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. 

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.

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. 

American Chestnut Trees

Here's something you don't see every day...

American chestnut (Castanea dentata) leaves.

 ....an American Chestnut tree!  My parents are growing American Chestnuts (Castanea dentata) on their property to help bring this great American tree species back to prominence.  Mom learned about the American Chestnut and wanted to get involved, and Dad was game to help Mom put in the work needed to plant and protect these trees as they try to survive.  They are both pleased with their American Chestnuts.

Dad with an American Chestnut on Father's Day.

Eastern forests in the United States were once dominated by this tree species.   If you think about how prevalent oak trees are in the Eastern forests of this country, it gives you a sense of the size of the American Chestnut's niche.  Its nuts provided great quantities of seriously delicious food for people, deer, bears, squirrels and many other animals.  It is a fast-growing member of the oak plant family (Fagaceae), and its wood is strong and particularly resistant to decay, so it was an extremely useful lumber-producing tree.

Why am I speaking in the past tense?  Because this tree species is now mostly gone due to the chestnut blight.  Chestnut blight (Cryphonectria parasitica) is a fungal disease that evolved in Asia and was accidentally brought to the U. S. in the late 1800's, probably on furniture, lumber or nuts.  Chinese Chestnut trees evolved with the disease, so they are resistant to it, but our trees were not resistant, and they succumbed to the disease as quickly as Native Americans died from European diseases introduced by the first European settlers of this land.  The disease was discovered in 1904, and by 1950, almost all the American chestnut trees were dead, with only small shrubby root sprouts left surviving.

Chestnut catkins (flowers).  The narrow ones have only male flowers, the upper one has some pollinated female flowers, which will produce nuts.

Several organizations, including the American Chestnut Foundation, are trying to breed blight-resistant chestnuts and repopulate our forests with this missing foundation species.  Chestnut-lovers are using existing trees to search for blight resistance.  They are also breeding Chinese Chestnut trees with American chestnut trees, eliminating those that don't survive the blight, and crossing the offspring back with American Chestnuts to result in trees that are mostly American but with the Chinese blight-resistance genes.  Right now, there exist trees that are 98% American with 2% Chinese genes.  These mostly American Chestnuts are responding well to blight exposure.  Nothing against Chinese Chestnut trees - they are great, but they're adapted to Chinese ecosystems.  Chestnut-lovers and ecologists want to maintain both species - with the American Chestnut trees back in the ecosystems here.  In the mean time, many people have planted Chinese or European Chestnuts in their yards in order to have some chestnuts to eat in the fall. 

Chestnut catkins with pollinated female flowers that have become burs, and male flowers above them.

Chestnuts have either all male flowers or male and female flowers.  Mom and Dad obtained dozens of chestnuts from the American Chestnut Foundation so they would have many trees and guarantee that they could have cross-pollination between the trees.  Chestnuts cannot self-pollinate.  This time of year,  pollinated female flowers are enlarging into burs.  Burs are spiky fruits that contain chestnut seeds.  In the fall, the seeds will be mature, the fruits will crack open, and the whole bur will fall to the ground.  As soon as the burs crack open, the seeds are mature and ready to overwinter and grow into new trees or to be eaten.

There are a few remaining adult American Chestnuts in North America.  Many of the surviving ones are outside the former range for American Chestnut trees, so the blight hasn't spread easily to them.  Also, there are different climactic conditions outside our chestnut's normal range, which cause the blight fungus to be weaker, or hypovirulent.  Mom and Dad's chestnut trees are outside the normal range, so they may survive longer than other American Chestnuts.  Of the original seeds they planted, about half remain.  Their trees probably didn't die due to blight, but to non-ideal climactic conditions.  Blight tends to affect teenage trees, and these trees are younger.  It is likely that all my parents' trees will eventually die, unfortunately.  Still, they may have a resistant tree, and their trees help maintain living tissue, help educate people about the trees, and help scientists learn more about what these trees need to survive.  With so many people working to solve this ecological tragedy, it appears likely that American Chestnuts will eventually recover.  I'm so proud of my parents for helping the American Chestnut!

