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.

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. 

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