Science High School Reviewer | Grade 4 Science | Life Science | Kingdoms of Life | Lesson 7: Life Cycle of a Plant

Like animals, plants have a life cycle. Whether a plant grows
from a seed or a spore, it goes through several stages of growth.
Plants, unlike animals, continue to grow as long as they live.
Life Cycle of a Flowering Plant
How long does a plant live? That depends on the kind of
plant. Some bristlecone pine trees are more than 4,000 years
old. A tomato plant lives for only a few months. A plant’s
life cycle includes all of the changes the plant goes through.
A seed needs the right conditions to begin growing. When
the seed has the right amounts of oxygen and water, and
the right temperature, it sprouts. The young roots grow
downward because of gravity. The new stem looks like
it’s reaching for the sunlight as it pushes up through
the seed coat. Perhaps you have seen a plant that is
bending toward the light.

Seeds on the Move
What would happen if all the cherries on a cherry tree fell to
the ground and stayed near the tree? Many of the seeds in the
cherries would start to grow. But roots from the parent tree had
already spread over a wide area. They would absorb most of the
water and nutrients from the soil. The shade of the parent tree
would allow only a little sunlight through to the young plants.
The fallen seeds would not have enough living space and
resources to grow properly. They would grow a lot better if their
seeds were scattered farther from the parent tree. That is why
plants have adaptations that help them scatter their seeds.
Animal Helpers
Many animals eat fruit. Since the seeds of the fruit have
a protective covering, they can pass unharmed through an
animal’s digestive system. The seeds can end up on the ground
in the animal’s droppings far from the parent plant.
Some fruits are covered with tiny hooks that catch on an
animal’s fur. As the animal moves, the fruits go along for
the ride. When the tiny hooks break, the fruit falls to the
ground. In this way, the seeds can be scattered far and wide.
You probably know that some animals gather and bury
nuts and seeds for the winter. Some of those nuts and
seeds start to grow after they have been buried.
Wind as a Helper
Have you ever blown on a dandelion puff? You
probably noticed the small white threads that floated
like parachutes. These threads catch the wind and
carry the fruits and seeds far away. Milkweed plants
and cottonwood trees also have “parachutes” that
carry fruits in the breeze.
The wing-shaped fruits of maple trees also take
advantage of the wind. You might call these fruits
“whirly-birds” as they twirl through the air like
a propeller.
In the southwestern United States, wind blows
tumbleweeds across the land. Seeds fall off as the
plant goes rolling along.

Water as a Helper
Some fruits and seeds get around by floating on water.
Perhaps you’ve seen one drifting along in a lake or river.
Coconuts, which are the fruits of one type of palm tree,
can float between islands. Storms can wash the fruits from
one island onto the beach where a new palm tree sprouts.
Even with all of the ways seeds can be scattered, most of
the seeds that plants make never grow into new plants.

Starting to Grow
When a seed drops to the ground, it may or may not
begin to grow right away. It depends on whether the seed’s
environment has everything the seed needs to grow.
A seed needs water, oxygen, and the right temperature.
A seed contains a small, young plant. Food stored in the
seed gives the young plant energy to start growing. If the
seed does not get what it needs, it stays dormant, or in
a state of rest. A seed might remain dormant for days,
months, or even years.
Spores
Some plants do not make flowers or cones. These
plants grow from spores instead of seeds. A spore is much
smaller and simpler than a seed. A spore is only one tiny,
microscopic cell. You need a microscope to see it. Because
it is just one cell, it has very little stored food. Animals
eat many kinds of seeds, but they usually do not eat
plant spores.
Spores are very hardy. They can stay dormant in dry
conditions for many years. Just like a seed, when the
conditions are right, a spore grows into a new plant.
In order to grow, a spore must land on wet ground.
Once it germinates, it needs almost constant moisture.

A Two-Step Cycle
Mosses and certain other plants with spores actually
reproduce in two steps. In the first step, the plant produces a
spore. If the spore germinates, it grows into a plant that has
male and female cells. The second step begins
when a male and a female cell combine. The
fertilized egg grows into a plant that produces
the next generation of spores.
Spores grow inside spore cases. When the
cases burst, the spores explode into the air.
Some spores fall near the parent plant. The
wind may carry others away. When the spore
lands, it remains dormant until it has the right
temperature and moisture. Then it swells with water.
The next step in the complex life cycle begins.

New plants from plant parts

Not all plants grow from seeds or spores. Some plants
grow from stems, roots, or leaves. Plants that grow this way
are usually identical to the parent plant.
Have you ever planted tulips? You do not start with seeds.
You start with tulip bulbs. A bulb is a type of underground
stem. A bulb is made of thick, fleshy layers of leaves that
store food. When these underground leaves push above
the surface of the soil, they turn green and start making
their own food. Other plants that grow from bulbs include
lilies and daffodils. Onions are bulbs too. Their layers are
actually leaves.
Sometimes new little plants begin to grow right on the
leaves of the parent plant. For example, plantlets form on
the leaves of a houseplant named the piggyback
plant. Maybe you have seen a potato with
sprouts growing from its buds. In time, the
sprouts will grow into new potato plants.

New Plants from Stems
Some plants, such as strawberries, have
stems called runners. Runners grow along the
ground. At certain points along the runners,
roots grow into the soil and leaves develop.
These are new plants that grow from the
runner. Soon the new plant may send out
runners of its own.
If you cut a section of stem from an African
violet plant and place it in a potting mix,
it will grow into a whole new African violet
plant. These sections, or cuttings, can be taken
from roots and leaves too.
Grafting
Suppose an apple grower has some apple
trees that grow good apples but have weak
roots. The trees might die easily because
the roots are not able to absorb water and
nutrients from the soil. Other trees have strong
roots but do not produce very good apples.
The grower can take branches from the trees
with good apples and join them to branches
of the trees with strong roots. This process is
called grafting. Grafting works only if the tubes
that carry water, mineral nutrients, and food
between the two plant parts match up. Then,
new tubes can grow between the two parts.

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How do seeds grow?
Inside each seed is an unformed
plant and a food source. A layer
called the seed coat surrounds and
protects them.
Germination
Under the right conditions, the
seed will sprout. First, the seed coat
splits open. A root pushes through
the crack. Then, a tiny stem grows
upward. One or two leaves appear
on the stem. All seeds develop
this way. The process is called
germination (jur•muh•NAY•shuhn).
Seedlings
A seedling is the young plant that
grows from a seed. Seedlings need
water, light, and nutrients to grow.
If its needs are met, the seedling will
grow into an adult plant. The adult
reaches maturity.

Life cycle of a plant diagram

Life Cycles
For some plants, it takes only days
for the seed to reach maturity. For
others, it takes years. No matter
how long it takes, all seed plants
go through the same stages.
Look at the diagram above. It
shows the life cycle of a berry plant.
A life cycle includes the stages of
germination, maturity, reproduction,
and death. Death marks the end
of a plant’s life. Some plants grow,
reproduce, and die in a single season.
Others, like some conifers, can live
more than 5,000 years.
When a plant dies, its life ends
but its life cycle does not. Seed
plants make new seeds. The life cycle
continues.
Seeds on the Move
Seeds need space to grow, but
they cannot move on their own.
Some, like dandelion seeds, are
easily carried by wind. Others have
prickly surfaces that can be caught
on animal fur.
Many seed plants bear fruits.
When an animal eats the fruit, the
seeds pass through the animal’s
body. Then, the seeds can germinate.