Massive Pollen Overload: Hello Spring Allergies

Brace yourself.  These pictures are of terrifying plant structures that cause much anguish to their human victims, and the overall post is frankly somewhat disturbing.  Be brave.  For the first picture, imagine some ominous, slow, suspenseful music, because a threat is just beginning to emerge from its lair.  Yes, the picture below shows flowers from a wind-pollinated tree emerging out of a bud.  The tiny green globs in the picture below will open and shower the world with..........pollen!!! No!!!!  These innocent-looking little guys are going to make you miserable for the next few weeks.

Leaf and flowers emerge from this hornbeam maple tree bud.

"Can those really be flowers?" you ask. "They're so small and boring-looking." Well, not all spring flowers are beautiful and showy.  When flowers are conspicuous, you can rest assured that they are not causing your spring allergies.  Beautiful, sweet-smelling flowers are attempting to attract insect pollinators, and insect-transported pollen sticks to the flower, then to insect legs, and it does not blow in the wind.  Tiny, green, anonymous-looking tree flowers are usually wind-pollinated, which means their pollen is dusty, copious, and perfect for floating along on a breeze to any location, including your sinuses.

"Why do plants make all that pollen?  What's the purpose??!!"  You're asking a lot of questions today. Unfortunately, if you are reading this and sniffling due to a nose full of pollen, you might find my answer to be a little disconcerting.  Pollen is the plant equivalent of sperm.  So, yes, your sinuses are clogged with plant sperm.  Pollen is produced by the male parts of flowers, and it combines with the ovule in the female part of flowers to produce a fertilized cell that will develop into a new offspring plant.  In this picture, you can see the female parts of tiny winter hazel flowers reaching into the air to snag pollen grains to make some new baby hazel seeds that will grow into new hazel trees.

Winter hazel flowers with stigmas reaching out to catch wind-borne pollen.

If you were to look at grains of pollen under the microscope, they wouldn't look much like sperm.  Pollen has varied shapes, depending on the tree.  Here is a book of scanning electron microscope images of different types of pollen - amazing stuff!  My favorite is pine pollen, shaped like Mickey Mouse's head (Google it).

Pollen does a very, very, very strange thing when it fertilizes plant ovules.  When pollen lands on a female flower structure, it divides into three sperm cells, with actual flagellae.  The sperm swim down a channel in the female structure of the flower.  One sperm fertilizes the ovule, as we would expect based on what we learned about human anatomy in 7th grade.  The other two sperms combine with another cell near the ovule to make a substance called endosperm.  The endosperm is genetically the combination of two parents, but it is not really an offspring.  Endosperm is the structure inside the seed that stores food for the new growing plant.  For example, in a corn seed the endosperm is the starch in the corn kernel (yes, popcorn is exploded endosperm, and the little nubs in popcorn are toasted corn embryos....mmmmm!). Now you know why I put three "verys" in the first sentence of this paragraph.

New leaves and flowers hanging in clusters called catkins on a red oak tree.

Oak trees (pictures above and below) are my favorite trees, so don't think I'm picking on them.  They are pretty bad at making giant clouds of pollen.  Pines are even more intense.  There are a few days in spring in Georgia that you really don't want to be outside because the pine trees seem to spew pollen like snow-making machines spew snow.  If you catch a tree as it's releasing pollen and shake one of it's branches, you can make a nice, yellow cloud in the air.

When pollen lands inside your nose, the membranes in your nose recognize it as a foreign object to be removed.  Your body leaps into action with sneezes, mucus production, and swelling (which can cause headaches) in order to get rid of the pollen.  This immune response can make you tired and uncomfortable.  Fortunately trees only make pollen for a short period of time.  The benefits of having lots of trees near you (shade, habitat, aesthetics, food, property values, reduced heat bills, etc.) vastly outweigh the annoyance of allergies.  If your spring allergies are really bad, stay inside and be sure to wash your hair and clothes after you go outside to keep the pollen away from your nose.  And just wait around a few days for a spring rain shower, and the pollen will be gone.

