Friday, March 08, 2013

Little Green Cells, Part Four (Natural History Lesson)

Adapted from "The Life of a Primrose" in The Fairy-Land of Science, by Arabella B. Buckley. 
Part One
Part Two
Part Three

Part One:

Can you tell the life story so far of the imaginary primrose plant?  Do you remember how the water gets "pumped up" into the leaves?  What is the name for that process? 

Before we read the first part of today's lesson, we need to talk about (or review) some ideas about the way we see colours.  In an earlier chapter of the book, the author says this:
Reflected light-waves not only make us see things, but they make us see them in different colors. What, you will ask, is this too the work of the sunbeams? Certainly; for if the color we see depends on the size of the waves which come back to us, then we must see things colored differently according to the waves they send back. For instance, imagine a sunbeam playing on a leaf: part of its waves bound straight back from it to our eye and make us see the surface of the leaf, but the rest go right into the leaf itself, and there some of them are used up and kept prisoners. The red, orange, yellow, blue, and violet waves are all useful to the leaf, and it does not let them go again. But it cannot absorb the green waves, and so it throws them back, and they travel to your eye and make you see a green color. 
 There's another good explanation of how we see colours at Art Smarts 4 Kids.  Then should we have a musical interlude with Harry Chapin?

More from Arabella Buckley: 
If you have ever tried to grow a plant in a cellar, you will know that in the dark its leaves remain white and sickly. It is only in the sunlight that a beautiful delicate green tint is given to them, and you will remember that this green tint shows that the leaf has used all the sun-waves except those which make you see green; but why should it do this only when it has grown up in the sunshine?

The reason is this: when the sunbeam darts into the leaf and sets all its particles quivering, it divides the protoplasm into two kinds, collected into different cells. One of these remains white, but the other kind, near the surface, is altered by the sunlight and by the help of the iron brought in by the water. This particular kind of protoplasm, which is called "chlorophyll," will have nothing to do with the green waves and throws them back, so that every little grain of this protoplasm looks green and gives the leaf its green color.

It is these little green cells that by the help of the sun-waves digest the food of the plant and turn the water and gases into useful sap and juices. 
When this book was first published in 1888, the name for this process didn't exist; it was first used by American botanist Charles Barnes in 1893.  Maybe you already know its proper name.  If you don't, see this page.  (Link fixed.)

Narration to follow.

Part Two:

What is carbon?  Where do you ever hear that word?  Do you know what a carbon copy is?  What are the bubbles in gingerale?  Look at this page about carbon at the Chem4Kids website.  Did you know that humans are about 18% carbon? 
When we breathe in air, we use up the oxygen in it and send back out of our mouths carbon dioxide, which is a gas made of oxygen and carbon.   Now, every living thing wants carbon to feed upon, but plants cannot take it in by itself, because carbon is solid (the graphite in your pencils is pure carbon), and a plant cannot eat, it can only drink in fluids and gases. Here the little green cells help it out of its difficulty. They take in or absorb out of the air carbon dioxide gas which we have given out of our mouths and then by the help of the sun-waves they tear the carbon and oxygen apart. Most of the oxygen they throw back into the air for us to use, but the carbon they keep.

If you will take some fresh laurel leaves and put them into a tumbler of water turned upside-down in a saucer of water, and set the tumbler in the sunshine, you will soon see little bright bubbles rising up and clinging to the glass. These are bubbles of oxygen gas, and they tell you that they have been set free by the green cells which have torn from them the carbon of the carbon dioxide in the water.

But what becomes of the carbon? And what use is made of the water which we have kept waiting all this time in the leaves? Water, you already know, is made of hydrogen and oxygen, but perhaps you will be surprised when I tell you that starch, sugar, and oil, which we get from plants, are nothing more than hydrogen and oxygen in different quantities joined to carbon. 
In the "don't try this at home" category:  the teacher proves that when you take the water out of a plant, what's left is mostly carbon.
It is very difficult at first to picture such a black thing as carbon making part of delicate leaves and beautiful flowers, and still more of pure white sugar. But we can make an experiment by which we can draw the hydrogen and oxygen out of sugar, and then you will see the carbon stand out in all its blackness. I have here a plate with a heap of white sugar in it. I pour upon it first some hot water to melt and warm it, and then some strong sulphuric acid. This acid does nothing more than simply draw the hydrogen and oxygen out. See! in a few moments a black mass of carbon begins to rise, all of which has come out of the white sugar you saw just now. You see, then, that from the whitest substance in plants we can get this black carbon; and in truth, one-half of the dry part of every plant is composed of it.

Now look at my plant again, and tell me if we have not already found a curious history? Fancy that you see the water creeping in at the roots, oozing up from cell to cell till it reaches the leaves, and there meeting the carbon which has just come out of the air, and being worked up with it by the sun-waves into starch, or sugar, or oils.
Narration to follow.  Have a good weekend!  Keep your beans moist!

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