Ferns Unfurling into the Future

Since spring is the time of transition, this blog is transitioning too.  It is time for my final outdoor classroom blog post.  Next time, I will return to my original "Biological Thinking" blog of discovering biological phenomena in everyday encounters with nature.  I hope you stay tuned if you have enjoyed my site-specific posts.  Thank you for reading!

A Fern Leaf

Our final topic: ferns.  Ferns are so common in the South that it is easy to overlook their strange beauty.  In the summer, many people display ferns in baskets outside their homes or apartments.  If you walk through the woods, ferns contribute to much of the background green of the understory.  Their repetitive intricate form makes them ideal for adding greenery to bouquets.  They are so familiar, yet they are so unusual. 

Fern spore-producing structures called sporangia.

Most plants we know of produce flowers and seeds to grow their offspring.  Seeds are tiny plants with a starter food supply wrapped in a protective coating.  They are produced from flowers.  When a seed grows, the tiny plant in it gets larger until it is a hackberry tree or a clover plant or, well, you get the idea.  Ferns, however, do not make flowers or seeds.  Fern plants make spores.  Spores are microscopic bits that are so light they can blow for miles on the wind.  When they land, if conditions are moist, they can grow into a small green leaf-like flat structure called a gametophyte.  The gametophyte looks nothing like a fern - just a tiny green patch.  The gametophyte grows for a while on its own, then it produces structures that can grow into a full-grown fern.

It's difficult to find gametophytes, but its easy to find spores.  Just look for dots on the bottom sides of fern leaves - those are pockets filled with microscopic spring-loaded spore-launchers called sporangia.  If you could shrink yourself, you could hang out under a fern leaf just for the fun of watching the tiny sporangia catapult their spores.  Nature's microscopic fireworks!

How leaves of flowering plants open up.

Another difference between ferns and flowering plants is how their new leaves open up.  The above plant, a flowering plant, has new leaves that are enlarging and opening by unfolding.  The young leaves are small and creased, the mature leaves are open and flat.  Below you see a fern leaf opening up.  Young fern leaves are small and curled up.  When they grow, they unfurl or unroll.  Young fern leaves look like the tops of violins (also called fiddles), so young fern leaves are called fiddleheads.  Fiddleheads might be green or brown or hairy or look quite different than the leaves they mature into.  It's easy to find fiddle heads in the spring and early summer - just look around the base of fern leaves, which we have in abundance all over our outdoor classroom.

Fern fiddlehead with a spore-producing leaf behind it.

There is a term for the unfurling of fiddleheads, of course, since there is a term for everything in science.  It is perhaps my favorite scientific term: circinate vernation (pronounced SIR-sun-ate ver-NAY-shun).  Look at the first word: circinate.  What other word starts with circ-?  Circle!  Circinate means circling or spiraling.  The second word might be tougher to figure out.  It derives from the word 'vernal', which means spring.  So circinate vernation is literally the unfurling of the spring, which is what ferns do.

More fern fiddleheads.

Ferns are ancient plants on earth.  Well, the plants in our outdoor classroom plant beds aren't ancient - they are the same age as their flowering neighbor herbs.  What I mean is that ferns were present on earth before flowering plants evolved.  They were some of the earliest plants in the fossil record, and they used to be the dominant type of plant on earth.  Now ferns are all fairly small plants, but before there were trees like we have now, ferns could be any size up to as large as small trees.  Ferns spores are a limitation that requires moist habitat, so ferns can't grow everywhere.  When some plants evolved the ability to form flowers and seeds, they could live in many types of habitats, so the flowering plants out-competed the ferns and became the dominant types of plants on earth. 

This fern leaf is almost completely unfurled.

All plants, including ferns, use photosynthesis to capture the sun's energy and make food.  Like flowering plants, ferns have xylem and phloem vessels for transporting food and water.  There are even more simple and ancient plants still easily found, even in our outdoor classroom: the mosses.  Take a look at the plants covering the rocks behind with waterfall.  They are mostly mosses, which don't have flowers, seeds, xylem or phloem.  They still photosynthesize, and they make spores like ferns do.

Moss - an even simpler plant than ferns.

If you want a more ancient plant than moss, you have to look into the pond at the algae.  Which brings us back to the very first topic we started with in August! 

I will leave you with one last picture of a fiddlehead getting ready to open up and live its life in the world.  It's so full of possibility!

A young fiddlehead.

Hello! I'm Veronica. I'll Be Your Nectar Guide Today

The plant below is called Veronica.  That's its scientific name and its common name.  In our outdoor classroom, it's located on the rocks behind the pond, and it's blooming like crazy right now.  Veronica flowers have something really neat called nectar guides. 

Nectar Guides on Veronica Flowers

Nectar guides help bees find flowers.  They point to the part of the flower that contains the nectar, which is what bees are looking for.  Nectar is sugary plant sap found in the base of many flowers, and it is perfect bee food.  I like to think of nectar guides working a lot like the stripes and lights on airport runways telling airplane pilots where to land their planes.

