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.

http://education-portal.com/academy/lesson/structure-of-plant-stems-vascular-and-ground-tissue.html

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.



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