Mrs. Schlumbohm's class is a 2nd/3rd grade class at Salcha Elementary that has been working with OneTree Alaska for several years. Each year, Mrs. S's class builds on the interests and questions generated during the previous year. This year her students are studying the effects of different light treatments on Alaskan birch tree seedlings. In their day-to-day observations, the students find they have many questions. Some they discuss and resolve through hypothesizing and continued observations; others they send along to us, Dr. Jan Dawe and Nicole Dunham. We post our answers here, with the intention that this become an ongoing dialogue. Please join in!
These are photomicrographs taken by Mrs. Schlumbohm's class in Salcha while observing their birch tree specimens through a microscope.
Upon closer investigation of the amazing photomicrographs Mrs. Schlumbohm's class sent us, we see stomata on the bottom of the birch leaf surface! Stomata are critical to photosynthesis and plant function because they allow carbon dioxide into the leaf, and oxygen and water out. We marked many of the stomata with arrows, but can you find more in the picture?
Questions from 2/1/16
Our Feb. 1 observations are yielding more questions.
Gabby: Why are two of my leaves turning white? They are both at the bottom and small and up against the stem.
Great job describing what you’ve observed, Gabby!
Those two leaves are turning white because the tree turned off its water supply to them. In birch, fresh green leaves signal the presence of active chlorophyll. If the chlorophyll isn’t replaced each day as the old chlorophyll breaks down, the leaf becomes loses its green color. And with water supply turned off to the leaves, they couldn’t photosynthesize and make food for their own growth, so they stayed small.
It’s not bad that your seedling turned off the water supply to those two leaves. In fact, it’s probably a signal that your seedling is growing so rapidly that it has to make decisions about where to send its limited resources of water and minerals. Trees are very efficient: they send their resources to where they’ll do the most good for the overall organism. The leaves at the top of your seedling, and the ones on long branches growing away from the stem, are the ones getting the most light, so they can be super food factories, making food for the tree. Your tree turned off the water supply to some of the least productive and most shaded leaves: those on short branches at the base of the stem.
Anthony: Why is my leaf soft?
Your leaf is soft because is young and full of water, and especially because it’s covered with soft hairs.
William R. Why are there hairs on the leaves? We noticed the hairs are only on the top.
Anthony and William R: you two have done a good job of building on each other’s observations. The hairs on the leaf protect the leaf surface from munching insects, and they also help keep the warmth absorbed from the light source inside the leaf, so it can grow faster. The hairs are on the top because those are the surfaces that collect light and warmth, and make great landing sites for insects.
Eli: Why are the leaves at the tippy top small? Rusty says it's because they are new and just started growing.
Good observation, Eli; and Rusty, you’re right! The leaves at the top are the closest to a special region of the shoot called the meristem. The meristem produces all the new leaves on the shoot. The leaves closest to the meristem start out very small and, as they move away from it, they grow larger.
Eli, thanks for giving credit to Rusty for his idea. That’s always important to do in life, and especially so in science. By naming Rusty, you can go back to him with the new information you’ve gained. The two of you can discuss it, and you’ll very likely come up with new questions as a result of your discussion. That’s how science is done.
Bertie: Why is there red in the new growth? It is on the top of my plant on the leaf and the stem and the stems of the leaves.
Good job describing what you’re seeing and where its occurring, Bertie! The red color is due to pigments in the leaf and stems of the leaves. The pigment in the leaf acts as a sunscreen to protect the delicate chlorophyll that’s just beginning to be made. Later, when there’s enough chlorophyll in the leaf, the red pigment will fade away. The red in the stems of the leaves may or may not be due to the same pigment as in the tops of the leaves.
To the next three questioners: Lylah, William R, and Bertie. Lylah: Why are there white dots on my leaf? William R. Why is there yellow on the midrib? Bertie: Why are the edges of the leaves darker than on the inside?
To the three questioners: Lylah, William R, and Bertie: We will need to see the leaves that are showing these characteristics to be able to give a good answer. Great job at looking closely!
Eli: Why are there little veins coming off the veins coming off the midrib?
Nice observation Eli. The network of veins you’re describing is essential for getting water to all parts of the leaf blade, where the work of photosynthesis is being done. The veins act as a two-way highway, taking water and nutrients to every part of the leaf, and taking sugar—the product of photosynthesis—away from where it’s been produced and to other places where it’s needed.
Rusty: Why is there a white dot on my tree (on the bottom)?
Let’s look at the dot. Is there just one or are there dots scattered on the stem? You might be seeing a resin dot, which is where the seedling sends chemicals to help keep away insect and animals that would otherwise want to eat the stem! The resin dots (or glands) taste bitter and nasty!
Questions from 1/25/2016
Bertie: What are the little parts on the stem where the leaves have come out? Is it left over from the node?