Why would flowers advertise where their nectar is?  It turns out flowers are offering the nectar in a bargain.  Do you see those tiny white structures poking out of the flowers in the picture above?  Those are anthers, and they contain a dust called pollen.  Flowers must have pollen moved from one flower to another in order to be able to grow seeds to grow a new generation.  While a bee sips the nectar in a flower, those anthers are in the perfect place to dab some powdery pollen onto the bee.  Then the bee rubs off the pollen at the next flower it goes to.  The flowers are giving the bees a meal in return for moving their pollen from one place to another.  (Do you remember the name for a close relationship between two organisms where both organisms benefit?  The answer is at the bottom of this page.)

Bee-pollinated flowers usually have nectar guides.  Flowers that are small, fragrant and with a shallow cup for nectar are usually bee-pollinated, and we have lots of bee-pollinated plants at our outdoor classroom to discover.  You may not always notice nectar guides on bee flowers because sometimes the guides are invisible to human eyes.  Strangely enough, there are more colors of light than humans can see.  Rainbows actually have more stripes than humans see, in colors we haven't imagined.  Bees can probably see one more stripe on the rainbow than we can.  We call this color ultraviolet, and know it exists because we can detect ultraviolet with machines.  But how do we know bees can see ultraviolet?  Because scientists have given bees eye tests!  Scientists tested bees' eyesight by making fake flowers.  On some flowers, they painted nectar guides with an ultraviolet dye.  When you offer fake flowers to bees, the bees are way more curious about the ones with the ultraviolet nectar guides than the ones without nectar guides.  Scientists have detected ultraviolet nectar guides on real flowers (sunflowers have ultraviolet nectar guides).

Go back to the picture above and see if you can find the nectar thief.  A nectar thief is something that steals nectar without moving pollen - a parasite!  The nectar thief on our Veronica flowers is an ant toward the left of the picture.  As we learned last week, ants eat sugar, so it's no surprise that they would like flower nectar.  The ants are too small for the anthers to dust them with pollen, so they slip in, drink the nectar and slip out again without helping the plant.  Ants mostly use smell and taste to find their way in the world, so they probably don't even notice the nectar guides.  If you are jealous of bees for seeing one more color than we do, then you're really going to be mad at ants.  Scientists think ants can smell and taste an enormous number of things humans can't, with much more precision.  If you don't believe me, try closing your eyes and finding your way around using just your nose and tongue, and you will appreciate just how much better ants' senses are.

Answer: Mutualism

Down on the Aphid Ranch

Originally this post was just going to be about vines.  There are four basic plant forms: trees, shrubs (small, branched trees), herbs (non-woody, soft plants), and vines.  Vines are woody but not strong, and they grow as tall as trees by climbing up other structures, often trees, and they usually have roots that are good for holding on, like this:

English ivy with roots that are good for holding on to buildings or trees.

 Some vines are evergreen like the English ivy on the back wall of our classroom:

English ivy doesn't lose its leaves in winter.

And some are deciduous like the milkweed vine on the lamppost by the playground:

Milkweed vines lose their leaves in winter.

Like I said, I was originally going to post about vines.  BUT when I was looking through my vine pictures, I noticed a very lucky but accidental detail on this picture of the English ivy:

New growth on the English ivy with some curious dots on the stem.

Look very closely at the stem, and you will notice there are ants walking on that stem:

Ants going up and down the ivy stem.

I started to wonder what ants were doing walking on an ivy stem, since there is not likely to be much ant food at the top of an ivy vine.  Then I looked even closer and I saw this:

An farmer ant and her aphids.

Now we have a story!!!  The picture is a little blurry, which means you're going to have to go out to the outdoor classroom and see this for yourselves.  The dark spots are insects called aphids (which can also be whitish or greenish), and the reddish brown spot is an ant.  What the ant is doing is called aphid farming.  It's a bit gross, but it's so amazing that it's completely worth learning about.

To explain aphid farming, we have to go back to plant sap.  Remember plants do photosynthesis and make sugars, which are dissolved in plant sap, making plant sap slightly sweet?  Aphids have pointy, needle-shaped mouthparts they poke into soft plant tissues, and they suck the plant sap out of plants for their own food - much like the psyllids we learned about back in the fall.  Since aphids live closely with plants, they are said to be in a relationship called a symbiosis.  In this relationship, the aphids are harmful to the plants because they 'sap' their energy.  The aphids benefit by getting food.  A symbiosis where one organism benefits and the other is harmed is called parasitism. 

