Leaf science: adventure and art of the splendour
Turn a leaf-collecting trip into an adventure into biochemistry ... and art. It's an activity that's most dramatic in the fall, but you can do variations throughout the year
Why leaves usually look green
Most leaves appear green during most of the year. This is because chlorophyll (which uses sunshine, carbon dioxide, and water to produce sugar and oxygen) reflects green light. This masks the other pigments in the leaf. Almost all leaves have another set of pigments called carotenoids, which are yellow and orange. These pigments help with sugar production.
In the fall, we get to see more of the leaf pigments because the chlorophyll factories are shutting down for the winter. Chlorophyll breaks down in the sunlight so if there is not a constant supply, the green colour disappears. In fall, many species of trees prepare for the winter by breaking down chlorophyll and other cell components and moving them into the woody part of the tree. In the winter, the sunlight does not have as much of the blue spectrum available to make chlorophyll. The leaves would also be a drain on tree resources, as the fluids in the thin leaves tend to freeze. Therefore, it makes sense to close for the winter. As the nights get longer, a chemical called phytochrome triggers the activity to prepare for dormancy (good oxymoron – dormancy activities). The leaves are not dying; the tree is actively shedding them. As part of the process, the veins that carry fluids into and out of the leaf gradually close off as a layer of cells forms at the base of each leaf. This is called the abscission layer. When this layer is finished, the leaf drops off the tree.
Winds and rain hasten the process.
Colours of fall
Many leaves seem to turn yellow as fall progresses. Actually the yellow has been there all the time. We can now see it because the chlorophyll no longer blocks it. The bright reds, purples, and crimsons come from a different source – anthocyanin pigments. These form when there is a lot of light and a lot of sugar trapped in the leaf. If the days are bright and sunny, sugar production is high. If the nights are cold, but not freezing, the movement of sugar-rich fluid slows down. These conditions result in the beautiful blazing reds, oranges (red and yellow), deep purples, and intense crimsons. Anthocyanin's colour varies with the pH of the leaf sap. If it is acidic, the colour is red and purple if it is base. Dry weather also serves to intensify the colour because the concentration of sugars in the leaves is less dilute. Some leaves turn brown while on the tree. This is due to the buildup of tannins in the leaves.
Peeping at pigments
Using paper chromatography, you can see separate the pigments in leaves. This works at any time of year, but if you want to separate anthocyanins, you will need fall leaves or leaves from plants naturally rich in anthocyanins. The experiment below describes how to use coffee filters and rubbing alcohol to do this. Chromatography works because the pigment molecules dissolve in the alcohol-water mix, but each has a different affiliation for, or attraction to, the cellulose matrix of the coffee filter paper. This means the colour moves up the filter at differing rates leaving fuzzy bands of different colours. These strips dry, leaving a permanent record of the varying pigments.
Gather the following items:
· Fresh leaves
· Isopropyl alcohol 50 per cent (rubbing alcohol)
· Coffee filters
· Glass cup or jar (one cup to one pint)
· Paper clips or bobby pins
Cut the coffee filters into strips. I recommend the round filters. Flatten them out. Cut off the edges of the circle and discard, then cut the remaining portion into strips about _‰ wide (I iron mine to flatten them). Label each strip with the source of the leaf, the date, and the colour of the leaf. About 1/2 inch from the bottom, rub some of the colour off the leaf in a line across the filter strip using the back of a spoon. The best results come from a thin line between 1/8‰ and 1/4‰ across. Then suspend the strip in alcohol. Place just the end of the strip, below the colour stripe, into the alcohol. Use a bobby pin or paper clip to hold the strip firm to the edge of the jar or glass. It takes five to fifteen minutes for the alcohol to wick up the paper and form the colour stripes. When it finishes remove the strips and let them dry. We ironed ours (again) and pasted them to a page for labeling and preservation.
Collect fresh leaves with plenty of sap. Picking leaves from the tree is preferable. If you do pick leaves off the ground, make sure they are pliable and fresh. I suggest using plastic bags to store them. If you are not going to do the experiment right away, put the leaves in the refrigerator. As you pick up the leaves, you may want to keep a record of where you got them. Keep a log with the date, location, and type of tree – there are many good field guides that will help you. Put a number by the log entry and write that number on the plastic bag using a Sharpie or other pen that writes on plastic. You can take samples from the same tree over a period of time to track the changes in pigment. It might be interesting to take a sample in the spring, summer, and fall from the same trees. While you are collecting, you might want to gather some extra leaves for art projects or to preserve.
One way to preserve leaves is to iron them between sheets of wax paper. Place a towel on the ironing board, then a sheet of wax paper, the leaf, and another sheet of wax paper and another towel. Iron firmly. The wax paper will fuse. Let the paper cool, then peel apart. You will have dried leaves.
Press leaves by sandwiching them between sheets of paper (I write the leaf information on the paper) and putting the sheets of paper into a book. Then place the book on the bottom of a large stack of books. You can also make a press with two pieces of cardboard, newspapers, and paper towels. Lay out the first piece of cardboard, add a layer of newspaper, cover with paper towels, lay out the leaves ensuring they do not overlap, add a layer of paper towels, more newspapers, and so on, ending with the cardboard. You can then tie the bundle tightly with twine or weight it down with heavy objects. It takes about ten days for the leaves to dry.
To save dried leaves, glue them on heavy paper stock, writing who collected the leaf, when it was collected and what it is. Cover with contact paper for added protection. You could also use a photo album to store the leaves.
Leaves can be used in a variety of art projects. Select pliable leaves with strong veins and use them as a stamp. Put the leaf on a piece of newspaper on a hard surface. Paint it using acrylics or fabric paint. Lay paper or fabric over the leaf and rub with a rolling pin. You can also attach several leaves to the paper or fabric with masking tape and sponge paint over them, leaving negative leaves. This also works well with chalk. You can also make leaf rubbings, again this works best with leaves that have strong veins. Put the leaf under the paper. Rub over the top with chalk, crayons, or coloured pencils. You can also make an interesting picture by pounding the leaf pigment onto fabric. Place the leaf on a piece of plywood. Put the fabric on top. Use pushpins to hold the fabric in place. Stretch out any wrinkles. Using a hammer, bang firmly. This drives the pigment from the leaf into your fabric forming a delicate leaf picture. Ironing helps set, and enhance, the colour.
If you use multiple leaves, pound one at a time. It does take a lot of banging. Very fresh leaves are best. This project is not limited to fall leaves, of course. Green leaves will also make interesting pictures.
*This article originally appeared in the online journal Family Explorer and is reproduced by permission of the editor, Larry Sessions