Wednesday, July 31, 2013

Plants Class #5: Flowers and Pollination

This was a fun class, and not quite as packed with information. The timing was about right, though maybe a little early, as we did this class in the Spring. By early March flowers were starting to bloom and the yellow pollen was thick on everyones patios and cars. I was able to collect a nice bunch of flowers, plus I went to the store and got a few cut flowers that we could take apart.

As I usually do with these classes, I started with an essential question. Why do plants make flowers? What are they for? I got back a range of answers. Some were "Ummm... because they are pretty?" and some were more, "So bees can have food." or "To make more plants!" Bingo! 

Essentially, flowers are where the plant makes seeds. The point of a flower is reproduction. Many plants have flashy colorful petals to attract pollinators, but some have very unnoticeable flowers. They may rely on smell, wind or pollinators that don't fly.

I gave them a hand-out at this point and we went over the different things that pollenate plants and the different ways plants get those things to do that job for them. 

First off, we have wind pollinated plants. This has a fancy name: anemophily. Most gymnosperms and a few angiosperms use this method. Many wind-pollinated plants are also allergens. Some common wind pollenated plants in our area are pines (that lovely yellow coating we get every Spring), birches, oaks, ragweed and grasses.
pine                                                                                        ragweed

You'll notice that most of these have flowers that are not showy. There is no reason to spend energy on showy flowers if you are just counting on the wind to move the pollen around. Pollen from male cones or spikes needs to somehow get to the female cones or spikes in these cases, so that seeds can form. In some cases, both will be on the same plant, and in some cases there will be male plants and female plants. 

Other kinds of plants have both male and female parts on each flower, and flowers can be self-pollenated or cross-pollenated.

Other than the wind, insects are the most common pollinators, and of these, bees and butterflies do 80% of the work. Bees are by far the most important insect when it comes to pollination, and we rely upon them for many of our crops.

The single largest pollination event in the world is in the California almond orchards, where about one million hives are trucked in each spring to do the job. New York's apple crop requires about 30,000 hives, and Maine's blueberry crop uses about 50,000 hives each year (from Wikipedia: Pollination).

Day pollinated flowers tend to be colorful and flashy. Many offer sugary nectar as a reward. In return, the insect unwittingly carries the pollen from flower to flower. Bees drink nectar but also gather pollen to feed their young and make honey with (which is food for getting through the winter). There are many types of bees. You many also find beetles or other bugs also feasting on what the plants have to offer.

Long tubular flowers can only be pollinated by things with long tongues, like hummingbirds. Hummingbirds also tend to like the color red, so many of these flowers are also red. 
Some plants like moonflowers use moths to pollinate, and many only open their flowers at night.
The rafflesia flower (Rafflesia arnoldii) is the largest in the world and it smells like rotting meat to attract flies.

Occasionally, bats, geckos and mammals are also known to be pollinators.

The next part of the class involved going over the parts of a flower and doing a flower dissection. I handed out some little booklets I had made up for them. On the front was a flower diagram with the parts of the flower labeled. Inside was an unlabeled diagram. I gave them each a flower or two to pull apart and they were to find each flower part and label the inside diagram of the booklet. I asked them to also tape one of each part onto the inside as well. 

They really enjoyed this, and I enjoyed the big pile of flowers all over the table. 

While they did this, I explained that the pollen grain would go down the stigma to the ovary and then the seed could start forming once the pollen and egg had merged.The diagram they had had both "male" and "female" parts labeled on it. I also explained that flowers like sunflowers were actually multiple flowers in one.

They had tulips and another kind of flower (I can't remember the name) to compare and I asked them if they could tell me if the flowers were monocots or dicots. The tulip is a monocot and has flower parts in threes. Some of the kids had brought some additional flowers to class, and we looked at these as well. I asked them to notice differences between the different flowers. 

For homework, they each got a flower parts sheet to color. This way, hopefully they would review the flower parts I had just given them. I got the sheet at, which seems to have a good selection of materials. Here's the link.

