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Patterns - LEAVES: NATURE’S SUNCATCHERS - Background
Leaves are all around us. They are the first signs of spring, brightening the landscape
with fresh green. They give us shade in summer and color fields and hillsides emerald.
Every fall they dazzle us with their fiery display of color. Leaves come in a variety of
shapes and sizes, yet a close-up look reveals patterns that help us identify them. Despite
differences in form, all leaves perform the same vital function- capturing sunlight and
turning it into food for the plant.
At its most fundamental, a leaf is a flattened green surface or blade, supported by a
slender stalk or petiole. It is made up of layers of cells sandwiched between outer waxy
coverings that help keep water in and harmful bacteria and fungi out. These surface
layers are dotted with tiny pores called stomata, which can open and close to regulate gas
exchange, moisture and temperature within the leaf. A network of veins carries water and
nutrients into and out of the leaf and provides a supportive framework for these delicate
tissues. A simple leaf has a single blade while a compound leaf, like clover, is made of
several smaller blades, called leaflets, attached to the petiole. Leaves are arranged on the
stem to maximize their exposure to sunlight.
Just as we can recognize people by small facial differences, we can identify plants by
studying their leaf shapes and arrangement. Each leaf has a unique set of features, and
there are many variations in leaf designs. Leaves can be broad like maple leaves, narrow
like blades of grass or even needle-shaped like the leaves of evergreens. Leaf edges,
called margins, can vary, too. Lilac leaves have smooth margins; elm leaves are toothed,
like a saw blade; while dandelions are jagged and oak leaves are divided into lobes. Some
leaves, like those of carrot, are so finely dissected they look like lace.
Another distinctive identifying feature of leaves is the pattern of veins that run
throughout the blade. These act as the leaf’s plumbing system carrying water and
nutrients in and food, in the form of sugars, out of the leaf. There are three main types of
leaf venation--pinnate, palmate and parallel. Pinnate venation consists of one main
vein, called a midrib, running through the middle of the leaf with side veins extending to
the leaf margins, similar to the structure of a feather. In palmate leaves, such as maple
leaves, the main veins radiate from the petiole attachment and fan out through the leaf
blade. In the grass, lily and related families, parallel veins are the rule, running side-byside down the length of the leaf blade.
Environmental conditions influence leaf features in many ways. Shaded leaves tend to be
larger, thinner and darker green than those in full sunlight. Quaking aspen grows in open
sunny places and has many small leaves with flattened petioles that let each leaf tremble
in the slightest breeze, allowing sun to filter to the leaves below and keeping the leaf
from overheating. Leaves of tropical plants often have “drip tips” that allow water to
drain from leaf surfaces thus preventing mold growth that could damage the leaf.
Common mullein grows in sunny open places and their fuzzy leaves are thought to trap
moisture, reflect sunlight, and reduce water loss.
Leaves come in all shapes and sizes, yet they all have the same function--to produce food
for the plant. For unlike animals, plants don’t raid the cookie jar or go hunting for food
when they are hungry; they make their own food. And their green color is key to this
function. The green comes from the pigment, chlorophyll, which is contained in the
upper surface of the leaf. To understand this food-making process, think of it as a recipe-the plant’s version of making a batch of brownies. Along with chlorophyll, the main
ingredients are: solar energy, from the sun; water, absorbed by plant roots; and carbon
dioxide, a gas absorbed from air through the leaves. Chlorophyll absorbs sunlight and
acts as a catalyst, transforming solar energy into chemical energy used to make plant
food. This energy is then used to split water into its component parts, hydrogen and
oxygen. The hydrogen joins with carbon dioxide to form a simple sugar called glucose.
This sugar is carried in the veins to other parts of the plant and used to help the plant
grow, making new leaves, flowers, fruits and seeds. As a by-product, plants give off
oxygen, an essential element needed by living things. This chemical reaction, called
photosynthesis, is thought to be one of the most important processes on Earth, for it
provides food, not only for plants, but ultimately, for most animals who depend on plants
for energy to live and grow.
All summer, leaves are busy making and storing food. But the shortening days of fall
signal a change in the seasons. Tree leaves have to cope with less light, cold temperatures
and frozen water. Tough waxy evergreen needles are resistant to the cold and droughtlike conditions of winter and remain on the tree. But broad-leaved trees seal off and shed
their leaves. Special cork cells grow over the veins into the leaves, cutting off their water
supply. Without water, chlorophyll production starts to dwindle. Now the other pigments
that have been there all along start to show through. The orange colors come from
carotene and yellow from xanthophylls, two pigments common in familiar fruits and
vegetables, like carrots, corn and bananas. While yellow and orange colors are simply
unmasked, red and purple colors are produced in response to bright light and sugar in
leaves. As veins get sealed off, sugars get trapped in the leaves. On cool nights, a
chemical reaction converts sugars in the sap to a new pigment, anthocyanin, tinting
leaves red to purple.
Although not completely understood, temperature, light and water supply are thought to
influence the degree and duration of fall color. An early frost weakens the leaf, turning it
brown before the best color is developed. Drought tends to delay or halt color, causing
pigments to fade, leaves to dry out and turn brown. A series of rainy or overcast days
limits light, hence photosynthesis, resulting in less sugar production and fewer red leaves.
The best colors are produced when a warm, wet spring is followed by a summer that is
not too hot or dry. These combined with sunny fall days and cool nights above freezing
produce the best display of colors. Fall leaf color can also be used to identify trees, as
each type of tree has it own special range of color.
Leaves make up our natural world for without them there would be no forests, no grassy
meadows, no gardens of leafy greens. Through the amazing process of photosynthesis,
they provide the oxygen we breathe and the food we eat. We couldn’t live without them!
Suggested Readings
Bang, Molly and Chisholm, Penny. Living Sunlight—How Plants Bring the Earth to Life.
New York: Blue Sky Press, 2009.
Maestro, Betsy. Why Do Leaves Change Color? New York: Harper Collins, 1994.
Silverstein, A. et al. Photosynthesis. Minneapolis, MN: Twenty-First Century Books,
2008.
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