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 Food
Oak Embryo for Embryo

Introduction
Plop. The cool autumn day is interrupted by
a ripe acorn falling to the forest floor. A squirrel
snatches the acorn and carries it to an open spot,
where it is buried snugly beneath the leaves. Meant
to be a winter snack, the acorn remains beneath a
carpet of leaves and snow.
The acorn contains a tiny, living, embryo plant
and its food supply. At the pointed end of the
acorn, there is a tiny root, stem, and a tuft of white
leaflets. Just below the tiny embryo are two big
cotyledons—the food supply for the embryo when
it starts to grow.

2
 In the spring, the snow
melts and the days get
warmer and longer. The thick
acorn shell becomes wet
and soft. It swells and bursts Shoot
open. Then water easily seeps
into the acorn and reaches
the embryo. After absorbing
the moisture, the embryo
begins to swell. A tiny root
pokes out and begins to
grow downward. The tiny
shoot thrusts upward. The
cotyledons inside the acorn
shrink as the growing root
uses this food supply. The Root
root grows downward,
probing for water deep in the
soil. It becomes an anchor
for the young oak. Then the
first tiny leaves unfurl. As
soon as sunlight strikes the
leaves, they begin to make
chlorophyll and turn from
white to bright green. The
chlorophyll will allow the
plant to make its own food.

3
 This is not the only acorn to have germinated in this opening in the forest.
An old oak tree once stood here, but it has recently fallen, leaving a break in
the forest canopy. Now there is a race among oak seedlings to claim this spot
in the sun. To survive, the young oak must rapidly put out new leaves so that
it can make enough food to grow. By growing faster and bigger than the
other seedlings, it wins the competition for sunlight and grows into a sapling.
There are other hurdles that the sapling must overcome during its first
years of life. It must compete with other plants for water and nutrients. The
sapling is still young and tender, so it might be eaten by a caterpillar or a
deer. But an extra dose of a bitter-tasting chemical called tannin discourages
browsers from taking more than one bite. There are periods of no rain in
summer and intense cold in winter. Most oak saplings do not survive these
hardships, but this one grows into a strong, young tree.

Roots
The sapling’s main root grows four or five feet deep. When there is no
water at the surface, the root can draw water from the soil below. The water
is in small holes between the soil particles. The root also stores food over
the long winter. In the spring, this ready food supply can be used for speedy
growth. Feeder roots grow many branches just beneath the soil surface. Here
they have a supply of nutrients from the layer of dead leaves that covers the
forest floor. Dark surface soil also is rich in nutrients. The roots can get water
from rains and melting snow.

4
Sapling with
root system
below ground

5
 The roots grow rapidly at their tips. A tiny root cap protects each delicate
root tip as it pushes through the soil. Farther from the root tip are the mature
root cells that make up root tissues, each with its own role to play. A thin outer
layer keeps the roots from drying out. At the center of the root are the xylem
and phloem. These make up the tubes through which water and minerals
flow. Food also flows to every part of the young tree. Encircling the xylem and
phloem is a layer of cells that produce new roots. Around this is another layer
of cells that regulate the flow of water into and out of the tubes. Food is stored
in the cortex. A furry coat of root hairs projects from the mature root tissue.
The root hairs absorb water and nutrients from the soil.

Xylem
Phloem
Cortex

Root
Hairs

Root Cap

CXENL07ARD53_ALR_4A
6
 Water Column

CXENL07ARD53_ALR_5A
Once inside the root, water and minerals move into the xylem tissue. The
xylem tissue is part of a complex system of tubes that extends from near the
tips of the roots all the way to the surface of leaves.
Water absorbed by the roots must reach all the way to the highest leaves.
Water molecules attract each other strongly. Water molecules also stick to the
smooth walls of the xylem. These two forces hold the water together so that it
forms one continuous column inside the xylem, from the roots all the way to
the leaves. On the undersides of the leaves are small pores. Water evaporates
from the pores. More than 95 percent of the water taken in by the roots
passes directly out of the tree through these pores. This evaporation, called
transpiration, is what pulls water and dissolved nutrients up through the tree’s
xylem, from the roots all the way to the leaves.

7
 Trunk, Limbs, and Branches
The oak’s trunk, limbs, and branches are the framework that holds the
leaves up to the sunlight. They also contain the tubes for carrying water,
minerals, and food from one place to another inside the tree. Water and
minerals travel through the xylem tissue from the roots to the leaves. Food
from the leaves moves through the phloem to places where the tree is
growing, or to the roots, where it is stored.
At the center of the tree’s trunk, limbs, and branches is the heartwood.
The heartwood is made up of dead xylem tissue that no longer transports
water. The dead cells are reinforced with a substance called lignin, which
makes them strong and stiff.

Phloem

Sapwood

Heartwood

8
 Thus, the heartwood is a strong central column that supports the tree.
Outside the heartwood is the sapwood, so named because its xylem cells still
transport water and minerals. Each year, a new layer of xylem grows that is
larger in circumference than the previous year’s xylem. This enables the xylem
to transport more sap each year, which provides water and minerals to an
increasing number of leaves. Just under the bark is the living phloem tissue,
which transports food from the leaves to other parts of the oak.
Growth also occurs at the tips of the branches, where new cells are made.
This is also where buds develop. The buds give rise to leaves and flowers.
Each autumn, before the leaves fall, buds for the following spring have
already been made.