Sunday, July 28, 2013

Plant Class #4: Leaves and Photosynthesis

For this class on leaves and photosynthesis, I first asked the kids what a leaf was for. What does a leaf do? Most of them were able to answer at least within the correct ballpark. 

Leaves are the food factories of the plant. They make food from the sun and store it, sometimes moving it to stems and roots by osmosis.

I gave them a hand-out at this point, the contents of which I will duplicate here. Handouts are good in terms of giving them something to look at during and after class and also to let the parents know what we have covered when all is said and done. 

I also gave them each a leaf from the lamb's-ear plant. They are so soft and fuzzy, it's fun to hold them, and it served as an example of what we were talking about. 

While they were holding their leaves, I pointed out the parts of a leaf: tip, base, midrib, lateral veins, petiole and margins. I also showed them a picture from a plant key (How to Identify Plants) and explained that there is great deal of language wrapped up in identification of the parts of the leaf, though they need not worry about this right now.

Next, we looked at a diagram of the inside of a leaf and I pointed out some major features. There is the upper epidermis (skin) covered by a cuticle, or waxy coating, to limit water loss. Under that are the palisade cells, and under the palisade cells is where the xylem and phloem will be in a matrix of spongy mesophyll cells. Finally, the bottom is where the stoma are located, with their guard cells, and this is what regulates the rate of water loss for the leaf and lets oxygen in and out.

The stoma are very interesting. When there is plenty of water, the guard cells get very full and open up, letting moisture and oxygen out. When there is less water, they go limp and close up, conserving water.

The process of water being lost by the leaves and pulled up from the roots is called transpiration, and can be a very important process for various habitats. Sometimes in moist forests, you can even see the moisture in the air as a form of fog above the trees.

I had tried to do an experiment at home demonstrating the production of oxygen by some plants, but it didn't work very well for me. I brought it in for them to see anyway. In this experiment, some aquatic plants or algae (conveniently gotten from our fish tank at home) are put in a glass jar with water. A funnel is placed over the plants and a test-tube is inverted on top of this. If you assemble it all in a bucket, the test tube and funnel will begin full of water. Over time, you should notice an air bubble forming at the top of the test tube. This would be a clear example of oxygen being given off by the plant. 
Again, this didn't work for me, but I encouraged them to try it at home if they wanted too.

Next, I needed to try to explain photosynthesis. Photosynthesis is how plants make food. They need Carbon Dioxide (what we breathe out), water and light to make sugars and oxygen. I went ahead and showed them the chemical formula for this and explained each part. 

6 CO2 + 6 H2O → 6(CH2O) + 6 O2

The sugars are the food the plants make for themselves. Plants also use some of the sugars in their cells, reversing the process, but overall they produce more oxygen than carbon dioxide.

Plants have various pigments that help them to absorb light and use it. Chlorophyll is by far the most common of these. It is green because it reflects green light and absorbs mostly blue and red wavelengths.

Light is energy of a certain wavelength, and our eyes can see only a small range of this. Plants can only use light of a particular wavelength. Many plants have additional pigments to help broaden their ability to absorb energy from the sun.

Some other pigments include:

Caroteniods: carotine (reflects orange), lutein (reflects yellow), lycopene (reflects red - tomatoes), and xanthophyll (yellow)

Anthocyanins: reflect red or blue (in many flower petals)

Betalains: red or yellow (the color of beets)

Many of these pigments are what you see in the fall when the leaves turn colors. The chlorophyll is the first to go away and the other pigments are left behind as the leaf starts to shut down and die.

You can do an experiment to see the different pigments in a leaf. This is a very simple version of a test that is used in laboratories, called chromatography. I got this experiment from here.

In this experiment you take samples from various plants, mash them up with some acetone and then use a coffee filter strip. If you dip one end of the strip in the leaf/acetone solution and let the capillary action migrate the solution up the strip, in the end you will get a series of stripes. Large molecule pigments will migrate slower than small molecule pigments. Here is a picture of my results. I think the best one was the beet greens. You can see the red pigments at the bottom, followed by a band of green, and at the very top, a brown.

