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.

Meet the Juniper (AKA Eastern Red Cedar)

Middle Tennessee is known for its cedar trees.  We even have a state park called Cedars of Lebanon.  The trouble is, the trees we call cedars are actually a type of tree called a juniper.  Our cedars have all the plant parts and structures of junipers, yet we call them Eastern Red Cedars.  Names are difficult to change once we get used to them, but I'm going to call our cedars junipers in this post.  The scientific name of our Middle Tennessee junipers is Juniperus virginiana, meaning juniper of the Virginia region.

Meet one of the junipers at our outdoor classroom.

Junipers have a million interesting characteristics.  My favorite thing about them is the difference in their immature and mature foliage (leaves).  Their mature leaves are smooth, rounded overlapping scales like you see in the picture below.  Junipers' immature foliage is sharp and spiky.  The easiest way to tell if foliage on a juniper is young or older is to close your eyes and feel the difference.  Fortunately, junipers are evergreen trees, so this is a great time of year to investigate their foliage.

Mature growth on a juniper.

Below is a juniper tree whole foliage is almost entirely immature.  It is about as big as the juniper in the picture above, so it must be about as old.  Spiky immature foliage protects young junipers from being eaten by deer or other animals.  I noticed the tree below was damaged and had its main stem cut.  Perhaps the tree is maintaining spiky, defensive foliage in response to what must have felt like a big bite to the tree (if trees could feel).

Spiky immature growth on a juniper.

On the branch below, you can see both mature foliage on older growth and immature foliage on new growth.  You can also see the cutest, tiniest cones you ever saw on an evergreen tree.  Juniper trees have two varieties: just like humans, they come in male and female forms.  Males trees produce cones like the ones you see below.  Female trees produce slightly larger purple cone-structures we call berries (because they look like berries).  Female trees must be larger before they can produce berries, so I didn't see any berries on our juniper trees, but we should have some in a few years.  We'll have to wait a few years to find out if any of our junipers are females.  Juniper berries are technically somewhat edible, but they have such a strong flavor that they are used mostly as a spice or flavoring.

Male cones, mature growth and immature growth on a juniper.

Below is the real prize.  I found one of these at the outdoor classroom, and I'm not telling where - you're going to have to find it!  No, it is not a piece of gum that someone stuck in the tree.  It is a fungus called cedar apple rust.  It grows on junipers (ok, cedars) for half its life, and it grows on apple trees for the other half of its life.  On junipers, it forms brown hard globs for most of the year.  After a warm spring rain, each of the dimples in the brown glob will sprout a bright orange spaghetti-shaped strand (I'm not making this up!).  The orange things produce spores that float away on the wind, land on a growing apple, and make the surface of the apple look splotchy.  The apple-stage of the fungus then makes spores that float and land on a juniper.  And you thought human life was complicated!

Juniper with cedar apple rust.

Why Don't Fish Need Mittens?

Brrrr!  It's been cold the last few nights!  Air temperatures dropped into the mid 20's, which is way lower than freezing.  I bundled up in many layers to survive being outside for about an hour last evening.  I felt a little bad for the fish in our outdoor classroom - they are stuck in cold water without any hats or mittens or even hot cocoa to warm them up.

A mosquitofish alive and well after several nights of freezing temperatures.

Humans are like tropical animals in terms of their thermal comfort zone.  We are comfortable living in temperatures in the 60's to 90's on the Fahrenheit scale.  We have created many devices to keep ourselves at a comfortable temperature: clothes, buildings, heat, air conditioning, insulation and ice cubes all help us maintain comfortable body temperatures whether we are in the tropics or in the Arctic.  Animals can be classified as endotherms or ectotherms, and we are of the endotherm variety.  Endotherms use some of the energy in the food they eat to keep their bodies warm.  Even though the temperature of the air inside our buildings is usually around 72 degrees, our bodies stay at 98.6 degrees.  Mammals, birds and even some fish like tuna can keep their body temperature warm using energy from food.

Mosquitofish are happy as clams in a much broader range of temperatures than we can stand.  They can live in the very warm water of shallow sunny pools in the summer, and they can survive a fairly cold winter too.  Mosquitofish are ectotherms, like most fish, amphibians, reptiles, insects and mollusks.  They don't keep their body temperature warm - they let it cool off when the environment cools off.  And as the temperature drops, they simply slow down.  Their bodies move more slowly, they eat less food, and they stay more hidden.  Many ectotherms hibernate, essentially sleeping in a cold state until the weather becomes warm enough to move around again.  If you watch our mosquitofish, you will notice that they are much slower on cold days than warm days. 

Mosquitofish can't survive if the pond freezes all they way through.  Fortunately for them, water temperature usually doesn't get as low as air temperature, so the pond is going to be warmer than the air temperature, and it won't usually freeze.  Also, ponds freeze at their surface, then the ice acts as an insulator, keeping the lower layer of the pond from freezing.  So even if you see ice on our pond this winter, it is likely that the mosquitofish will be swimming slowly in the water under the surface. 

Do mosquitofish feel cold?  I don't know.  I suppose you would have to put a mosquitofish in a fish tank with a cold area and a warm area and see where it chooses to spend its time!