New leaves and catkins on a white oak.

Maples in Early Spring

If you live in the temperate eastern United States, and you only know one kind of tree, it's probably going to be a maple.  Everyone knows maples.  People either recognize and love maples' unique, pointy leaves, enjoy maple syrup, admire bright fall maple trees, or played with maple helicopter seeds as kids.  Few people know what maples are up to this time of year, though.

Even without leaves, maples are very busy this time of year.  Look at the ends of the branches on this maple tree below:  there are lumps all along the branches.

Swollen maple buds ready to pop.  Early March.

Those lumps are flower buds.  Many maples flower and fruit before they leaf out.  Here is a closeup of maple flowers:

Mid March, maple flowers.

Maple flowers are pollination generalists.  Some are pollinated by insects and bees, some are wind pollinated, and some are self-pollinated.  From the tree's perspective, it pays to be flexible with pollination strategies if you bloom very early in the growing season, because it's difficult to insure that insects will be out when you're ready to bloom.  Insects are really the best pollinators.  They are great at pollinating over long distances with small amounts of pollen, but they require warmer temperatures to do their work.  Wind pollinates cheaply - you don't have to feed it nectar or a portion of your pollen to get it to carry your pollen to another flower.  But wind isn't very specific in direction, so you usually need to make a lot of pollen if you are using wind (more on this next time!).  Self pollinating is convenient, but let's face it, you don't get much genetic variety if you make kids using only your own genes. 

Either way, lots of pollination has happened, because the maples in Chicago are LOADED with maple fruit.  Notice I called these helicopter things seeds earlier in the post, and now I'm calling them fruit.  I didn't want to alarm you earlier, but here's how this works:  fruits are plant parts that hold seeds.  An apple fruit has seeds in it, and so does a cucumber, and so does a maple fruit.  The maple fruit consists of a wing and a case around the actual seed.  Open up the swollen end of the fruit, and you will find a sticky seed (and you can stick the fruit on your nose or fingers like we did when we were kids).

Maple fruits (samaras) in late March.

Maple fruits are winged, and they are adapted to being carried far away from their parent tree by the wind.  They do indeed work like helicopters - their wing catches the wind and spins them along to hopefully sunnier ground than the ground just under their parent tree (maples are indeed shade trees).  There are many types of fruits out there: berries, capsules, hesperidia, drupes, pepoes, etc.  Fruits with wings are called samaras.  Both maples and ash trees have samaras to carry their seeds away.

New (red!) maple leaves, plus some maple samaras, late March.

Above you can see some new leaves just starting to grow on this maple. I had to look hard to find maple leaves on this type of maple tree - they mostly have only fruit right now.  Below you can see two pictures of early leaf growth on a Japanese maple.  Japanese maples seem to usually leaf out before they set fruit.

Japanese maple leaf buds opened and showing the new expanding leaves, late March.

Slightly older Japanese maple leaves, late March.

Buds of Spring

A few weeks ago, I explained how to tell the age of a twig, and last week, I showed you the amazing colors of photosynthesizing spring twigs.  Well, it's apparently twig month.  But don't worry, twigs are fascinating, and we haven't wrung all the goodie out of them yet.   

Buckeye leaf buds.

Today we'll focus in on buds, since that is what seems to be the most active biological phenomenon in Chicago at the moment.  Buds of many trees and shrubs are starting to grow, and I have become fearful of checking the weather report since we're well before the last frost date for this growth region (April 20).  I know we're going to see lots of branches with melty, drippy dead leaves one of these days.

As you can see from the picture above, I finally found a buckeye, the best twigs for learning twig structures.  Even with the mediocre picture quality, you can see the giant bud scales and leaf scars.  The terminal bud has almost doubled in size and is about ready to pop out some leaves.