What you see on the stem where the leaves came from is likely the stipules. Stipules are outgrowth appendages that come in various shapes and sizes for different plants. Even though there can be a range of shapes and sizes, most perform the same function: protecting the young leaves as they are emerging from their buds. They stay on the stem for some time, but eventually fall off in the case of our birch.
Matthew: How come in the winter squirrels like to bite on the bark of the trees?
Squirrels bite and strip the bark off trees in winter for a variety of reasons, some which we can only speculate about as no one knows for sure. The most likely reason is that winter is very long and difficult, so food becomes hard to find. If squirrels pull the bark away, they can get to the inner bark, where the cambium (somewhat softer living tissue inside the bark) and sugar are, good winter snacks. Scientists also think that pregnant squirrels may rip at bark as a way to deal with pain caused by the pregnancy, and that they may use some of the harvested bark to help make their nests. Chewing on the bark helps keep the squirrels' teeth strong and sharp, as they need big, strong teeth to get their food and make their homes.
Eli: Why is the top of the leaves darker than underneath? (Looking at one leaf. I'm thinking like a shark is light on the bottom and darker on the top?)
Before I answer this question, let's think about the three things essential for a plant to do in photosynthesis. Do you remember the ingredients a plant needs to make food for itself? They are sunlight, water, and air (carbon dioxide specifically, the gas that human beings breathe out). Think about which part of the plant gathers these ingredients? The leaves gather the sunlight and carbon dioxide, while the roots pull in water from the soil and transport it up to the leaves. The leaves are the center of photosynthesis, because the chloroplasts (where the chlorophyll is) process the ingredients to make the food, sugar.
We can observe that our birch trees are darker on the topside of the leaf than on the bottom. This is because the sun shines on the top of the leaf, and the leaf needs to be equipped to handle the rays of the sun. If we don't wear sunscreen when we go somewhere very sunny, we can get a sunburn. A leaf protects itself from the sun's rays by making the top of the leaf resistant to the sun’s harsh rays. Additionally, it is a darker color so the leaf can absorb the exact rays it needs to do photosynthesis. As your leaves get older and stronger, you will notice the top of them becomes waxy. This waxy layer protects the leaf from the sun, so that water doesn’t evaporate out from the top due to the sun’s heat. The waxy layer is not present on the bottom of the leaf, so water can still escape from the bottom. In fact, the bottom of the leaf needs to have openings to the air, because that is how carbon dioxide gets into the leaf. Tiny pores on the bottom of the leaf called stomata open and close, allowing carbon dioxide in, and oxygen to escape. When the leaf has water, carbon dioxide, and sunlight, photosynthesis can happen in the leaves and the plant is able to make itself food.
William: When will the whole stem turn green?
We think that the stem shows a green tinge as it becomes hydrated, but just because the stem isn't showing green doesn’t necessarily mean it isn’t hydrated. Hydration happens at different rates for each plant, so we can only hypothesize and observe, as you are currently doing.
Rusty: Are some leaves rounded and not pointed at the end?
The shape of our species of birch (Betula neoalaskana) is usually the same; pointed near the end, with double serrated margins (meaning the teeth on the edges of the leaf have little teeth on them!). Sometimes, a mutation — a change in the plant’s DNA — will cause a variation in a characteristic, life leaf shape. Your birch tree may have such a mutation. The offspring from one of the mother trees in the T-field have leaves that curl towards their tips!
Matthew: Why are some leaves at the top rounded and the rest are pointed?
The smallest leaves at the top sometimes appear rounded until they grow to their full size: then they look more pointed, like the older leaves, further down the stem.
Bertie: How big do the leaves normally grow and how fast?
You are in an excellent situation to answer this question by making your own observations! Consider doing this: choose a leaf toward the top of your seedling. Make a sketch that locates that leaf on the seedling in your Grinnell Journal: you should mark down which node it comes from. If it’s on a long branch, count the number of leaves from the bottom of the long branch, and mark your leaf in its position. That way, if the long branch continues to grow, you’ll still be able to tell which is the leaf you’re measuring. Next, make a sketch of the leaf you’ve chosen right next to the sketch you already made of your seedling. Measure the length and widest point of the leaf you’ve chosen. Write down the measurements in your Grinnell Journal, next to the sketch of your leaf. Measure your leaf each week for the next 4-5 weeks. Remember to date your sketch each time! After your close observations, you’ll be able to give the starting and ending length and width of your leaf, and answer the question of how fast it grew!
Botanists are always measuring and describing the shapes of leaves and flowers. They use their measurements to describe the range in normal growth patterns for the species they’re observing, and we go to certain books to find the most authoritative information about the species—and how it differs from closely-related species.
Jack: Something has been eating off my leaves. How did the bugs survive dormancy? They would have frozen.
