WHY DO LEAVES CHANGE COLOR IN THE FALL?
by
SCIENCE MADE SIMPLE
http://photoscience.la.asu.edu/photosyn/education/colorchange.html
COPYRIGHT © 1995, 1997 BY SCIENCE MADE SIMPLE,
INC.
INDEX: Why do leaves change color
in the Fall? --- I Can Read --- Word Puzzle
Learn More
About: How plants prepare for winter --- Projects
We all enjoy the beautiful show of colors as leaves
change each autumn. Did you ever wonder how and why this happens.
To answer that question, we first have to understand what leaves are and what
they do.
Leaves are nature's food factories. Plants take water from the ground through their roots. They take a gas called carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into glucose. Glucose is a kind of sugar. Plants use glucose as food for energy and as a building block for growing. The way plants turn water and carbon dioxide into sugar is called photosynthesis. That means "putting together with light." A chemical called chlorophyll helps make photosynthesis happen. Chlorophyll is what gives plants their green color.
As summer ends and autumn
comes, the days get shorter and shorter. This is how the trees "know"
to begin getting ready for winter.
During winter, there is not
enough light or water for photosynthesis. The trees will rest, and live off the
food they stored during the summer. They begin to shut down their food-making
factories. The green chlorophyll disappears from the leaves. As the bright
green fades away, we begin to see yellow and orange colors.
Small amounts of these colors have been in the leaves
all along. We just can't see them in the summer, because they are covered up by
the green chlorophyll.
The bright reds and purples
we see in leaves are made mostly in the fall. In some trees, like maples,
glucose is trapped in the leaves after photosynthesis stops. Sunlight and the
cool nights of autumn turn this glucose into a red color.
The brown color of trees like oaks is made from
wastes left in the leaves.
It is the combination of
all these things that make the beautiful colors we
enjoy in the fall.
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I CAN READ
WHY DO LEAVES CHANGE COLOR IN THE
FALL?
Plants make their own food. They take water from the ground through their roots. They take a gas called carbon dioxide from the air. They turn water and carbon dioxide into food using sunlight and something called chlorophyll. Chlorophyll is green. It gives leaves their green color.
Winter days are
short and dry. Many plants stop making food in the fall. The chlorophyll goes
away. Then we can see orange and yellow colors. These
colors were in the leaves all summer, but the green
covered them up.
Some leaves turn
red. This color is made in the fall, from food
trapped in the leaves. Brown colors are also made in
the fall. They come from wastes left in the leaves.
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How many
of these words can you unscramble? |
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trawe |
Answers follow Projects Section.
LEARN MORE ABOUT:
HOW PLANTS PREPARE FOR WINTER
All summer, with the long hours of sunlight and a good supply of liquid water,
plants are busy making and storing food, and growing. But
what about wintertime? The days are much shorter, and water is hard to
get. Plants have found many different ways to get through the harsh days of
winter.
Some plants, including many garden flowers, are called
"annuals," which means they complete their life cycle in one growing
season. They die when winter comes, but their seeds remain, ready to sprout
again in the spring. "Perennials" live for more than two years. This
category includes trees and shrubs, as well as herbaceous plants with soft,
fleshy stems. When winter comes, the woody parts of trees and shrubs can
survive the cold. The above ground parts of herbaceous plants (leaves, stalks)
will die off, but underground parts (roots, bulbs) will remain alive. In the
winter, plants rest and live off stored food until spring.
As plants grow, they shed older leaves and grow new ones. This is
important because the leaves become damaged over time by insects, disease and
weather. The shedding and replacement continues all the time. In addition,
deciduous trees, like maples, oaks and elms, shed all their leaves in the fall
in preparation for winter. "Evergreens" keep most of their leaves
during the winter. They have special leaves, resistant to cold and moisture
loss. Some, like pine and fir trees, have long thin needles. Others, like
holly, have broad leaves with tough, waxy surfaces. On very cold, dry days,
these leaves sometimes curl up to reduce their exposed surface. Evergreens may
continue to photosynthesize during the winter as long as they get enough water,
but the reactions occur more slowly at colder temperatures.
During summer days, leaves make more glucose than the plant needs for
energy and growth. The excess is turned into starch and stored until needed. As
the daylight gets shorter in the autumn, plants begin to shut down their food
production.
Many changes occur in the leaves of deciduous trees before they finally
fall from the branch. The leaf has actually been preparing for autumn since it
started to grow in the spring. At the base of each leaf is a special layer of
cells called the "abscission" or separation layer.
