule, called oxaloacetic acid, travels deep into the plant away from air.  All the plant's RUBISCO is stored near the center of the plant's leaves and stems, and the oxaloacetic acid drops off the trapped carbon to RUBISCO, where it goes through the rest of photosynthesis like normal.  So you have all of the photosynthesis with none of the waste from exposure to oxygen.  And thus the crabgrass flourishes in the middle of summer when photosynthesis is happening at its fastest.

* Most of the photosynthesis that occurs on earth is done by algae in the ocean.

In Which I Attempt to Fascinate You with a Minor Plant Pigment

The Amaranthaceae, or Amaranth Family, is a somewhat obscure plant family, but you'd never know it on the farm right now.  There are amaranth crops, escaped amaranth hybrids from last year, and native amaranth weeds taking over the farm this time of summer.  It's a colorful explosion of dramatic bloomers with other meek yet ubiquitous volunteers growing between the rows of actual crops.

The hoop house, a sort of open-sided green house, is bursting with ornamental cock's combs right now.  Cock's combs are in the genus Celosia, and they come in amazing colors: eye-searing red, blazing peach with yellow, glowing whitish-green and orange.  They are such shockingly bright colors due to the fact that they are fluorescent.  Fluorescent colors absorb light energy from outside the visible spectrum (like UV light) and then emit that UV light as visible light...so they do actually glow.  Fluorescence is most noticeable when visible lights are turned off and black lights are shined, but cock's combs are so fluorescent that you notice them in full sunlight.

Hoop house full of Celosias.

A Celosia close-up.  It's even brighter in real life.

Members of the Amaranthaceae and a few other closely-related plant families can fluoresce because they have an unusual class of plant pigments.  Most plants that have red parts use a plant pigment called anthocyanin.  Think maple leaves in the fall and apple skins.  The amaranths use a group of pigments called betalains for all their red and most of their yellow coloration.  Betalains are antioxidants, so they may have anti-cancer properties.  Betalains are also useful as dyes for food and cloth, but I doubt they are what make highlighter pens fluoresce. 

The fluorescence of cock's combs is useful for the plant - it attracts pollinators.  In the hoop house, the peach cock's combs were the hands-down favorite of bees and wasps.  Each plant was swarmed with pollinators large and small.  The peach sector of the hoop house was buzzing, audibly as well as visually, with insect activity.  Good thing I got over my giant ground hornet fear in the previous post. 

Peach Celosia, source.

Amaranths in the U.S. are herbs, though there are some tropical shrubs.  They have tiny flowers, usually clustered all together.  The flower parts are so tiny, they are best seen with a hand lens or dissecting microscope.  Other ornamental amaranths include Gomphrena and Iresine.  Edible amaranth, genus Amaranthus, is used as a grain.  It is an important high-protein cereal native to South America.  Weedy pigweeds, in the same genus as edible amaranth, are found here in the U. S., and though their seeds and leaves are edible, it is much more of a nuisance than a valued crop. 

On the farm, half of our interactions with amaranths involve planting and harvesting the Celosias and Gomphrenas and the other half are killing the pigweeds, spiny amaranths and escapees from last year's crops.  The escaped plants from last year are seeds that have fallen and overwintered in the soil.  They are usually crosses between different types of Celosias, so they have a blend of their parents' traits.  That means they might have unpredictable colors, small flower heads and irregular growth forms.  They don't usually make good cut flowers, and they have to be treated as regular weeds. 

An escaped and hybridized Celosia from last year's crop growing among the zinnias.

And here is your reward (or punishment, depending on your sensibilities) for reading to the end of the post:  Beets are in a closely-related plant family, the Chenopodiaceae, and their red pigment is also a type of betalain.  I have never really noticed fluorescence in beets, but I haven't tried them with a black light.  If you eat a lot of beets, you may have noticed one of the disconcerting properties of betalain.  Betalain is not readily digested by humans, and it either passes straight through the digestive tract or is absorbed into the blood and eventually filtered into the urine.  Either way, the betalains end up in the toilet bowl, the same color as when they were swallowed.

Vegetarian Vampires

There is a particularly interesting killer on the farm. It creeps silently toward its prey, searching victims by their shadows and their smell. It kills its victims gradually, sometimes over months, slowly drinking their fluids until there are none left and the dessicated victims expire from exhaustion.

Terrified? Don’t be. These predators are vegetarians – vegans even. They are actually parasitic plants called dodder, in the genus Cuscuta. picture They are the ghostly bleached cousins of morning glories. Dodders comprise approximately 150 closely-related species of flowering vines with no green chlorophyll. Their appearance reminds me of a childhood poem I once loved about spaghetti trees, because they drape over their victims with vast quantities of whitish-yellow strands.

Dodder produces flowers, fruits and seeds like any normal plant. Their flowers are small white clusters that develop in to fruits with multiple seeds. The seeds drop in groups onto the soil to germinate up to 5 years later. Even as the seeds germinate, the plant seems normal. The seedlings grow using the energy stored inside the seed by the mother plant. Since seedlings of most plant species grow from stored energy, it is common for seedlings to be white or yellow like those of dodder. Most plants quickly grow leaves that fill up with chlorophyll, and they start making their own food before they run out of what’s stored in the seed. But here is where dodder is unique.

If dodder seedlings don’t find a victim within 10 days of germination, they will starve. They are somewhat picky eaters, as dodder won’t parasitize grasses or corn. Fortunately, they have evolved the ability to find their preferred foods. The seeds germinate faster when a preferred host is near. They grow toward host stems by growing toward shade (most plants grow toward light), and by curling when they touch something. The most amazing feature of dodder is that it can find its hosts by smell. Dodder can detect and grow toward the volatile compounds produced by its food. Ever smell a tomato plant? Dodder has too!