Here's where it gets slightly gross.  Aphids drink a lot of plant sap, and they digest most of the sugar in it, but not all.  The leftover sap with a tiny bit of sugar in it goes on through and out the other end of the aphids' digestive system.  In all other organisms, this substance would be called feces or poo, but in aphids, the substance is a clear and sugary liquid, so it's called honeydew.  (Do NOT confuse this kind of honeydew with the delicious green melon you find in the produce section.)  If you have ever noticed sticky, clear spots on the hood and windshield of your car if you park it under a tree in summer, you have seen the results of the mist of honeydew that rains from the aphids in the tree.  Take a deep breath...it's really only plant sap run through an aphid!

OK, here's where it gets really gross, but also really, amazingly neat.  The sugars in the honeydew are technically a food source (like any other sugar), and ants eat sugar.  Put those two facts together, and you know what that ant is doing with the aphids on the leaf in the picture above.  Yes, some ants eat honeydew.  The ants know a good food source when they find it, so they protect the aphids and fight off aphid predators.  They even move the aphids around to better sap sources if their honeydew production slows down.  Some ants even keep aphid eggs in their ant nests in the ground during winter and place them on new plant growth in the spring so the aphid eggs have food when they hatch.  The ants are said to farm the aphids - just like humans farm cattle!  Dairy cattle ranchers protect the cows and move them around to new pastures with more food.  Cattle farmers also protect calves and raise them to adults.  And cattle are farmed to provide a liquid food source: milk! 

It's amazing to me that humans are not the only type of organism that does farming.  Ants live in groups and have very complex behaviors, as exhibited by the aphid farming.  Their behaviors often mimic human behaviors: they have 'jobs', fight battles, farm aphids (and fungi - neat story for another time), build large complicated structures and much more.

Ants and aphids are in a symbiosis - they live very closely together.  Their symbiosis involves both organisms benefiting from each other.  The aphids get protection, and the ants get food.  A symbiosis where both organisms benefit is called a mutualism.  Humans and cattle are in a similar mutualism.

The new growth on our ivy is very likely to have a constant supply of aphids and ants, so you should be able to find them any time!

The Hackberry: A Lesson in Bark

Do you remember looking at the hackberry tree last fall?  It's the big tree in our outdoor classroom by the entrance to the parking lot.  The hackberry is the tree whose leaves had hackberry leaf galls.  I promised I'd come back to the hackberry because of its unique bark.

The stem (ok, trunk) of our hackberry tree with its great hackberry bark.

The hackberry is generally an unnoticed tree in most parts of the U.S., but here in Nashville, it's so common it could be our city's official tree.  Hackberries have a huge influence on our lives here.  They provide wonderful shade for us in the summer and help keep our city air cooler and cleaner.  Hackberries can grow big and strong in the toughest of urban conditions as long as there is enough water.  They also are among the most common wild tree outside the city.  Hackberries provide lots of wildlife habitat and food.  Hackberries often have hollow portions which provide excellent homes for birds and small mammals.  The trees' berries, which are technically edible to humans, are a good source of winter food for birds.  You may have noticed when the late winter robins and starlings arrived a couple weeks ago that there were lots of purple bird bombs (bird droppings) on cars and sidewalks.  Most of those were the result of birds eating hackberries.  Nashville has felt the downside of hackberries too.  Hackberries can be a bit brittle, especially if they are hollow inside.  Tornadoes and great storms sometimes break off hackberry limbs, which can land on power lines or houses. 

Close-up of the edge of a ridge in hackberry bark showing the layers unique to hackberry bark.

To really appreciate the hackberry, you have to get up close and personal to it.  Specifically, look at its bark.  The bark of hackberries varies widely from almost completely smooth to almost completely bumpy, but there are two things it always has in common.  First, hackberry bark is always the same steely whitish gray color.  Second, the bark always has at least some bumps or ridges, which are made of layers and look somewhat like topographic maps.  No other bark that I have ever seen has layered bumps like this.

Our giant hackberry tree with very rough bark for a hackberry.

We have investigated bark before when we looked at scars in the sourwood tree, but let's dive in a little deeper - there's more good stuff here.  Bark is a plant organ.  Organs are structures that accomplish some function in an organism.  You may be more familiar with animal organs like the brain, the stomach, the skin, the lungs, etc.  Bark has two basic functions in plants: it protects the stem and it transports food all around the tree.  We have two separate organs for protection and food transport in our bodies.  Our skin provides protection from the outside, and our blood vessels transport blood around the body.  Blood carries digested food and many other things all around the body.  Let's investigate bark's two functions a little closer.

First: protection.  Bark seals off the tree from the environment.  It prevents the tree from drying out in the heat or getting soggy in the rain, just like our skin protects us.  Bark also keeps out insects and diseases, also like skin.  The stuff in bark that forms a seal against the world is called cork.  Cork is a spongy, softer material found in most types of bark, and it is waterproof due to the presence of a wax called suberin.  Some trees make more cork than others, and humans harvest cork for sealing bottles from the corkiest tree - the cork oak.  In most trees, the cork is interspersed with harder material in the outermost part of the bark.  The ridgy bumps as well as the smooth parts of hackberry bark both contain enough cork to protect the hackberry tree.

