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.

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. 

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!

Winter, Spring, Summer, Abscission

It's happening everywhere right now!  Plants are chopping off their own organs, and they are piling up in yards all over town!  How come no one is worried about this epidemic of leaf death??!!  Well, it happens every year, so I'm pretty sure the plants are going to recover.  Still, why on earth would plants get rid of their most important organs?  That's what we'll address in today's post.

Closeup of leaf abscission zone on sourwood.

In the picture above, you can see the color difference between the pale pink of a leaf petiole (technical term for a leaf stem), and the bright red of a sourwood twig.  The line between those two differently-colored plant parts is called the abscission (ab-SIZH-uhn) zone. 

Fresh leaf scar where the abscission zone dissolved and the leaf fell off.

This time of year, the layers of abscission zones are changing.  One layer is hardening and filling up with a corky substance called suberin.  Suberin is waterproof and heals what would otherwise be a wound where the leaf falls off.  The leaf scar in the picture above is dry and not losing sap because suberin has sealed the wound.  The second layer in the abscission zone is made of thin-walled, weak cells that self-dissolve when the plant is ready to shed its leaves.  Abscission zones are usually quite noticeable this time of year on any plant that is in the process of losing its leaves.  Take a look at the next two pictures and find the abscission zones.

Sourwood leaves and petioles (stems) about to undergo abscission.

The abscission zone is at the base of the leaf petiole where it attaches to the twig.

It is extremely unusual for living organisms to shed any part of themselves except for the production of offspring.  Some lizards have tails that fall off to distract predators, and many plants lose their leaves in the fall - but I can't think of other examples of falling-off body parts.  Of course, most organisms constantly rebuild their outer-coverings and some organisms can replace body parts that are bitten off, but voluntary amputation is strange, indeed! 

The loss of body parts comes at a huge cost.  Plants work all summer to catch enough sunlight to grow more leaves and get bigger, and leaf abscission every fall would seem to waste that energy.   But as with the lizards that lose their tails, there are also benefits.  Lizards' bodies escape to live another day and regrow another tail.  Plants benefit from shedding leaves by not having to maintain those leaves during the winter.  Leaves are tender tissues that would become disfigured and die when frozen.  Try putting some lettuce leaves in the freezer over night and then take them out to thaw.  You will notice they turn to mush when they return to room temperature.  In order for plants' leaves to survive winter, they would have to be tough, like holly, magnolia or spruce leaves, which take much more energy to produce.  Plants with leaves that survive freezing grow more slowly than ones that shed their leaves.

Dogwood with remnants of chlorophyll along veins and lots of anthocycanins (red pigment).

Plants have many ways to minimize the costs of losing their leaves.  They move all available nutrients out of their leaves and down into their roots to save the food for the next growing season.  Leaves fall near the plant that grew them and decompose, releasing their nutrients into the soil and further increasing the amount of nutrients recovered by the plant.  In this way, deciduous plants grow their own mulch.  Some plants, like walnut trees, even deposit compounds in their leaves that suppress the growth of competitor plants as the leaves decompose throughout the winter and spring. 

Rainbow of fall colors.

As leaves senesce (slow down and die) in the fall, they turn the variety of amazing colors we are so familiar with.  Plants' normal color is green, due to the most important compound in the world: chlorophyll.  Chlorophyll is the substance in plants that allows them to absorb sunlight and use the energy from sun to make food, a process called photosynthesis.  In the fall, chlorophyll breaks down, revealing other colorful substances plants use for photosynthesis: xanthophyll (ZAN-tho-fill), a yellow pigment, and carotene (CARE-oh-teen), an orange pigment.  As temperatures drop, some plants make anthocyanin (AN-tho-SIGH-uh-nin), a red pigment that helps the plants store sugars for winter.  Some plants reveal tanins (TAN-ins) in their leaves in the fall.  Tannins are brown in color and are thought to be waste molecules produced by plants.  They have a bitter flavor, though some tannins are pleasant, including the ones found in tea leaves.

Leaf scar on a buckeye showing scars where the leaf veins were sealed off with suberin.

So leaf abscission is a trade-off that works in parts of the world with four seasons.  Plants in the tropics and plants in colder regions keep their leaves.  Tropical plants don't have to deal with cold, so they don't shed their leaves unless there is a yearly dry season.  Plants nearer the poles of the planet don't have a long-enough growing season to start from scratch every year, so they have to grow slowly and produce evergreen leaves and needles.  We lucked out, and we get to see the beautiful fall colors that accompany leaf abscission.