When dodder finds a host, it punctures the stem and grows into the food veins, or phloem, of the plant. It absorbs water, sugar, minerals and everything else it needs straight from the plant. It discards its own stem and root connecting it to the ground and goes completely airborne as it quickly covers its host plant with spaghetti stems.

Dodders are annuals, meaning they die back each year and regrow from seeds. Some, however, have figured out another way to survive the winter. If they colonize a host plant that survives the winter, the dodder growing inside the host stem can survive and resprout the next spring.

The best parasites don’t usually kill their hosts, because then they would be out of a food source. This is usually true with dodder too. It may kill some of the plants it is in contact with, but it usually maintains at least one live host. It certainly reduces host plant size, which is a problem for farmers when the host plant is a crop plant. Farmers don’t like dodder, and they often have to work hard to get rid of it. On the farm where I work, we hand-pull the dodder and throw it onto mowed grass, where it can’t regrow. Desperate farms can switch to non-host plants for a few years in particularly infested fields.

As much as I know I should hate the plant, I can’t quite. It’s just so darn strange. There are very few truly parasitic plants, and this is one. People always think Venus Fly Traps are the consumers of the plant world, since their little traps look so much like mouths, and they move (!), but sadly, they are not consumers. They are green, indicating chlorophyll, and they get all their energy from the sunlight. They do, however, absorb nitrogen, which is like a plant vitamin, from the flies they trap. Dodder, though, has discarded its ability to photosynthesize. The loss of a trait is fascinating from an evolutionary perspective. Organisms evolve new traits all the time, but they don’t usually lose them. Of course, dodder probably still has most of the genes necessary for photosynthesis, because it is so closely related to and descended from photosynthesizing plants. But there is really no need to spend the energy to maintain those genes. The dodder plants that spent more energy parasitizing host plants and less energy maintaining their ability to photosynthesize found the successful strategy.

Are their any plants that are parasitic on animals? No. But the laws of natural selection say that will be is variation. Doubtless there is some dodder plant out there that might just maybe be able to puncture human flesh and collect its energy from me. So you’ll find me moving just a little bit faster past the dodder plants!

Weedy Strategies

The onions have weeds. LOTS of weeds. As we weeded for a couple of hours, I contemplated the varying successful strategies of the primary weeds we pulled. Weeds are any organism humans have deemed annoying, and we are annoyed by lots of organisms! Usually weeds compete with humans for food. For example, Monks and Schultheis (1) demonstrated that for every week crabgrass was allowed to grow alongside watermelons, the watermelon yield per hectare (2) was reduced by 716 watermelons (that's almost 4000 kg, or 8800 pounds, equivalent 2 average American cars)! So weeds GREATLY decrease productivity.

Why do weeds decrease productivity? Well, they are the greedy kids that grab the most M&M's at snack time. They are better at using available resources quickly. Resources for plants include sunlight, water and soil nutrients. Weeds out-compete crop plants for some or all of those resources. Crop plants are usually somewhat wimpy competitors. Plant breeders have selected for tender, juicy fruits or pretty flours or large seeds, which often come at the expense of defensive or offensive plant growth strategies.

Our hours of weeding enabled us to become intimately familiar with three weeds: crab grass (3), yellow wood sorrel (4) and spiny amaranth (5). Oddly enough, each of these weeds has a different strategy for success as a weed.

Most of us know crab grass, even if we don't realize it. Crab grass is a vicious, ubiquitous, nonnative weed found throughout most of North America. It is a weed of agriculture, lawns, sidewalks, and vacant lots. Crab grass's strategy is to dominate territory throughout the summer. It puts as much energy into roots as it does leaves as it develops its death grip on the soil. It is an annual that leaves holes in lawns when it dies back in the Fall after it has deprived all neighboring plants from all necessary resources. Removal of crabgrass requires tough hands and strong muscles and also a strong tolerance of soil disturbance by neighboring plants as the crabgrass clings to the soil even as it leaves the earth.

Wood sorrel is ironically the sweetest of the three weeds to pull. While it would taste sour due to its oxalic acid content, it comes out of the ground as smooth as honey - a great relief to a weeder who is fed up with crab grass. Wood sorrel grows quickly, and it puts little energy into maintaining its turf. It just needs a little patch of soil to get in, grow some seeds and get out. It is done with its life cycle early in the summer.

Spiny amaranth blends strategies of the other two plants and adds its own twist to the mix. It grows quickly but also holds on tightly to the soil. For a little flourish, it grows sharp spines all along it's stems discouraging predators and pullers.

From an evolutionary perspective, each of these plants has found a successful strategy. Their growth patterns yielded plants that could use available resources to produce offspring with similar characteristics to the parent plants. Each survives in a similar habitat with a different answer the the ultimate biological question, "How do I survive and pass on my DNA?". After yesterday, at least a few of those weeds lost the evolutionary battle to their wimpier onion competitors. Onions, though, have their own secret weapon. They have managed to be useful and appealing enough to humans that humans are willing to fight off their competitors for them!



(1)

David W. Monks and Jonathan R. Schultheis, 1998. Critical Weed-Free Period for Large Crabgrass (Digitaria sanguinalis) in Transplanted Watermelon (Citrullus lanatus)

Weed Science. Vol. 46, No. 5 (Sep. - Oct., 1998), pp. 530-532
(2) A hectare is about 2.5 acres, which would take about 5 hours to mow with a push mower. No breaks.
(3) http://plants.usda.gov/java/profile?symbol=DISA
(4) http://plants.usda.gov/java/profile?symbol=OXST
(5) http://plants.usda.gov/java/profile?symbol=AMSP