A Little Bit More
Although the autumn color change of tree leaves and their subsequent fall are two of the most obvious phenomena in plants, there is still no clear understanding of the entire process. It is the leaves of the deciduous plants – those that fall off the tree after each growing season – that go through this dramatic color change. It may appear that the evergreens don’t lose their leaves, but they too lose them; they just do so more gradually.
In most plants, the process leading up to leaf loss seems to be stimulated by decreasing day-length. Photoperiodism, (amount of daylight in a 24 hour period) regulates many of the life processes of both deciduous and evergreen trees. With decreasing day-length in autumn, certain cells at the base of the deciduous petioles (leaf stalk) change and form a layer of thin-walled cells called the abscission layer. Soon after formation, this abscission layer begins to disintegrate as enzymes attack this layer’s cell walls. Once this layer is weakened, the leaf may fall from its own weight or the result of wind and weather. In nature there are often , “variations on a theme” and trees such as the red oaks and American Beech keep most of their leaves through the winter and lose them in the spring. These trees usually lack an abscission layer, and the petiole remains durable for months after the leaf dies.
Most growing leaves owe their green color to the presence of chlorophyll – the main machinery for food production in plants. Two other pigments are also present – yellow and yellowish-orange, but not in great enough quantities to be evident in the presence of chlorophyll. As the leaf is cut off from its food and water source, however, the chlorophyll breaks down and as it does, the underlying pigments begin to show through. The deep reds and purplish colors found in sumacs, hard maples, oaks, Sweetgum, and others are believed to result from the accumulation of certain chemical products in the leaves. Colors may be brighter in some years than in others and the role of environmental stress that may affect these differences is not well understood. Sometimes even the simplest sounding questions such as “why do leaf colors vary in intensity from year to year?” are extremely complex and not easy to answer.
Activities
Fall Colors
Objectives: Focus on the colors of fall
Materials: Paint samples from paint store, assorted fall leaves
Since color is focused on in this unit and certainly the prime beauty of trees in fall, how about having students pay special attention to color. This is obviously a seasonal activity and will vary depending on where in North America (or the world) you live. Students living in northeastern North America (and in parts of China) will have the greatest range of colors to work with but everyone should be able to participate.
1. Bring in an assortment of paint color samples (the larger, the better) from your local paint store – you be the judge of which ones to bring in (I think most stores will be happy to supply these but if not, you can always promise to return them when finished). Bring in those colors that are most like the trees around where you live but be sure to be expansive with your color choices as the variety of leaf colors brought to you will be great.
2. Post the color samples where all of the students can see them so they can be thinking about color and the colors they’ll be looking for in leaves.
3. Have your students select leaves (whether they remove them from a tree or pick them off the ground) they find around school, at home, or in local parks that they think have “good” or “interesting” color and bring them in to class. The fresher the leaves the better so students will find their leaves will retain their color best if they place them in a sandwich bag with a wee bit of moisture and place them in the refrigerator until they can be brought to class.
4. Once a leaf is brought in to class (this can take place over weeks if you wish), have that student try to match up as closely as they can, their leaf with one of the color samples in the classroom (your color samples may have a number or name that should be noted along with the leaf). Over time, as more and more leaves come in for matching, you may want to segregate the different leaves and their color matches. The leaves will quickly fade but not your color samples.
5. It should soon become apparent that the colors of the leaves, from even the same tree, do vary and students will also very quickly realize how difficult it can be to match colors – they’ll have to use a very discerning eye but this should heighten their sense of color and maybe even their appreciation of fall leaves.
6. Once you decide that you’ve brought in enough leaves, you can spread out all of the different color samples for all to see – there should be lots of different ones! Fall’s riot of color is just that, a very wide assortment of different colors!
Key Concepts
Structures and Functions, Life Cycles
Questions
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Posted by Aria Srinivasan on February 02, 2005 at 04:04 PM
Q: My teacher told me that leaves change color because chlorophyl breaks down and you are then able to see more pigments and not just the green ones. It looks as though you are saying this may not be correct. What do you think?
Aria
A: Your teacher is correct for some leaves. As the chlorophyll breaks down in fall on certain kinds of plants, the underlying colors of the leaf are able to show through and you now see them - they were masked by the chlorophyll before.
Other kinds of leaves however, actually change their color over time due to chemical changes that take place in fall and are not simply the result of the chlorophyll being removed.
This whole process is quite complicated and scientists are still trying to work out details of the process.