Shrub buds leafing out.

In the photo above, of a mystery shrub I have not yet identified, the double terminal buds have expanded so much that you can see individual leaves.  I didn't stop to get a better picture because the house owner came home while I was photographing his buds, and we had an awkward moment.  In a better picture from buds at the Lincoln Park Zoo, below, you can see miniature leaves, and the terminal bud scales are still present at the base of the leaves.  No doubt zoo patrons were wondering why I was taking pictures of a shrub and not the demonstrating chimpanzees right behind me.  They'll just have to start subscribing to my blog to find out.  Loyal readers (hi Mom and Dad), any guesses why these new leaves are reddish?

Shrub buds leafing out.

The picture below introduces you to a new plant structure, the flower bud.  There are a cluster of tiny dogwood flower buds between my fingers, surrounded by four flower bud scales.  Any plant bud is just a beginning of a new plant structure.  So you can have leaf buds, flower buds and even root buds.  Inside each bud is a small cluster of the plant equivalent of stem cells.  Plants' stem cells are called meristematic tissue, and a cluster of these cells are called a meristem.  Meristems are the specific cells capable of growing new plant tissue.  Most plants have meristems in their tips, like in buds.  Plants like grasses have meristems near their bases, which means they can easily grow back after you run the lawnmower over them.

Dogwood shrub flower buds.

Below is a gorgeous magnolia flower bud.  You can see hairy bud scales and light pink flower petals beginning to emerge.

Saucer magnolia flower buds.

Next are the flower buds of a plant so famous for buds that it is named after them: the redbud tree.  Redbud buds are a little strange - they can grow out of a mature twig or even out of the tree trunk.  This phenomenon of flowers emerging from the mature wood instead of from new green growing tips is called cauliflory, and it's quite unusual.

Redbud buds.

I've saved the best picture for last.  I found these strange objects on the ground in the perennial garden section of the Chicago Botanical Garden.  There are red, papery bud scales and bunched up green leaf babies crammed inside the red buds emerging straight from the ground.  I had to ask, since I've never seen these before, and it turns out they are rhubarb buds!  The leaves will expand and grow remarkably quickly, since they are so fully formed inside the buds.  Then I will turn the stems into a pie.

Rhubarb buds.

Gettin' Twiggy With It

A trip to the Chicago Botanic Garden this morning provided me with much blog fodder for this and the next few posts.  Spring is early this year, bringing a bounty of beautiful sights to the Botanic Garden. 

Greenish yellow weeping willows and orange willow shrubs on the left side.

Many trees and shrubs have responded to the spring weather, even if they haven't leafed out yet, by becoming quite colorful.  Their twigs have begun to manufacture photosynthetic pigments near the surface of the bark, making for yellow, orange, red and green twigs.  Forget everything you ever learned about plants - they photosynthesize using bark! (OK, don't forget anything, but you can add on.)  The picture above shows a lovely spring scene with willow trees and shrubs revealing their spring pigments.

Crimson tipped shrub willows.

The brilliant colors of the shrub willows drew me in for a closer look.  Up close, they have yellow stems with bright red tips.  The greenish yellow of the lower stems is probably a mix of chlorophylls and xanthophylls (here is an explanation of pigments in this earlier post).  The red is likely due to anthocyanins, but there is almost certainly chlorophyll also present in the twigs masked by the stronger red pigments.

Willow twigs with crimson tips.

Red is a common pigment 'choice' for plants that are active in cold weather.  The red may act to filter out some excess light and act as a sunscreen for the plant.  Plants can't photosynthesize as quickly when it's cold out, and too much light can overload the slow system.  Red pigments also tend to absorb more heat than other pigments, and even a tiny increase in temperature can increase the rate of photosynthesis.  In this crimson-tipped willow, the narrow tips would be especially likely to freeze, so red pigments there could help them be more active in the cold.  Alternatively, since this plant is growing in a botanic garden, it is likely the product of selective breeding for aesthetically pleasing but physiologically useless traits - so the colorful twigs could just be pretty and not useful at all.