The second function of bark is food transport.  Trees and plants use the sun to make their energy in a process called photosynthesis.  Photosynthesis is the name for the chemical reaction that plants do to make sugar, and that chemical reaction is powered by sunlight.  Plants' basic food is sugar, which they can use for energy (just like you do) or for building other necessary plant parts.  Trees do photosynthesis in their leaves, but they need food in all parts of the plant.  The sugars from photosynthesis combine with water in the tree to form a liquid called sap, and liquid sugar is easy for trees to move around.  Tiny tubes in the bark transport dissolved sugars in the form of tree sap from the leaves to the rest of the plant, which is very similar to how the tiny tubes called blood vessels transport blood (which contains dissolved sugars too!) all around your body. 

If you've ever tasted maple syrup, you have tasted the concentrated tree sap taken from maple bark.  Maple syrup is sweet because maple trees' leaves did photosynthesis using the sun to make sugar.  Unfortunately the way I've explained this makes me think of maple syrup as tree blood, but that's really not quite true.  Blood is way more complex than sap, and blood has many more functions in our bodies than sap has in trees, but that's a story for another day. 

Tadpoles in the Pond

We have several tadpoles in our classroom's pond.  To see them, you have to be a little bit lucky.  Now that it's warm, the fish are easy to see - they are out in the open water, swimming smoothly and darting skillfully.  But tadpoles are a different story.  They lurk near the bottom in the leaves and muck.  Every once in a while, they clumsily wriggle from one spot to another.  The best time to see them is when you first walk up to the pond - they will wriggle to a hiding spot in response to the shadow you cast over the pond (they respond as if you were a predator!).  If you don't see one right away, wait around and watch - it might be your lucky day.  You could also take matters into your own hands and scoop through the bottom of the pond with a net.  If you do, please keep the tadpole in the water - they are very fragile and can't survive being dry or being squished.

A tadpole in our pond at the outdoor classroom.

Tadpoles are truly strange creatures.  They are the larval (young) form of frogs.  Frogs lay eggs in water, each of which will hatch into a tiny tadpole.  The tadpoles use gills to breathe water - just like fish do.  Tadpoles swim and eat and grow larger and larger, all underwater.  Eventually when conditions are right and they have had enough food, the tadpoles' bodies change form completely.  Their bodies digest and absorb their tail, and they grow tiny forelimbs (arms) and hind legs.  As they change external forms from tadpole to frog, their internal structures change too.  They grow lungs for breathing air!  Adult frogs hop out of the water and live their adult lives mostly on land but near water.  A change in body form like tadpoles have is called a metamorphosis.  If your body changed and suddenly grew wings for flying, that would be a type of metamorphosis.  Other animals that do metamorphosis are insects (maggots become flies, caterpillars to butterflies, etc.).

Tadpoles and frogs are vertebrates.  Vertebrates are any animals that have an internal skeleton with a backbone.  That means mammals, birds, reptiles and fish are also vertebrates.  Can you think of animals that don't have backbones? (Answer below*.)  Any vertebrate that starts its life in the water and undergoes metamorphosis is called an amphibian.  The word amphibian makes sense if you know what the parts of it mean: amphi- means both, and -bian means life form.  Amphibians include frogs, toads, newts and salamanders. 

Why do you think fish are such better swimmers than tadpoles?  Compare the body shape of a tadpole to a fish, then try an experiment.  First, find a pool and a life guard.  Then jump into the swimming pool and swim like normal using your arms and legs to help you.  Then hold your arms and legs into your body and try to swim - it's not so easy without appendages, is it?  Tadpoles do not have fins like fish do, and fins are great for steering while you're swimming.  They have only a tail to help push them along.  That's why tadpoles wriggle around so strangely and fish swim with ease.

How do you think the tadpoles got to our pond?  Frogs had to have laid the eggs that grew into these tadpoles, but how did the frogs get to our pond?  They have to stay near water, and there aren't other ponds nearby.  Frogs could not have gotten to our pond to lay eggs!  One possible answer could be related to the fact that frog eggs are somewhat sticky.  If a bird stood in a different pond and frog eggs stuck to its feet, then the bird came to our pond, it could have brought the eggs that hatched to our tadpoles.  That makes frog eggs disperse just like the kinds of seeds that stick to animal fur - which would be animals imitating plants!

*Invertebrates include insects, spiders, clams, snails, sponges, jellyfish, sea stars, and thousands more types of organisms.

Sycamores, Trees of Wonder

If you were to decide to learn only one tree in your life, I would recommend learning the sycamore (though I don't recommend learning only one tree).

A stately sycamore between our outdoor classroom and playground.