While in the refrigerator, the trees were kept at 38°F, just above freezing. The temperature was chilly enough for both the trees and the bugs to slow way down for winter—to go dormant—but still perk up when they were put out into warmer temperatures, like that in your classroom. Your seedlings were growing outside before we dug them up and put them in the conetainers: likely there were bugs in the soil that became chilled in the refrigerator, but were still alive!
Eli: Why are all the long branches light green?
The long branches are light green because they are made of brand new, rapidly-growing tissue. They contain a lot of water and are fragile, as the new cells haven't had time to build up any protection. As the tissue gets older, it will become more tan, then brown and woody.
Eli: Why does it grow brown around the holes that the bugs have been chewing?
There is brown around the holes that the bugs have chewed because as they chew, they damage the leaf by tearing into the cell walls of the tissue. As the interior of the cells are exposed to air, they turn brown. As long as the damage doesn't kill the entire leaf, the leaf can survive by blocking off the area where the bug damaged it. The brown area is likely where the leaf has protected itself from the damaged area so it can retain partial use of the leaf.
Abby: Will we have catkins? Last year we had them.
The trees you are working with this year are young seedlings; they only germinated May 2015. It is impossible for such young birch seedlings to create catkins for reproduction. They don’t have the resources—the energy stored in food they’ve made through photosynthesis—to reproduce yet.The trees you worked with last year were actually branches clipped from fully grown trees that were reproductively active.
Questions from 1/21/2016
Why do spruce trees have photosynthesis in winter?
Great question! You would think spruce trees photosynthesize in winter since they keep their green leaves and we know that green leaves are the only ones that can photosynthesize. Spruce trees should be able to photosynthesize if we think only about that (the color of the needles). So why aren't all those green needles acting like food-making factories, and photosynthesizing in winter?
Well, to answer that, we have to think of all the things the spruce tree needs for photosynthesis to occur in their needles: 1) Do they have bodies to capture the sun's light energy? Yes!! They have plenty of chlorophyll-rich chloroplasts. We can see that,, because the needles are green.
2) Do they have carbon dioxide they can absorb from the air? Well, there's plenty of carbon dioxide available in our Fairbanks winter air, but the spruce needles close their stoma -- their carbon-dioxide absorbing pores— down in the cold, dark winter weather conditions. The trees breathe much more slowly in winter: neither bringing in much carbon dioxide from the air, or sending much oxygen (which is a by-product of photosynthesis) back out.
3) Do they have water circulating throughout the tree and needles?
NO, and this is the main reason our spruce trees don't photosynthesize in winter. There's no water circulating in the tree, because our air temperatures generally stay below freezing, and the soil is similarly very cold. If the temperature climbs above freezing each day for a week or so and stays that way, the spruce trees might begin to photosynthesize.
That's one of the concerns with climate changing: there are more winter thaws in the past 10-20 years than there were before that time. If a thaw lasts for several days, the trees do begin to warm up. Once a certain amount of warming has occurred, the water that's trapped in the spruce stems and needles begins to circulate, Once that happens, the needles can begin to photosynthesize, even if at a lower rate than summer, when the temperature is warmer.
As soon as cold temperatures return, photosynthesis will again stops. That's because ________ (now you fill in the blank. What happens to the water in the stem and needles once the temperature goes below freezing again?) Mathijs filled in the blank with "water freezes!"
Is this normal, a tree peeling? It's birch.
It is normal for our birch trees to peel! That's because of the way birch trees produce bark each year. The tree creates bark in big continuous sheets, that are laid down tight against the growing wood of the tree. As the tree grows from one year to the next, it adds more wood and the diameter of the trees grow outward. But the bark that's been laid down some years earlier has no way to grow. The wood pushes against that bark, which stresses the tree. It's something like how uncomfortable you get if you wear a tight belt and then go eat a big meal. Something has to give! As the tree grows, cracks begin to form in the sheets of bark, and finally it starts to peel in large sheets--revealing fresh white bark underneath! The process repeats over and over.
I think my branch is growing faster than the stem.
Cool!! I bet you're right. Is the branch that's growing faster than the stem close to the top of the tree? If so, it may take over being the leader of the tree. Consider measuring the length of that branch each week, as well as the overall height of your seedling. That'll give you proof about where growth is happening fastest: on a branch below the top of the tree, or in the branch that grows up from the apical bud of the whole tree.
Why are our leaves fuzzy?
The fuzziness on your young birch leaves has a lot of the same functions as the hair on your head: It protects the leaves from insect attacks by limiting where insects can land and start chewing or sucking on the leaf.
The fuzziness also insulates the leaf: so there's not such a big difference in air temperature inside vs. outside of the plant.
As the leaves age, they lose some of their fuzziness. Mostly hairs are only kept only along vein surfaces on the underside of the leaf, and sometimes along the leaf margins as well.
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