Here are some other ways you can see organisms responding to the temperature at the outdoor classroom this week:

It's easy to see which plants survive freezing right now.  I'll write about this more in the deep winter, but it's probably easier to see now before the dead plants blow away and decompose.  The dead leaves in the picture below didn't survive freezing.  Either their seeds will survive the winter or their roots will survive in the ground, but it will not grow again until the spring.  The plant on the left is just fine with freezing temperatures, and it will stay growing, though very slowly, through the winter.  there are lots of winter-growing plants in our classroom.

The fern on the left survived freezing, the plant on the right did not.

The honey bees are still drinking at our pond on warm days!  They must have a fairly warm location for their hive.  Bees do some temperature regulation of their hives by eating food then shaking their wings really hard inside the hive to generate heat.  Our bodies do a similar thing - they shiver to generate heat.  Bees also flap their wings to fan the hive if it gets too hot.  Even though insects are ectotherms, bees have some endotherm ability.  Neat!

Honey bees are still drinking from our pond on warmer days despite the freezing nights.

Help! I've Got Hackberry Leaf Galls!

Last week students in our outdoor classroom sent the picture below, wondering what it was.  It's a very logical question, since those...things are growing out of what is obviously a leaf, but no normal leaf has weird miniature mushroom-shapes growing out of it.

Hackberry Leaf Galls (photo: M. Sherman)

The leaf above comes from a hackberry tree, whose bark I think is fantastic, and which we will explore later in the winter.  We have a gigantic hackberry tree in the outdoor classroom, and it's at the end of the row of parking spaces near the road.  Here's our hackberry:

Hackberry tree with most leaves already gone for the winter.

If you search through the fallen leaves around the classroom, you can find lots of hackberry leaves right now.  They have toothed edges (lots of tiny points), and they narrow to a tip.  Also, the wider end of the leaf usually is lopsided with one side larger than the other.  Most of the hackberry leaves have one or more of those big lumps on the lower side of the leaf.  The lumps are called leaf galls, and they are scar tissue the tree has grown in self defense against a parasite.

Three hackberry leaves, two with galls, one without.

So, what exactly is a parasite, you ask?  A parasite is a small organism that lives on a larger organism and often uses the larger organism for food, harming the larger organism in the process.  The larger organism is called the host.  There are lots interesting types of parasites in this world.  Dogs and cats sometimes have fleas for parasites.  Deer often have ticks.  Humans can sometimes have lice.  And plants can have parasites too.  The parasite on our hackberry leaves can only live on hackberries, not humans.  It is a type of insect called a psyllid (SIL'-id).  Psyllids look just like tiny cicadas - smaller than a grain of rice.

I broke open this gall, but it was empty.  The adult has already emerged from it.

Hackberry leaf psyllids lay their eggs on the underside of hackberry leaves in the spring.  The eggs grow into immature psyllids that look like this.  The psyllids damage the leaves, which causes the leaves to grow a lump of scar tissue (a gall).  The psyllids eat hackberry sap and live inside the gall as they grow larger through the summer.  In the fall, the psyllids grow into adults and drill out of the gall.  They fly or crawl to find crevices in bark or buildings to overwinter safely.  When the weather warms up in the spring, they lay eggs and start the cycle again. 

Hackberries grow well in Middle Tennessee, yet they almost always have hackberry leaf galls damaging their leaves.  The trees don't seem overly harmed by the gall psyllids' damage.  This is normal parasite behavior.  Most parasites don't cause extensive harm to their hosts.  They take just a little food from them but not enough to kill them.  If a parasite ate too much of its host and killed it, the parasite would be out of food and would die too.  Parasites use their hosts in a sustainable manner so that their food source will be available in the future.

Nature's Crayons

I promise this is the last post on plant pigments!  It's a short one, too.  I just want to take advantage of the last of the glorious fall colors. 

The first thing I noticed at the outdoor classroom this week is that the pond has turned brown.  The brown color is from tannins in the oak leaves that have fallen into the pond.  Our pond water has basically turned into oak tea.  The mosquitofish in the water don't seem to mind.  They are a very tolerant species.  High concentrations of tannins in water can alter the water chemistry, changing the types of organisms that can live in the water.  If we had fragile aquarium fish in our pond, we would want to change the water, but our organisms (fish and snails) are adapted to a wide variety of water conditions, including tannins.

Tannins from oak and maple leaves have turned the water brown.

The next thing I noticed at the classroom was leaves of every plant color you can think of.  See if you can name the pigments in the leaves from our classroom.  Here's a reminder of the plant pigments:

• Chlorophyll = green
• Tannins = brown
• Carotene = orange
• Xanthophyll = yellow
• Anthocyanins = red and purple

Xanthophyll in a maple leaf.

Tannins in a maple leaf.

Chlorophyll in magnolia leaves, xanthophyll in the leaf petioles.

Anthocyanins in sourwood leaves.

Chlorophyll and anthocyanins in oak leaf hydrangea leaves.

Anthocyanins, carotene and xanthophyll.

Anthocyanins, carotene and xanthophyll.

These pictures are just the tip of the iceberg.  When you walk through the outdoor classroom this week, see how many differently-colored leaves you can find.  Bring your fall Crayons if you like, and draw what you see.

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.

Weeds: The Superhero Gang of the Plant World

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

A clover plant thriving in a nutrient-depleted patch.

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

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

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

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

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

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

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

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

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.