How to Read Bark Scars in Sourwood Trees

Sourwood trees are among the first to turn colors in the Fall.

The sourwood trees in our outdoor classroom are the first to put on their fall colors for the season.  Sourwoods are wonderful, smallish trees with beautiful foliage and interesting bark.  They are named for the sour taste of their leaves, which you can experience if you touch a bit of torn leaf to your tongue.  The leaves contain oxalic acid, which tastes pleasantly sour (all acids taste sour).  Tasting the leaf is not harmful, but the leaves are not considered edible and shouldn't be eaten.

The small orange-leaved tree in the picture is one of our sourwoods.

We have two sourwood trees.  Above you can see the location of one sourwood - it's the orange-leaved small tree in the center of the picture.  See if you can find the second sourwood tree when you visit the classroom.

Lenticels in young bark of the sourwood tree.

Sourwood bark is wonderful - it has so many visible features that give clues to what the tree is doing and what it has gone through during the tree's life.  A lot of people think tree bark is a dead part of the tree, but the opposite is true: tree bark is a living, important tree tissue that changes as trees grow.  Bark is mostly responsible for moving sugars (a tree's food, made from photosynthesis) between the leaves and roots.  Bark is filled with phloem tubes for transporting the sugar.  The above picture of a young twig contains tiny spots called lenticels.  Lenticels are tiny holes in the bark to allow air to get into and out of the inner tissues of the twig.  Compare the above twig to the one below.

Sourwood twig with cicada damage.

The twig in the picture above is about the same age as the one in the previous picture, but something looks wrong!  This giant gash in the bark is the healed wound cut into the bark by one of last year's cicadas.  Cicadas cut into young bark and lay their eggs in the gash where the developing offspring can feed on tree sap.  The living bark responds by slowly growing a scar to heal the wound and seal off the wood, which is what you see above.  

Older twig with young bark splitting as the twig grows larger.

Bark naturally stretches and tears and re-heals to allow tree twigs and trunks to increase in girth.  The twig above shows the first tears in young bark as the twig is getting thicker through the years.  The stretch marks get bigger as the tree gets bigger, and large branches and trunks might have deep furrows in the bark.

Scar from where a branch broke off the tree.

When branches fall off or are broken off, the bark around the broken area swells up and heals over the scar.  The scar above looks like a pretty big scar, so I suspect the branch that used to grow here was torn off unevenly.  Notice the larger tears in the normal bark above and below the branch scar.

Large gash in bark that is healing over - possibly damage from planting the tree.

Here is an even bigger scar from some major damage to the trunk.  Something cut into the bark of this tree.  Perhaps it was damaged as it was being transported or planted here.  Such an injury can weaken or kill a tree, because it can let diseases into the tree, just like a wound in our skin can become infected.  I wish more people realized this so they wouldn't carve their initials into trees' bark.  Nevertheless, this tree appears to be healing from its damage.  You can see exposed wood through the gash in this bark.  If the wound to this tree were to have cut through the bark all the way around the tree, the tree would have died, since the bark would be unable to move sugars up and down the tree.  Plant managers who need to kill trees use this technique - it's called girdling a tree.

Check out the bark on our sourwood trees and look for lenticels, branch scars, normal tears in the bark, and possible injuries to the bark.  Then take a look at other types of trees and see if you can read the scars in the bark.  Can you tell where branches used to be?  Can you see how the bark split at the tree got bigger?

Click to zoom in and see how the leaf veins connect.

When you check out the bark on our sourwoods, be sure to look at the leaves too.  Sourwood leaf veins are large, and it's easy to see the network of how the veins connect.  Also be sure to look for the remnants of flowers, now turning into fruits, at the ends of some of the branches. 

What is a Leaf Skeleton?

Leaf skeleton floating just below the surface in our pond.

For some reason, floating leaf skeletons are most visible on gloomy days.  Perhaps the filtered sunlight doesn't glare as much off the surface of the pond, and the light-colored skeletons stand out against the dark background of the pond depths.  Needless to say, leaf skeletons are strikingly beautiful, and they are lovely to find on a gray day.

Leaves ranging from living to dead floating in our pond.  The dead ones are starting to decompose.

