Why do the leaves change?
-Biology-
Introduction
The weather is not the only process affecting the foliage.
In fact, the weather is a distant second in importance to the biology
of the trees and leaves.
Trees
There are many species of trees and shrubs, each with different
characteristics. There are two types of trees, evergreen and deciduous.
Evergreen trees (e.g., pine, spruce, fir, etc.), sometimes referred
to as conifers, keep their leaves (needles) year round. Deciduous
trees however, shed their leaves during the winter or during prolonged
droughts.
Leaves
The leaves are responsible for producing food and nutrients for
the trees and shrubs. Deciduous trees produced enough nutrients,
which are stored by the tree in the trunk and roots, to last throughout
the winter. The leaves also allow the trees and shrubs to release
excess moisture. The process through which leaves produce food is
known as photosynthesis. Photosynthesis breaks down carbon dioxide
(CO2) into organic materials such as carbohydrates (e.g.,
sugars and starches). In order for photosynthesis to take place,
energy is required. For plants, this energy is provided by sunlight.
Pigments
To use the sunlight, the plant must have a mechanism for absorbing
the light. Plants use chlorophyll and carotenoids, both chemical
pigments, to absorb light. Light has color components collectively
known as the spectrum. The spectrum consists of red, orange, yellow,
green, blue, indigo, and violet. The green light portion of the
spectrum is not effectively absorbed by chlorophyll in plants. The
green light is either reflected by or passed through the leaf. This
is why leaves generally appear green.
Carotenoids represent a collection of two groups of pigments, carotenes,
and xanthophylls. Carotenes are similar to vitamin A and are responsible
for the coloration of carrots (the name carrot is derived from this
pigment), pumpkins, and yellow and orange leaves. Xanthophylls are
yellow pigment responsible for the coloration found in dandelions,
sunflowers, corn, egg yolks, and yellow leaves.
In addition to carotenes and xanthophylls, three other chemical
pigments play a role in foliage coloration. Anthocyanins produce
blue, red, and violet colors. If the plant cell fluid is acidic,
the coloration will be red. Conversely, if the fluid is basic, then
the color will be blue. Tannins produce brown colors often seen
in tea, chestnut bark, and in oak leaves in the fall. The final
pigment is a group known as flavones. They are also yellow and are
found in sumac, horse chestnut, tea and onions.
Autumn Change
Now that you have a basic understanding of trees, leaves, and the
chemical pigments found in the leaves, we can now look at how leaves
alter their colors in the fall. So often, people talk about the
"changing of the leaves". It may be more accurate to say that the
leaves are actually losing their color. Leaves contain the pigments
mentioned in the earlier section but are overshadowed during the
spring and summer by an abundance of chlorophyll, which is green.
As autumn approaches, the amount of available sunlight decreases.
This signals the tree that winter is approaching and that it is
time to begin the process of shedding its leaves. As a result, the
production of chlorophyll ceases and breaks down. Because of this,
the green color of the leaf disappears allowing the other pigments
to show off their hues.
A maple leaf loses its chlorophyll and other pigments become
visible.
Other changes are also taking place during the autumn season. At
the point where the leaf stem joins with the branch, a layer of
cells develops which blocks the transfer of water to and from the
leaves. The veins that carry fluid into and out of the leaf gradually
close, trapping sugars and promoting the production of anthocyanins
in the leaf. Once the cell buildup is complete, the leaf is ready
to fall.
We hope that this brief tutorial has been helpful in explaining
the reasons for and the factors which influence the fall foliage.
If you have any questions, please e-mail
us and we'll do our best to answer them.
Other "About Foliage" Sections
A Guide to the leaves
How to preserve leaves |
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