A friend once told me that sycamores are easy to recognize because they are the only tree that wears pants.  A sycamore's upper limbs are white and appear to be uncovered, and its lower trunk is covered with brown, patchy bark - the sycamore's pants.  Sycamore bark consists of three layers: the outer is brown, the middle is greenish and the inner bark is white.  Sometimes you can see all three layers together in a camouflage-like pattern.  The outer two layers peel off of the upper limbs, leaving bone-like white branches that look spectacular against a blue sky.  Scientists aren't sure why the upper bark peels off, but some people think it falls off to prevent vines from being able to grow up into the treetops. 

Sycamores tend to grow near water, and since their white branches are visible from a distance, they are useful for finding water if you are ever lost in the forest.  Early explorers used them to find water sources across North America, but they used sycamores for lots of other things too.  Sycamores are very fast-growing, so they produce a lot of wood.  Though sycamore wood is twisty, it is extremely strong and light, and Native Americans and settlers both used it to make just about everything you can make out of wood.  Before North American forests were logged, most forests contained trees that were a lot older, therefore they were bigger than trees we have now.  Old sycamores tend to be shockingly enormous compared to other trees, and they are often hollow (here's a medium-sized one, and possibly the world's largest), so sometimes they were used by people as a shelter or to corral animals.

A drift of sycamore seeds and a few pieces of fallen sycamore bark.

The sycamore tree at our school is making a mess right now.  While more northern parts of the country still have snowdrifts, here in Middle Tennessee, we get drifts of sycamore seeds.  Sycamores hold their seed balls (technically fruits) up on their branches all winter, but now the seed balls are dispersing their seeds.  Every ball contains hundreds of wind-dispersed seeds that each have a few feathery hairs to catch the wind.  Some of the seed balls break apart while they are on the tree, and the seeds are dispersed from high in the sky.  Many seed balls fall onto the ground as well.  Mostly the seed balls break apart when they fall off the tree.  The seeds are only loosely-held together, so the seed balls usually smash to smithereens on impact with the ground.  If you're lucky, you may find a whole seed ball, which is exceedingly enjoyable to break apart for yourself.

A sycamore seed ball with a few seeds falling out.

 If you lightly crush the seed ball, you can see how the seeds fit together so tightly. 

A lightly-crushed sycamore seed ball.

Once you have completely crushed the seed ball, look for the hard structure inside.  That structure is the base of where the seeds are produced, and it looks like the strangest type of seed or fruit you've ever seen, but it's neither seed nor fruit, just stem.  Many naturalists have been confused when trying to identify these structures when they find them without the surrounding furry sycamore seeds.

A fully-crushed seed ball.

Soon our sycamore will look very different.  It will be covered with giant plate-sized sycamore leaves, but you will still be able to recognize it by its beautiful white branches.

Mosses, Lichens and Succession

Life has a way of taking over here on Earth.  Any surface without living things on it will eventually have life growing on it if you wait around long enough.  A new sidewalk of poured cement will have plants growing through cracks after twenty years.  New roofs will eventually become soft and covered with mold, moss and even plants.  Fresh lava from a volcanic eruption will cool, harden and in several decades be covered by a forest (Neat example here).  The invasion and growth of life on nonliving surfaces is called succession, and it's happening right here in our classroom.

Green and grey lichens growing on rock.

The nonliving surfaces we have at the classroom are mostly the rocks.  The big boulders and the flat rocks around the pond are too recently dug from the ground to have life on them yet, but they probably will by the time you graduate from high school.  The rocks with the waterfall behind the pond and the rocks that make up the wall at the back of the classroom have been exposed at the Earth's surface for long enough to have some neat life growing on them. 

Organisms that can colonize bare rock are called pioneer species.  Lichens are usually the first pioneer species, and they look like color splotches on the surface of rocks - white, green, grey, yellow or even orange.  Lichens are actually two organisms for the price of one: a fungus and an alga living together.  The fungus and alga form a mutualism - an interaction where both organisms benefit.  If you remember from the beginning of the year, algae grow in our pond - algae can only live where they don't dry out.  In lichens, they live surrounded by cells of fungus so they can live outside of a pond.  In return for this good protection, the algae provide the fungus with food from doing photosynthesis.  Together, the organisms that form lichens make acids that slowly dissolve the rock on which they grow, which makes tiny crevices in the rocks.

White, green and grey lichens plus dark green mosses growing on a rock.

Once lichens have been growing on rocks for a while, mosses are able to survive there too.  Mosses are plants that don't have flowers or stems or roots - just tiny green leaf-like structures and microscopic hair-like structures.  Mosses send their hair-like structures into the crevices the lichens made in order to anchor themselves on the rock.  Then the mosses grow bigger.  They die back during harsh weather and grow more in good weather.  As they die back, their dead parts decompose in place, and they turn into a tiny bit of soil.  After several years, mosses build up enough soil underneath themselves that other plants can move in.  Mosses can also start to grow in cracks and pockets in rocks.