Red dogwood twigs.

Above you can see entirely red twigs of a shrubby type of dogwood.  I can attest that many types of dogwood twigs are often red in the wild as well as in botanic gardens.  People and nature seem to favor red twigs for winter growth.  The overall effect (below) of these red twigs is startlingly beautiful.

Dogwood shrubs.

Many plants opt for green chlorophyll for winter twigs, as seen this variety of rose-related shrub below.  These stems can actively photosynthesize any time the temperature and light are favorable.  The tough, thick stems are able to survive freezing where leaves cannot.  When the temperatures rise to predictably non-freezing levels, these roses will leaf out and photosynthesize in earnest for the growing season.

Rose stems.

When we came to Chicago in October, forecasts said it would be the worst winter ever.  Instead, it's been a record-breakingly warm winter.  Spring seems to be competing to outdo winter's numbers.  It's been 80 degrees for days now.  Plants that use temperature as a trigger to emerge from winter's dormancy are already leafing out.  Those that use day length as the gauge for the start of spring still look like they should for this time of year - leafless and grey.  I suspect the day-length strategy will work better this year, since Chicago has been known to have freezes into April.  Trees that leaf out early stand a good chance of having to grow new leaves after their first ones get frozen off.  Late leaf growth combined with twig pigmentation is a good strategy for climates with unpredictable spring temperatures.  Using twigs to photosynthesize can give a tree a good head-start on the growing season without the risk of having tender plant parts frozen off.

How to Read a Twig

Everyone knows that you can chop down a tree, count the rings in the wood and tell how old the tree is.  Trees grow in spurts, usually one growth spurt per spring/summer, and they add a new layer of tree where the bark meets the wood.  They add a new, microscopic layer of bark, and a layer of wood thick enough to count when the tree is cut down.  More on bark and wood at this previous post.

I bet you didn't know that you can figure out how old twigs are too, and you don't even have to cut them down to tell.  Twigs have so much to say once you know how to read them.  This post is best if you have a twig and a magnifying lens nearby, but in case you don't, I've included some fabulous drawings to help you.  I hired a professional artist and spared no expense.  Twig details are usually too small to photograph well, unless you have a buckeye twig which are out-sized and easy to see.  I haven't found a buckeye in Chicago yet, so you'll have to make do with these drawings instead of photographs.  All the parts I'll discuss here are visible on all twigs, but most are a little smaller.

Artist's rendering of a buckeye twig on archival-quality notebook paper.

 First, find the buds.  Buds are live twig-tips covered in scales and protected to survive the winter.  The end buds, called terminal buds, are at the ends of twigs, and they are usually the largest.  If you cut open a terminal bud under a microscope, you would see layers of yet-to-be expanded leaves and stem material.  In the spring, terminal buds grow long and send out leaves, extending the twig.  There are also axillary buds, which grow just above every leaf a tree has (unless the tree is a sycamore, then axillary buds grow inside the leaf stem base).  Axillary buds can expand into twigs and leaves too, but they are usually much smaller and less mature.  They often only grow in spectacular growing seasons or if the terminal bud is cut off.

Buds - terminal and axillary

 In the winter, you can see scars on the twigs where previously-grown leaves have fallen off.  Axillary buds are above the leaf scars.  Buds are covered by bud scales, tough leaf-shaped brown things protecting the bud in winter.  Bud scales fall off in the spring when the bud starts to grow, and they leave scars that encircle the twig in tiny, bunched lines.  If you look back from the tip of a twig, you can find scars from last winter's bud scales.

Bud scales, bud scale scars and leaf scars.

 Since bud scales are produced each winter and fall off each spring, they leave annual scars on the twig.  You can tell a twig's age by counting back from the tip of the twig and seeing how many sets of bud scale scars are present on the twig.