One warning for anyone trying this. First of all, let those mashed leaves soak for a while (at least a day). Also, when you start your experiment, make sure there is plenty of ventilation. That acetone is not good for the brain cells.

Finally, I did a brief overview of some interesting leaf adaptations. Some plants have minimized their leaf surface to save water in dry places (cacti), while others have big wide leaves to catch more light in moist and shady places (under a rain forest canopy). Because it is hard to pull water up from the ground when it is frozen, many deciduous plants drop their leaves and essentially go to sleep until Spring. Pine leaves solve this problem by making their leaves small, so that rain, snow and wind falls off or goes thru them. 

As a final fun activity, we made some Sunprints. I got a packet of the 8x12 size sheets and brought in a large selection of leaves and flowers for them to choose from. If you had time, you could have your kids go out and collect leaves of their own to use. Each person was able to arrange leaves and flowers on a sheet and we took them outside to develop. This was a bit of a tricky process to do with 8 kids, but we did manage it with some extra cardboard backing and the use of some plastic sheets used for overhead transparencies. They really enjoyed this.

If we had had more time, we could have also collected leaves and did rubbings of our favorites. I suggested this to them for when they got home. In the meantime, I had given them enough information for one day. Again, there are many things I could have delved into deeper, but for a one hour class for a bunch of 9-10 year olds, this was more than enough!

Sunday, July 21, 2013

Plant Class #3: Plant Stems and How Stuff is Moved Around

This class was interesting because in order to understand how plants passively move nutrients and water around, I had to explain to the kids a couple of physical processes that I don't think most fourth grade science texts cover. I hate that this kind of thing gets left out of elementary science, because how can you understand what is going on unless someone explains these things to you? The books just say, "The plant 'pulls' the water up from the roots." They don't explain how. You are left to assume that this is something they do actively, like we pump blood, or just to wonder why. The unspoken thing here is that you (the forth grader) are too stupid to understand these things right now so you must just accept what we say and you may get to know it later on. Again, I always wished someone would explain the "how" and "why" to me, so I gave it a shot with my group of kids.

I'll get back to that part later, but for now I must start at the beginning. I started them off with a great video I found. It lasts about 7 minutes and gives a really nice overview on plant stems. The video is on the Education Portal, a place that has some nice free lessons on video accompanied by written copies of the lesson.

I told them not to worry about everything in the video, but that I wanted them to remember two things: xylem and phloem and how they are arranged in Monocots versus Dicots.

xylem: carries water and nutrients from the roots to the leaves
phloem: carries food throughout the plant, mostly from the leaves downwards. The phloem carries the sap. 

These two are arranged in a plant in pairs called vascular bundles. In a Monocot, these are scattered about the stem, but in a dicot they are arranged on the outer part, with the xylem on the inside and the phloem on the outside. For a tree, the phloem will be just under the bark. 

When we tap a sugar maple for sap we are tapping into the xylem (even though many sources will tell you it is the phloem). Early Spring with warm days and freezing nights is the only time the xylem will carry all of the extra sugars that we use for maple syrup. The reasons and mechanisms are complicated, but when we tap sugar maples for maple syrup we are essentially bleeding the tree.

I went over these pictures and diagrams with the kids (they each had a hand-out) and then we had some fun with a maple syrup taste test.

I had three types of maple syrup: Grade A, Grade B and Grade C. Grade A is the most filtered of the types of syrup you can buy, and Grade C is the least. Grade C is typically darker and still has many of the other compounds the tree makes. Maple syrup has vanillin and furanones, which give it it's distinct taste. I passed some unmarked samples around and each kid had 3 popsicle sticks with which to taste test them. I asked them to see if they could guess which was which. Most of them were right on the money. 

While they were doing that, I read them some facts from an article I found called the Biology of Maple Sap Flow, an article published in the journal Plant Physiology by Dr. Stephen G. Saupe. Here are some interesting facts:

* A single tap will produce about 10 gallons of sap per season and this produces one quart of finished syrup. It takes about 40 gallons of sap to produce one gallon of syrup.

* The distinctive flavor of syrup is caused by the heating, which changes certain nitrogenous chemicals in the sap. Part of this is the vanillins and furanones. The darker the syrup the more the furanones and the stronger the taste.