Just as mosses build habitat for small flowering plants, the flowering plants provide habitat and food for more creatures.  Flowering plants have roots that hold the soil in place, and they also add to the soil as they die back each winter and decompose.  Mosses and plants can host tiny insects, adding to the variety of life growing on a formerly bare rock.  As the years go on, the soil builds and builds and larger plants, shrubs and eventually trees can grow on what was once bare ground.  Eventually a mature forest might be found where once there was bare rock, and succession has been a success.

A rather large moss behind the waterfall.

Next time you are near an older neighborhood or a vacant lot in Nashville, see if you can recognize succession.  Old houses have mossy roofs.  Ancient stone walls are covered in plants with trees growing through them and lizards living between the stones and roots.  Old parking lots or yards are infiltrated with weeds and dotted with butterflies drinking from the weeds' flowers.  You can see the results of succession at the River Campus too.  Most of what is now the wetland used to be an open farm field with only grass - only 15 years ago!  Now it has grown into a young forested wetland with lots of plants and small trees.  Life certainly does take over!

Turdus migratorius, American Robin

The robins are here!  For such a pretty and lovable bird, they have a very unflattering scientific name: Turdus migratorius.  The migratorius part isn't so bad, and robins are indeed migratory birds.  But why the name Turdus?  Scientific names are in Latin, and 'turdus' means 'thrush' in Latin, which is a bad deal for the robin.  A thrush is a type of bird that is usually small and plump and searches for food on the ground.  Bluebirds and wood thrushes are other common Middle Tennessee thrushes.

A male robin.

Robins are grayish with rusty undersides.  The males' heads are darker than their backs, and their underbellies are usually brighter than females'.  Last week a male and female were getting to know each other at our outdoor classroom. 

A female robin.

 In Nashville, this time of year robins are just finishing up their winter migration.  Robins group together near the end of winter into massive flocks (did you notice them a couple weeks ago?), then they migrate to follow food sources and warm weather.  They may not migrate straight north like most other migratory birds do - they just go wherever life is good for robins, which explains why we have some robins here year-round.  At the end of the winter migration, birds form pairs and begin to find a home range to nest in.  They may come back to the same place as last year, but they may not.

A female robin between the two Japanese quinces.

Robins are generalists in both nesting and in feeding.  They build nests in a variety of habitats, from landscaped yards to meadows to forests.  They eat a variety of insects, worms and fruits, depending on what is available to them to eat.  In the spring and summer, they tend to eat more insects, and in the fall and winter they eat more fruit and berries.  This time of year, you are likely to see the cliché of a robin with a worm dangling from its beak.  Robins can usually find their food and shelter requirements around where humans live, so they tend to be very familiar to us.

A female foraging for ground insects or worms and a male sitting on the fence.

If the male and female I saw at our outdoor classroom become a pair, they will have 2-3 broods of baby robins this spring and summer.  The female will soon start building a cup-shaped nest of three layers: twigs first then mud then grass on the inside.  She will probably lay 3 light blue oblong eggs in the nest a couple of days after it is finished.  The female sits on the nest for 12-14 days, getting up every once in a while to turn the eggs or go get food.  The male might bring her some food or might not.  The female's belly has a patch of skin with extra blood vessels that keep the eggs warm as she sits on them.  If the eggs get too cold, they will die.  When the eggs hatch, the female tosses the egg shells out of the nest and broods (sits) for another 3-4 days.  After that, the hatchlings are able to keep themselves warm enough without being sat upon constantly.

Both parents feed the hatchlings after they escape their eggshells.  For the first few days, the menu is regurgitated food the parents already ate.  And if that isn't gross enough, after that the parents bring soft-bodied insects and worms to feed the poor little birds.  The hatchlings beg and peep like mad for their food, so they must like it.  Begging is an important skill for robin hatchlings, because the most aggressive peeper with the longest neck and widest-open beak will get the most food and is most likely to survive to adulthood. 

At two weeks old, the robins usually fledge (leave the nest).  They still don't fly well or know how to find food, so the parents hop around them on the ground alerting them to danger and bringing them insects and berries.  At first, the mom feeds the fledglings, but then when they are starting to become independent, the dad will feed them, and the mom will go and build a new nest for the next brood.  The fledgeling stage is very dangerous for the birds because the young ones can't fly yet.  They might become the food that a mother hawk brings home to her hatchlings, or they might fall prey to a cat.

There are many types of birds at our outdoor classroom right now.  Next time you go, try to count the different types of birds you see.  Last time I was there, I saw 3 kinds.  Pay special attention to the robins, and see if you can figure out what they are doing when you see them (feeding? gathering nest materials? searching for a good nest site? fighting off other birds? or are they watching you?).   

Hey Buds!

Today felt like winter, but if you read nature instead of reading the thermometer, spring is already here.  Tree buds are among the first things to reveal that winter is over, and many of the buds at our outdoor classroom are already saying spring.  Twigs are the ends of tree or shrub branches, and the caps at the end of twigs are called buds.  Buds are the most exciting things about twigs (and trust me, there are a lot of exciting things about twigs).