The pond in our outdoor classroom has many tall trees towering over it.  As summer turns into fall, it is going to fill up with leaves, and we will hopefully have lots more floating skeletons.  Ponds provide the perfect habitat for production of leaf skeletons, because they allow for fast decomposition of soft plant tissues.  Aquatic conditions provide lots of snails, insects and microscopic organisms that attack a leaf as soon as it falls, since a newly fallen leaf has lots of nutrients to feed pond organisms.  The organisms eat the softest tissues and leave the leaf veins behind, allowing us to marvel at an important plant tissue.

Leaf skeleton from a hackberry leaf.

Leaf veins are a part of the transportation system within the plant.  The veins in leaves connect through the leaf stems into the main plant stems and all the way down to the roots.  Veins in plants are network of tubes for moving necessary substances from anywhere in the plant to any other part of the plant.  My mind can't help but compare leaf veins to our system of roads.  All our houses are connected to them, and we can use roads to get anywhere else.  I also think of our system of blood vessels - a similar network of tubes for moving necessary nutrients from one place to another within our bodies.

Leaf skeleton showing network of veins.

Plant vascular systems (vein systems) contain two types of tubes inside each vein.  One type of tube, called xylem (pronouned zy-lum), is rigid and only runs in one direction.  Xylem carries water and nutrients from the roots in the ground up to the leaves.  The second type of tube is called phloem (pronounced flow-um).  Phloem tubes are soft and flimsy, and they carry sugar, the product of photosynthesis, to wherever energy is needed in the plant.  Phloem tubes run in two directions.

Leaf veins in living leaves.

Leaf skeletons can be preserved by drying them pressed between newspaper or paper towels.  Their patterns can be revealed by covering them with paper and making a rubbing.  Different types of leaves have different patterns of leaf veins - highly branched, long parallel lines or radiating out from a center.  All three of these patterns of leaf veins can be found at our outdoor classroom.  Another fun trick for learning about the function of plant veins is to place a white carnation in water with food coloring.  After a few hours to a day, the xylem will carry the food coloring up to the petals.  The same thing would work in a leaf, but the green color of living leaves would make it difficult to see the food coloring.

Green Algae

The school where I work has an outdoor classroom that invites nature into our urban schoolyard and creates a peaceful outdoor space for students to learn about life.  We have trees, wildflowers, and a pond, each hosting their share of associated organisms.  I'll be doing some nature blogging about the outdoor classroom for use by teachers at our school, and I'm posting the first outdoor classroom blog post here:

Welcome!  I'm excited about starting an outdoor classroom blog.  I can't wait to see what we find in our little urban nature oasis.  The subject of our first post is green algae.

If you look under the surface of the pond, you see a very busy ecosystem indeed!  There are plants, fish and insects, and those are just the visible organisms.  There are way more microscopic organisms than big ones, which I'll save for a future post.

Green algae growing just below the surface of our pond.

The strangest macroscopic (big enough to see) organism is the filamentous green alga that forms clouds of soggy green cotton candy.  But what on earth are green algae?  They seem a lot like plants: they photosynthesize, they have cell walls, and they have chloroplasts (green structures that photosynthesize).  There are also some features of green algae that we don't usually associate with plants: they live entirely under water; they don't form roots, leaves or stems; and some of the microscopic ones can swim!  Scientists have wavered a bit about whether algae are actually plants or protists.  The discovery of how to sequence DNA has allowed algal geneticists to confidently classify green algae as plants, though the other colors of algae (red, brown and blue-green) are not classified as plants.  I have had to re-learn my green algae taxonomy!

Interconnected strands of green algae pulled up from under the surface.

Our green alga (alga, hard g, is singular, and algae, soft g, is plural) is a filamentous type, meaning it grows in long strands.  The filaments (strands) of green algae are only one-cell thick, which is surprising considering how tough they are.  Each algal cell is cylindrical like a soup can, and the filament is arranged like an infinite strand of soup cans glued end to end.

Green algae out of the water.

The color of green algae comes from the pigment chlorophyll, just like in other plants.  Chlorophyll is the molecule that can catch light to allow plants to use its energy to build food.  Green algae cells contain structures called chloroplasts that hold the chlorophyll plus all the other machinery needed to conduct photosynthesis.  All plants' cells contain chloroplasts.

I looked at the algae from our pond using a microscope that magnified what I saw by a factor of 100.  In the picture below, you can see the cell wall between adjacent algal cells, just above the pointer.  Cell walls are rigid structures made of cellulose (a strong, rigid molecule), and they give plant cells their shape.  Paper is made by starting with plant material and getting rid of everything but the cellulose, meaning that paper is essentially squished, dried plant cell walls.