* Sap flow requires cool nights (below freezing) and warm days. In central Minnesota, this is typically mid-March to mid-April.

I also gave them some other random facts...

* Tree sap has also given us amber. Amber is a fossilized tree resin. Resin is different from sap and is mostly produced by conifers, probably as a defense mechanism. It's color has made it valuable for jewelry and we can learn about insects from the ones that got trapped in resin hundreds or thousands of years ago. (I had a few samples of amber and was able to pass these around).

* We also use turpentine, which is something we get from pine tree resin and is used to remove paint among other uses.

At this point I had them put everything away and we got down to some nitty-gritty.

The big question:
How does the stuff move? How does the water rise through the xylem and how does the food made in the cells make it to the phloem and the other parts of the plant?

There are two physical things that are happening. In the leaves there are little cells called stomata (something we will cover more when we talk about leaves). The stomata open and close like mouths. When the stomata are open, water evaporates, and the act of water leaving makes room and helps to pull more water up from below. We call this capillary action or cohesion (the tendency of like substances to stay together). This tendency of molecules to want to stay together can actually pull liquid up against gravity into a tube. The water leaving the plant through transpiration helps this along.

I had ordered a pack of capillary tubes from a science supply store and gave them each one to try. We had to be careful with these, because they were made of glass, and any stray tubes on the ground would have been hard to see and could be stepped on. They had some colored water and were able to see for themselves how the colored water went up the tube seemingly by magic. This is exactly how it can work in plant tubes. They may also have seen a nurse use a capillary tube to get a small sample of blood while at the doctor's office.

The second physical process that helps plants to move stuff around is called osmosis. Osmosis is "the net movement of solvent across a semi-permeable membrane". What this means is, if only some things can move across a membrane, things will want to try to even themselves out. If it is water that can move, it will try to even itself out across the membrane. In this way, water or other things can move out of plant cells to other plant cells just by basic physical laws and no extra energy.

As a demonstration of this, I had prepared some eggs for the classic egg demonstration. A quick overview of this can be found here. In short, you take two eggs and dissolve the shells off with vinegar (this can take a couple of days). What is left surrounding the egg is the inner membrane, which is semi-permeable. Next you place one in corn syrup, and one in water. The one in corn syrup will shrink as it looses water to the surrounding syrup and the one in water will swell as it takes on water. The kids got to see this and it's a great demonstration of water moving around. 

When I was done showing them this, I think they understood. It was a little bit of a stretch because these things can be so abstract, but hopefully, even if they didn't really understand, they will remember this lesson later when they go over it again in other classes. 

My final demonstration was some flowers and plants I had colored with food coloring in their water. It was just another fun demonstration of how stuff can move up a plant stem even when the plant is technically not alive. It's also a fun little thing they could do at home if they wanted too. 


These kids love food, so I ended with them getting samples of sugar cane (wonderfully supplied by one of the girls in the group who had just returned from Trinidad). While they had fun eating that I read them some more fun facts:

* William Harvey, the great 17th Century plant physiologist, thought that plants must have a circulatory system like ours. He abandoned the idea after plant dissection failed to reveal a heart.

* The largest stem in the world is the trunk of the giant sequoia tree. It is up to 115m tall and 8m wide. You can drive a car through some!

* Sunflowers bend towards the sun because the sun destroys a growth hormone so the shady side grows faster.

* Bamboo stems are extremely fast growing: giant bamboos can grow 91cm a day! They are also very strong, and can even withstand earthquakes. In Japan they produce square bamboo poles by placing a square wooden mould over the shoots as they grow.

* Although rhubarb stems are sweet to eat, the leaves are poisonous.

* Sugar comes from sugar cane stems (or sugar beets). Children in the tropics chew the stems.

* The sensitive plant, Mimosa pudica, collapses it's leaves when touched. How? It's stem behaves like a nerve and is sensitive to touch. 

I found a nice video of the sensitive plant and they enjoyed it. This is not the same one I used, but I'm sure any kid would enjoy it.