Xylem

9
 Sun

Oak Tree

Water
Leaves
The purpose of leaves is to make food for the oak tree. Leaves use a
process called photosynthesis. Sunlight falls on the oak’s leaves and is captured
by chlorophyll. Chlorophyll is a substance that gives the leaves their green
color. Energy from sunlight is used to combine water and carbon dioxide to
make food in the form of sugar. Water for photosynthesis is taken up from
the soil by the roots. The carbon dioxide enters the leaves through pores.
A product of photosynthesis is oxygen, which passes out of the leaves.

10
 Water

Carbon
Dioxide
Food

Pores
Oxygen

Food Underside of leaf

Water

The oak, like all other plants, respires. Respiration is almost the reverse
of photosynthesis. Oxygen is taken in and carbon dioxide is released. The
carbon dioxide and oxygen pass through the pores in the oak tree’s leaves.
The oxygen is used to break down sugars to produce energy the oak needs
to live and grow. Because respiration does not require sunlight, the oak tree
respires constantly. During the day, when photosynthesis is occurring, the tree
produces oxygen and uses more carbon dioxide than it releases. At night, the
tree stops producing oxygen. Only some carbon dioxide is released.

11
 An oak leaf is designed perfectly for its food-making function. It has a
protective outer surface covered with wax, which keeps it from drying out.
Just under this protective layer is a layer of cells filled with green chlorophyll.
This layer is where photosynthesis takes place. The next layer has loosely
packed cells and the xylem and phloem tissue that make up the leaf’s veins.
The phloem tissue in the veins whisks away the sugars that are made during
photosynthesis. On the underside of the leaf are hundreds of pores, through
which water and gases can pass in and out of the leaf.

Protective Outer
Cells Filled Surface
with Chlorophyll

Vein

Pore

CXENL07ARD53_ALR_9A
12
 Female Oak
Flowers

Male Catkins

Stigma

Ovary

CXENL07ARD53_ALR_10A
Maturity
In 15 years the oak is a mature tree, more than 30 feet tall. Each spring,
before tree leaves sprout, it is decorated with countless blossoms. The male
flowers are called catkins because they look like little cat tails. The tiny female
flowers are hardly noticeable. They are found in the angle between a new
leaf and twig. On windy days, clouds of pollen blow off the catkins. Stigmas
covered with a sticky glue protrude from the female flower. Pollination occurs
when oak pollen lands on the stigmas. After pollination has occurred, acorns
begin to form. The acorns get bigger as more and more food is stored in them.

13
 Through the years, the oak grows large. Its canopy of leaves reaches out
over the forest, casting the forest’s floor in dim shade. Its many leaves rustle in
the breeze each summer and fall to the ground each autumn. The fallen leaves
provide a carpet of litter that is home to insects, worms, and many other
organisms. In time, these leaves decay and make their nutrients again available
to the mighty oak.
During winter, the oak stands dormant. Its leaves have fallen, and its
branches are exposed. The oak does not make food during winter. There
are no green leaves within which photosynthesis can occur. Instead, the tree
remains in a state of dormancy until the days grow warm and long again in
the spring.

14
Old Age and Death
Six hundred years
come and go. The old oak
is five feet in diameter and
150 feet tall. Each spring
the mighty oak bears its
flowers, and each fall it
drops its acorns. But over
the years, old age, violent
weather, and disease take
their toll. There are jagged
holes in the bark where
limbs have fallen away. A
squirrel family makes its
nest in one of the holes.
Shelf fungus grows from
the dying trunk. Bark
beetles and grubs feed on
the bark. Woodpeckers
search for insects inside
the tree.

15
 Then one spring day the weakened tree can no longer stand. It crashes to
the forest floor. Millions of bacteria, fungi, earthworms, and other inhabitants
of the forest floor begin to digest the wood from the dead oak. They are
nature’s recyclers, making the nutrients from the old oak available for brand
new oak trees. Sunlight streams through the opening in the forest canopy
where the old oak used to stand. The sun strikes the leaves of a tiny sapling. It
has just sprouted up from an acorn that a squirrel has buried and forgotten.

16
Think and Write
1. Compare the life cycle of an oak tree to the life cycle of a human.
2. How is photosynthesis almost the opposite of respiration?
3. Describe how the structures of the leaves, the trunk, and the roots
of an oak are suited for their functions.
4. Persuasive Writing Trees are important to your community. During
summer, trees keep streets and homes cooler. Trees beautify yards,
parks, and roadsides. Do research to find out more about the
importance of trees. Then write a letter to the editor of your local
newspaper about the value of trees in your community.

Hands-On Activity
Color some water with red food dye. Make a fresh cut across the stem
of a stalk of celery. Leave the celery stalk in the colored water for a day.
What happens to the leaves? Cut across the celery stem and look at
it through a hand lens. Find the tiny red xylem tissue that carried the
colored water. Try to separate a single tube from the celery stem.