The pointer rests on a filament of green algae.  A broken alga releases its cell contents. 100x

Another interesting thing in the picture above is the broken cell.  A chloroplast is slipping out of the broken plant cell in the center of the picture.

Below is another view under the microscope, with one normal filament and one filament with shrunken cell contents.  The chloroplasts and other structures have been compressed into a central structure in each cell.  The strange filament may be undergoing reproduction or it could be stressed, but either way, you can see the beautiful cylindrical shape of each individual cell.

Shrunken cell contents in the lower filament allow you to see the cell walls.  The round object is an air bubble. 100x

In our pond, the algae seem to be growing quickly.  There is ample sun for food, plus decomposing leaves and insect/fish excrement providing nutrients, the equivalents of vitamins in our food.  When you see a pond with lots of algae in it, you can assume that there are a lot of nutrients in the water, either from natural sources or from pollution.

Why Do Sunflowers Follow the Sun?

Blooming sunflower.

It feels good to look at, doesn't it?  Something about bright yellow and radial symmetry is so pleasing to the eyes and mind.  Perhaps the visual buzz we get from gazing on sunflowers is due to the arrangement of the flower parts in what is known as Fibbonaci spirals, which I will not attempt to explain to you, but which you can learn about here in a fascinating animation.  No matter how much you understand Fibbonaci spirals, sunflowers are captivating.

The top picture shows a recently-opened sunflower inflorescence, or group of flowers.  Each dot in the face of the sunflower is actually a single flower, and the picture above shows sunflowers at the stage of pollination.  After pollen has been transported from the male parts of the flower to the female parts of the flower, the female parts of the flower begin to grow into what we know of as sunflower seeds.  In the two photographs below, you can see a mature sunflower inflorescence transitioning to sunflower seeds.

Sunflower finishing blooming and turning to seed.

Sunflower seeds not yet turned black arranged in a Fibbonaci spiral, allowing for maximum packing of seeds.

Above, you can see whitish green sunflower seeds packed together with their pointy end in.   The Fibbonaci spiral arrangement allows the plant to pack as many seeds as possible into the space available.  As this sunflower matures, the seeds will turn striped white and black like what we're accustomed to seeing in the grocery store or birdfeeder.  For a more detailed description of sunflower floral structures, see my older post on aster-type flowers, written when I wrote catchier though less-internet-searchable titles for my posts.

Sunflowers also do that amazing sun-following trick that makes these plants seem to possess some mystical powers.  Well, if you'd like to maintain your sunflower mysticism, I suggest you skip the rest of the text in this post and just look at the pretty pictures.

Sunflowers facing the sun.

What's really going on here is something called heliotropism, and lots of plants do it.  Heliotropism means moving toward the sun.  If you've ever repositioned yourself periodically during an afternoon of misguided youthful tanning in order to get even sun exposure on all parts of your previously cancer-free skin, you've done heliotropism yourself.  The puzzle with sunflowers is, why do the flowers need to face the sun?  To even out tan lines? To look good in a white dress?  To appear thinner?  To fit in with their friends?  Read on.

The truth is, the stems of all actively growing sunflower parts - flowers and leaves - grow to face the sun in order to maximize photosynthesis.  During the day, the stems elongate on the side away from the sun, tilting leaves and immature flowers toward the sun throughout the day and ending up facing west at sunset.  When there's no light (so...night time), the other side of the stem grows, pushing the leaves and flowers back to the east where they will be facing the sun at sunrise.  Growing leaves and immature flowers are green and actively photosynthesizing, and heliotropism provides them with 10-15% more sunlight than just sitting still.

Take a look at the picture below.  On the right, you can see an immature sunflower inflorescence covered in green bracts, which are obviously photosynthesizing since they are filled with chlorophyll and appear green.  The younger sunflower has immature leaves held up and facing the sun as well.  The lower leaves on the younger sunflower, as well as all parts on the older sunflower, have matured, and though they are generally facing up, they are not facing the sun.  The older sunflower is drooping from the weight of the developing seeds.

Young sunflower parts following the sun, old sunflower parts stuck in place.

So just-opened sunflowers like the gorgeous ones in the vase below (if they weren't cut off from their stalks) are still growing some, so they still face the sun.  As soon as they mature, they usually end up facing east and staying there.

Bouquet of sunflowers