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You know that plants need water. Water carries nutrients
throughout the plant so that it grows and reproduces. But did
you know that water is what holds a plant upright, even if it is
growing in the desert?
Plants, just like all living things, are made up of cells.
Cells contain a collection of tiny structures that keep
plants alive and growing. In each cell, a wall
surrounds these structures, giving the cell its shape.
Water fills the cell and helps support the cell wall.
If you look at a balloon, you will notice that when it is
empty, it has almost no shape at all. If you stacked
several empty balloons, you would have a flat pile of
objects. If you threw them at a target, they would not
fly far. If you tried to balance one on your head
during a relay race, it would not provide much of a
challenge. In short, an empty balloon is not very
useful at all.
The walls of a plant cell are supported
by fluid inside.
Suppose you were to fill the balloon with water. As more water
enters the balloon, it begins to take a distinct shape. The water
pushes against the inside, stretching the balloon tightly and
making its shape more rigid. If you take several filled balloons
and stack them in a container, the resulting structure will be
taller than if you were to stack the same number of empty
balloons.
Plant cells are similar to water balloons. If the plant does not
receive enough water, its cells cannot perform their functions.
Without water, nutrients are not carried efficiently into the cells.
and wastes are not carried away efficiently. In addition, without
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Another example of pressure occurs when a trumpet player's
lungs force air through the brass tubing of the trumpet--in this
ease the pressure produces a tone. With a balloon,
water pushes against its walls with enough pressure to
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inflate it and give it shape.
Pressure created by various forces affects the way
plants live, grow, and reproduce. Wind, rain, animals,
and people push and pull against plants. Gravity helps
determine the direction in which they grow. The
chemical bonds inside the plant's cells create forces
that keep the cells attached to each other.
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Yucca plants growing in the desert rely
on turgor xessure to stay upright,
Another important force is turgor pressure. Turgot
pressure is the force of water pushing against the
inside of cell walls. Turgor pressure enables each cell to
perform its functions. The pressure makes the cell walls rigid,
which supports the plant's structure and keeps it upright.
Have you ever heard someone say, "I'm under too much
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pressure"? When people use the word pressure in this way, they
are probably not talking about physical forces such as turgor
pressure. When people are "under pressure," they are usually
experiencing stress that leaves them feeling tired and worn
down. Plants are just the opposite! When plants are droopy and
wilted, a little water increases the turgor pressure in their cells
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and quickly helps them stand tall again.
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Sir Isaac Newton studied the way objects move and
developed laws of motion that today enable us to
describe the forces at work around us. For instance,
we know that inertia explains the motion of a football
lying on the ground; the ball will remain on the
ground until another force causes the ball to move.
When the ball is thrown through the air, we know that
the strength of the toss determines how quickly the
ball moves. And when two helmets collide, we know
that each player absorbs the same amount of force.
These forces affect the motion of living things as well.
When a football player leaps or dives into an end zone,
Many forces are at work on the football
field.
gravity pulls him down toward the ground. Inertia acts ...............................................................................................................
on his body, causing him to fall until his impact with the ground
stops him. The larger the player is, the more force it takes to
bring him down. And during a collision on the field, both players
experience the impact.
Newton also developed the concept of gravity, which is an
attractive force that occurs between two objects, whether they
are alive or not. The strength of gravity's pull depends on the
mass of the objects--therefore, a large object (like Earth) exerts
more gravitational pull than a smaller object (like a ball). This
explains why a ball that is thrown into the air will always fall
back toward Earth.
Gravity keeps living things on Earth close to the planet's surface;
gravity even acts on the cells within those living things. For
example, the grass growing on the football field always grows up
toward the sky, while the roots always grow down into the soil.
This is partly because plant shoots, which need radiant energy to
make their food, grow toward light. But most scientists believe
that gravity also lends a hand.
Plant cells contain a hormone called auxin that accumulates on
the downward side of the cell. This hormone seems to permit
the cells of roots and shoots to bend in a specific direction.
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Roots grow in the direction of gravity's pull; scientists refer to
this process as positive geotropism° Plant shoots and stems
grow upward, opposite to the pull of gravity, and thereby exhibit
negative geotropism.
When grass first begins to sprout, it exerts a force of its own. A
seedling must push through its seed easing and through the soil
above it. The emergence force that a seedling uses to move
toward the surface can be surprisingly strong, allowing plants to
force their way through dense soil and sometimes even
sidewalks or rocks.
Have you noticed that healthy grass always stands
tall and straight? Another force within the plant
ensures this outcome. A flexible wall surrounds all
plant cells, and water can move through this wall to
deliver nutrients and remove waste. If the plant does
not receive enough water, the cell's shape will lack
definition, and the structure will not be supported.
As water fills the cells, however, this water presses
against the cell walls from the inside, strengthening
the structure of the entire plant. This force of water is
referred to as turgot pressure; this pressure is
Turgor pressure supports cell walls,
what enables a plant to stand upright.
which in turn support the structure of the
entire plant.
Oeotropism, emergence force, and turgor pressure all
support a plant's survival, but other forces can be devastating.
Tension force is applied by pulling; if a plant is pulled too
strongly, its roots will be pulled out of the ground or separated
from the stem, and the plant will perish. Animals or humans
walking through a field apply a force that can break or crush
plant stems and leaves. The forces of wind, rain, snow, and hail
can cause similar destruction and death.
Every day, both beneficial and dangerous forces affect all living
things. At times, harmful forces cause damage to organisms.
When these forces are in balance, however, they provide an
environment in which organisms can thrive.
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