Buckeye twig with three buds.  The end bud is starting to open.

Generally, ends of plant parts are very important, because these parts contain the only plant structures that can make new growth.  Parts capable of plant growth are called meristems.  Look at the end of a tree’s twig and you will see a structure called a bud.  Each bud contains a meristem covered with tiny leaf-like things called bud scales.  Scales are the protectors that keep the meristem inside from dying in the freezing cold of winter.  This time of year, the meristem starts to grow, and it pushes the scales aside.  As the meristem grows, it starts to produce either new leaves, stems and twigs or new flowers.   If you’re curious, cut off a swollen bud, slice it in half from top to bottom, and look at the cut surface with a magnifying glass.  You’ll see sliced immature leaves or flower petals.

Large buckeye bud just starting to open.  Notice the bud pushing the bud scales aside.

Plants grow very differently than people do.  People get longer and wider in every area of their bodies as they grow from child to adult.  So your arm in first grade will be both shorter and thinner in all sections, upper and lower arm, hand and fingers, than your arm in the twelfth grade.  Most plants only grow longer at their tips. (Can you guess what plants don’t grow from their tips?)*  Imagine if your body only grew longer at the ends of your toes and fingers – your adult body would look VERY different.  Plants’ stems (including tree trunks) and roots can grow wider at any point, which is why it takes more people to hug around old trees than young trees, but they only grow longer at the tips.  People often think that if they were to carve something into a tree and come back in 20 years, the carving would be very high off the ground.  This is false, since trees only get taller at the ends of their branches.  [Just a reminder, please don’t carve things into trees – tree bark is the plant organ that carries food between the roots and the leaves, and damaging the bark can kill a tree.]

Back to the buds.  Look at several trees and shrubs at the outdoor classroom.  Right now many trees have some buds just starting to open and other buds in their closed-up winter stage.  It’s a great time to compare winter buds and spring buds on the same plant.  Notice the different shapes of bud scales – pointy or rounded, separate or overlapping, striped or not, green or brown.  You might also notice how some twigs become very colorful just as their buds begin to expand.

Rhododendron flower bud

My favorite thing about buds is that bud scales leave scars on the twigs when they fall off.  Bud scale scars look like tiny clustered rings around a twig.  If you look at a twig starting at the tip and move back along the twig, you will encounter several tiny rings in the same place running around the twig like tight bracelets.  Those rings are scars from where last year’s bud scales were, and everything from there to the tip is last year’s new growth!  Look further down the twig toward the main branch and you may find additional years’ of bud scale scars.  You can tell the age of a twig by counting back bands of bud scale scars from the tips to the trunk.

Rhododendron flower bud opening.

*Grass and other grass-like plants grow from meristems that are near the ground.  That’s why we can mow the grass and cut off all the leaf tips but the grass keeps growing and needs to be cut again soon.

Secrets in Sedementary Rocks

[This is a follow-up post to the first Geology post from a few weeks ago.] 

These rocks need names.  Each of the big limestone boulders in the circle in our outdoor classroom has something unique to offer the curious eye.  I'm sure by now most students have decided which rock is their favorite to sit on when they visit the outdoor classroom.  For now, I'll number the rocks 1-8, but hopefully students will come up with actual names for them. 

Rocks 1-4, left to right in the picture.

If you follow the main path into the circle, rock number one is on the left.  Then continue counting around the circle.  The fence is between 4 and 5.  Rocks 7 and 8 are the last ones, and they have been placed right next to each other.

Stand in the center of the circle and look around at the rocks.  What are the first things you see?  I see scrape marks, those lighter marks on the rocks where some serious equipment carried them from somewhere else in TN to here.  Next I notice that the rocks look layered.  These rocks are sedimentary rocks, formed over thousands of years as water dropped off layer after layer of tiny mineral particles (more below).

Let's take a look at a few specific rocks in more detail and see what they have to tell us.

Rock #1:
Most of the rock does not contain visible fossils, but there is a gigantic nautiloid fossil that is fairly easy to locate on the surface of the rock.  Hard structures of ancient creatures formed fossils easily, and nautiloids had hard, segmented structures that left fossils like the one to the right of my finger below.  Nautiloids were predators, and where there are predators, we know there has to be prey.  Nautiloids' prey organisms were soft-bodied, so they didn't tend to leave fossils.  The amount of rock that contains a nautiloid is small compared to the rest of the rock because nautiloids were fairly rare in their ecosystem.

Rock # 2:
This rock has a fossil from an organism called a bryozoan.  Bryozoans are filter feeding animals that live in colonies.  They are much like corals, though they are not closely related to corals.  It's common for organisms to be similar and yet unrelated.  For example, penguins and dolphins have many similarities (shape, color, swimming style), yet they are very different types of organisms: penguins are birds and dolphins are mammals.  In the same way, bryozoans and corals are similar in shape and habit yet unrelated.  Bryozoans were much more common when these rocks were formed (300-500 million years ago!) than corals were, and there are several bryozoan fossils on our rocks.