All summer, small tubes which pass through this layer carry water into the
leaf, and food back to the tree. In the fall, the cells of the abscission layer
begin to swell and form a cork-like material, reducing and finally cutting off
flow between leaf and tree. Glucose and waste products are trapped in the leaf.
Without fresh water to renew it, chlorophyll begins to disappear.
Other colors, which have been there all along
then become visible. The orange colors come from
carotene ('kar-uh-teen) and the yellows from xanthophyll ('zan-thuh-fil). They
are common pigments, also found in flowers, and foods like carrots, bananas and
egg yolks. We do not know their exact role in leaves, but scientists think they
may be involved somehow in photosynthesis.
The bright red and purple colors come from anthocyanin (an-thuh-'si-uh-nuhn)
pigments. These are also common in plants; for example, beets, red apples, and
purple grapes, and flowers like violets and hyacinths. In the leaves, these
pigments are formed in the autumn from trapped glucose. Brown colors come from tannin, a bitter waste product. Different
combinations of these pigments give us a wide range of colors
each fall.
As the bottom cells in the separation layer form a seal between leaf and
tree, the cells in the top of the separation layer begin to disintegrate. They
form a tear-line, and eventually the leaf is blown away or simply falls from
the tree.
One more important question remains. What causes the most spectacular
display? The best place in the world for viewing fall colors
is probably the
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FUN FACT |
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What do autumn leaves and
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PROJECTS TO DO TOGETHER
NOTE: ADULT SUPERVISION IS REQUIRED. Please read all instructions completely
before starting. Observe all safety precautions.
PROJECT 1 - Separate Colors in a Green Leaf
using Chromatography
What you need:
leaves,
small jars (baby food jars work well)
covers
for jars or aluminum foil or plastic wrap
rubbing
alcohol, paper coffee filters
shallow
pan, hot tap water, tape, pen
plastic
knife or spoon, clock or timer.
What you do:
- Collect 2-3
large leaves from several different trees. Tear or chop the leaves into
very small pieces and put them into small jars labeled
with the name or location of the tree.
- Add enough
rubbing alcohol to each jar to cover the leaves. Using a plastic knife or
spoon, carefully chop and grind the leaves in the alcohol.
SAFETY NOTE: Isopropyl rubbing alcohol can be harmful if mishandled or misused. Read and carefully follow all warnings on the alcohol bottle. - Cover the jars
very loosely with lids or plastic wrap or aluminum
foil. Place the jars carefully into a shallow tray containing 1 inch of
hot tap water.
SAFETY NOTE: Hot water above 150 F can quickly cause severe burns. Experts recommend setting your water heater thermostat no higher than 125 F. - Keep the jars
in the water for at least a half-hour, longer if needed, until the alcohol
has become colored (the darker the better).
Twirl each jar gently about every five minutes. Replace the hot water if
it cools off.
- Cut a long thin
strip of coffee filter paper for each of the jars and label it.
- Remove jars
from water and uncover. Place a strip of filter paper into each jar so
that one end is in the alcohol. Bend the other end over the top of the jar
and secure it with tape.
- The alcohol
will travel up the paper, bringing the colors
with it. After 30-90 minutes (or longer), the colors
will travel different distances up the paper as the alcohol evaporates.
You should be able to see different shades of green, and possibly some
yellow, orange or red, depending on the type of leaf.
- Remove the strips of paper, let them dry and then tape them to a piece of plain paper. Save them for the next project.
PROJECT 2 - Separate Colors in a Fall Leaf
using Chromatography
What you need: same as Project 1.
What you do:
- Repeat step
(1)-(8) from Project 1, this time using leaves that have changed color. You may have to wait much longer in steps (4)
and (7). There is normally much less of the other colors
in the leaves compared to the green chlorophyll.
PROJECT 3 - Observe how light affects color
development
What you need:
a
tree with leaves that turn red in autumn
aluminum
foil or heavy paper and masking tape.
What you do:
- Before the
leaves turn colors in the fall, find a maple
tree, flowering dogwood, sweet gum, or other tree or shrub that you know
will turn bright red or purple.
- Find several
leaves that receive bright sunlight, and cover part of them with foil or
heavy paper and tape.
- After the
leaves have changed color, remove the covering
and observe the different colors underneath.
These are the colors that were in the leaf all
summer. The bright reds and purples are only made in the fall, with
exposure to light.
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