Twigs also have tiny dots on them called lenticels.  If you zoomed in on a lenticel, you would see an opening with a tiny mouth-like structure that can actually open and close.  The opening is called a stoma, and it is for allowing exchange of oxygen and carbon dioxide into the air - basically it allows the plant to breathe.  You have probably heard of stomata (plural of stoma) on leaves before, and these are the same thing, just on bark instead.

Lenticels and age of twig

 The age of the whole twig can usually be determined by counting back from the longest branch.  You can tell the age of side branches too, by counting the bud scale scars back from their tips.  You can also tell how good the growing season was by the distance between bud scale scars.  A good growing season will have a long distance between bud scale scars.  A dry or cool year will have less growth.

Deer like to eat terminal buds off of edible trees because of the tender, living material inside.  When that happens, axillary buds are activated, and often several side-branches will grow during the next summer.  This can give a branch a bushy appearance.  When you trim a branch by chopping off the end of it, the same thing happens.  Whatever grows back will be bushier.  If you trim off an entire side branch where it joins a larger branch, the next year's growth will not be bushier - instead the terminal bud elongates and the branch just gets longer.  You can influence the shape your trees and bushes by trimming off side branches completely to encourage them to grow in the direction of the remaining terminal buds.

Twig life history

How Trees Work

Trees' general strategy as plants is to grow slowly, put a lot of time into building a structure that gets leaves closer to the sun, and eventually out-compete the fast-growing soft plants and shrubs that can't get off the ground.  The part that makes a tree a tree, namely the wooden trunk, serves as both support for holding up leaves and the means transportation of materials from the leaves to the roots and back. 

This is a tree.

 Tree trunks are strong because their cells have surrounded themselves with lignin, the hard material in wood.  There are two basic tissues in tree trunks: wood (the inside), and bark (the outside).  Guess which one of these materials is alive?  You'll have to wait just a minute to find out.  The wood part of a tree trunk contains mostly lignin, so it's very strong.  The bark contains more suberin and much less lignin.  Suberin, the subject of a previous post, is a softer substance, and if you have ever squeezed a wine cork, you know the texture of suberin.  Wine corks are cut from the bark of the cork oak. 

The wood of a tree is produced by plant cells growing very long, surrounding themselves with lignin,  leaving a few tiny holes in each end to connect to the next cells, then dying and leaving behind hollow tubes of lignin.  Yes, the interior of trees is mostly dead.  The hollow tubes connect down to the roots and up to the leaves.  The tubes are so narrow that water can pull itself up through the tubes by capillary action.  Capillary action occurs because water is pulled more by the chemical attractions of water to the lignin than by the pull of gravity.  Capillary action works in trees as long as the lignin tubes are very thin and the tree isn't too tall.  You may remember from 9th grade Biology that the material which transports water in plants is called xylem.  In fact, wood is almost entirely xylem.

Bark, the living part of a tree trunk, is composed of mostly phloem and suberin.  Phloem transports sugars, the tree's food, dissolved in water.  If you look at bark under a microscope, you would see that it, like wood, is composed of microscopic tubes running up and down the trunk.  Unlike xylem, phloem tubes can run up or down, depending on the season.  In the summer, the phloem is busy transporting extra sugars and nutrients down into the roots for winter storage.  In the early spring, phloem brings that stored sugar back up to provide energy for the new spring growth.

A tree will die if the bark is cut all the way around the tree.  The tree below has been girdled.  It is a white poplar, a weedy tree, that is probably interfering with the native plant restoration going on in Lincoln Park.  The ecosystem manager probably also applied an herbicide to the bottom cut on this girdle so it would be pulled down into the roots.  White poplars are notorious root-sprouters, and if you don't kill the roots, you will have new mini-trees coming up all over the place.

A girdled tree.

This tree has also been girdled, but not by humans.

A girdled green ash.

Bark is infinitely variable in its patterns and characteristics.  Here's my favorite bark - white birch.  You can tell a lot about a tree by looking carefully at its bark.  This will be the subject of a future post!

Beautiful birch bark.