T-shaped Bryozoan fossil on Rock 2.

Rock 3:
Find the crystal-lined hole in Rock 3.  You have found a geode-like structure.  Geodes can form in different ways, but it is likely that this one formed when part of the original sedimentary rock dissolved and washed away to leave a hole, or cavity.  Then over thousands of years, water seeped through the cavity.  Water usually has dissolved minerals in it, and sometimes those dissolved minerals crystallize into solid minerals.  If you have an older faucet at home, look for a whitish crust around it - the white crust is crystallized minerals from the water that is carried through the pipes.  I suppose if the conditions were right your faucet could be covered with pretty crystals in a few thousand years as more and more minerals are deposited on it.  The same process that makes the crust on your faucet made the geode you see in this rock.

A geode-like cavity in Rock 3.

Rock 7:
Rock 7 is my favorite for looking at the layers of sediment that formed the rock.  For some reason, this rock was placed on its side, so the layers are more easily visible.  Flowing water makes sedimentary rocks.  Fast-moving water picks up larger, sand-sized sediment and carries it until the water slows down.  Then the sediment falls out of the water and settles on the ground in a flat layer.  Slower-moving water picks up smaller particles, like the particles that make up mud.  When slow water stops moving, it drops its particles onto the ground.  If the speed of the water above the ground changes through the decades, the sediments that settle onto the ground will have different textures.  The darker, smoother bands in Rock 7 were formed from slow-moving water's particles.  The lighter, rougher bands were formed from faster water's particles.

Layers in this sedimentary limestone reveal the speed of the water that deposited the particles.

There are lots more treasures to find on these rocks, so look carefully.  There are many more fossils, interesting rock layers, geode-like structures and other strange features.  I would like to thank Mr. Smail for teaching me about our rocks.  Feel free to email him if you have more Geology questions!

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.

Nandina: A Berry Interesting Problem

Oh, nandina, nandina, you trouble my heart!  You are so beautiful all winter with your green foliage and bright red berries, but you are an invasive plant.  What is a good naturalist to do?

Nandina, AKA heavenly bamboo.

Invasive species (species = type of organism) are a problem in Tennessee, and in the rest of the world as well.  They are currently the #2 cause of extinction of other species, just after habitat destruction.   Invasive species are non-native organisms that grow like crazy and take over.  Non-native organisms are moved from one part of the world to another, mostly by humans.  This is usually not a problem, except that for some non-native species, the new habitat has none of their usual diseases and predators, and the habitat seems to fit just right.  In that case....they can take over and crowd out the habitat of other organisms.  Nandina is native in Asia - from Japan west to India.  It is a beloved plant there, also used in landscaping like we use it here. 

Poisonous beauty: these berries contain nandina seeds and cyanide!

In Tennessee, some of our most harmful invasives are bush honeysuckle and kudzu.  Bush honeysuckle uses up habitat for other plants.  Also, birds that nest in it are more likely to get eaten (not sure why).  Kudzu simply crowds out every living organism where it grows.  Nandina is not that bad!  It is usually only found growing wild near where humans have intentionally planted it.  The Tennessee organization that helps keep invasive plants under control (TN-EPPC)  wants more information about nandina in order to keep tabs on the problem.  If you are ever out hiking in the wild (not in a landscaped yard) and you see a nandina, TN-EPPC would like to know about it.  You can report an escaped nandina at the TN-EPPC website: http://www.tneppc.org/

A nandina draws your gaze from behind a sedimentary rock.

Biologists worry about invasive species because they cause the total number of species to decrease.  The loss of a species, or extinction, causes the loss of a participant in an ecosystem.  For example, when a bird species dies out, an ecosystem might lose a seed-disperser.  If honeybees died out, there would be way fewer pollinators and thus way fewer fruits and seeds.  If a type of beetle died out, we might lose a soil recycler. 

Nandina is guilty of taking up a tiny bit habitat that would otherwise be used by native species, though it doesn't appear extremely aggressive.  It has another problem, though.  Nandina berries contain a toxin called cyanide.  Birds in the US haven't figured out how to deal with the poison, and some cedar waxwing birds have died from eating lots of the berries.  The berries can be toxic to any other animal too, so don't eat them. (It probably takes a lot of berries to hurt a large animal such as a human...still...don't eat them.)

Back to the original question: what's a good naturalist to do?  That depends on who you ask.  Some will say to never plant nandinas.  Others say plant them but clip off the berries this time of year when birds start foraging.  Others say don't worry about it - eventually the other species will adapt and nandina will become another important part of our ecosystem.  The only problem with this last option is that adaptation takes hundreds to thousands of years, so we won't find out how nandinas mesh with our Middle Tenneessee ecosystem for a looooonnnngggg time!  What do you think we should do?