WHY LEAVES CHANGE COLORS IN THE FALL
1) Pass around leaf specimens and let students examine them. After about ten minutes, collect leaves and ask students to take out paper and pencil for note taking.
2) Discuss why leaves change colors in the fall. Say something like the following:
Everyone enjoys seeing colorful fall leaves like these -- especially when they are on the trees -- but how many of you know why some leaves change colors in autumn? (Allow time for answers.)
First, let's talk about what kinds of plants have leaves that change color and fall. They are perennial plants that are broad-leaved and deciduous. (Write these 3 words on the board.)
Perennial, which is derived from the Latin words per(through) + annus(year), means through the years, so perennial plants are those that live for many years. In order to survive for more than a year, these plants need some way to protect themselves from the extreme cold and other harsh conditions of winter.
Broad-leaved plants have leaves with some width; they don't have needles or scales. Because their many, broad leaves are filled with watery sap, winter presents two big problems for most of these plants -- the potential freezing and destruction of tender leaf tissues and a lack of water to make the required sap when the water is frozen in or on the ground.
Deciduous, which derives from the Latin decidere meaning to fall off, describes plants that lose their leaves before entering a period of dormancy.
Most of the trees and many shrubs and herbaceous plants that inhabit New England are deciduous, broad-leaved, perennial plants. The leaves of these plants have an abscission layer at the base of each petiole. (Draw a leaf on the board and label the blade, petiole, and branch.)
A petiole is the stalk or stem connecting the blade of a leaf to the branch.
The abscission layer is made of cells which die in the fall producing a corky layer that seals the leaf off from the branch. This stops the flow of water and sugars between the leaf and the branch and eventually separates the leaf from the branch. (Show the abscission layer on the drawing; show how it separates the leaf from the rest of the plant.)
With the leaves gone, these deciduous plants rest from the work of photosynthesis, don't require much water, and are much less likely to freeze and die. So God has made these plants to shed their leaves and become dormant in winter so they can survive for many years, but we still haven't said why the leaves change colors before they fall off. Here's a short poem that hints at three main causes.
Now do you have any ideas? (List their responses on the board then go on to explain)
The main factors which cause leaves to change colors are length of night, leaf pigments, weather, and genetics. The most important reason why leaves change color (and why the cells in the abscission layer start to die) is the increasing length of night. Steadily increasing hours of darkness signal the plant that it is time to prepare for winter, time for the cork layer to start forming and for other biochemical processes to begin. The date when this happens is consistent from year to year since it is caused by the earth's revolution around the sun and the tilt of the earth's axis. (Draw a picture on the board showing how the northern hemisphere is tilted away from the sun in winter.)
The second factor influencing leaf color is leaf pigmentation. The main pigments found in leaves include: chlorophyll, carotenoids, and anthocyanins.
Chlorophyll is the pigment which makes leaves green and is required for photosynthesis. During the growing season, chlorophyll is constantly being produced and broken down, but as nights become longer, chlorophyll production slows and then stops. Because chlorophyll is not stable, especially in cold weather and bright sunshine, it disappears from the leaves. When it does, other pigments in the leaf begin to give the leaf its color.
Carotenoids, which are found in some plants throughout the growing season, are more stable pigments that do not break down quickly like chlorophyll. Carotenoids can absorb some sunlight and transfer its energy to the chlorophyll for use in photosynthesis. Leaves with these pigments are yellowish-green throughout spring and summer and usually turn yellow, bronze, or orange in autumn when the chlorophyll disappears.
Anthocyanins are rarely found in leaves before autumn. These pigments are produced in the leaves of some plants when the sugar concentration in the leaves increases because the corky abscission layer prevents the sugars from flowing out of the leaves into the branches. In the presence of sunlight, the concentrated sugars react with proteins in leaf sap to form anthocyanins, and these pigments turn the leaves either red or purple. When the leaf sap is very acidic, anthocyanins make the leaves bright red; when the sap is less acidic, anthocyanins impart more purple. It is interesting to note that these pigments don't seem to serve any biological function. I think that God put them in these leaves just to make them more beautiful for us. He is such a creative and loving God that he's made dying leaves into a dazzling display of color when He could have let them all turn brown as they prepared to fall.
The third factor influencing leaf color is the weather both during and prior to autumn. Temperature, moisture, and sunshine effect the variety and brilliance of the colors that develop. A warm, wet spring followed by a pleasant summer and a sunny, dry fall with cool but not freezing nights produces the most dazzling colors. When autumn is sunny, dry, and cool at night, sugars formed during spring and summer become most concentrated in the leaves, and this favors the greatest formation of anthocyanins, the pigments which turn leaves red, crimson, and purple. There is less variation in the brilliance of the leaves which turn yellow because the carotenoids are always present in those leaves. Warm or unusually wet weather in autumn results in less intense colors, and late spring or summer droughts delay the onset of color in the fall. Because weather is so variable from year to year, its effects on the timing of color changes make predictions of peak colors difficult.
A fourth factor effecting color and timing of color change is genetics. Different types of trees growing in the same forest change colors at different times, and neither elevation nor habitat seems as important as genetics because all the trees of a particular species at a given latitude change color at about the same time whether in the mountains or the lowlands. A species which changes color earlier than most, often in late summer, is sourwood. Oaks are species which change color later than most, often after other trees have lost their leaves.
Lastly, I will note that each deciduous species turns the same colors year after year because its DNA produces only certain pigments. All green plants produce chlorophyll, but only certain plants produce carotenoids and/or anthocyanins. Let's go to the computers now and use the Internet to help us identify the typical fall colors of a number of tree species. Then we'll compare what we find there with the colors of our leaf specimens.
3) Go to the computers and help students find the computerized worksheet on species and fall colors. Have students access the Internet and complete the worksheet.
4) Pass out the leaf specimens again so students can compare these with their worksheet lists, and so they can simply enjoy looking at God's handiwork.
5) Read the poem Laughing Children and thank God for His gifts of leaves and love!
This lesson created by Darlene Smalley