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You know that plants need water. Water carries nutrientsthroughout the plant so that it grows and reproduces. But didyou know that water is what holds a plant upright, even if it isgrowing in the desert?

Plants, just like all living things, are made up of cells.Cells contain a collection of tiny structures that keepplants alive and growing. In each cell, a wallsurrounds 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 isempty, it has almost no shape at all. If you stackedseveral empty balloons, you would have a flat pile ofobjects. If you threw them at a target, they would notfly far. If you tried to balance one on your headduring a relay race, it would not provide much of achallenge. In short, an empty balloon is not veryuseful at all.

The walls of a plant cell are supportedby fluid inside.

Suppose you were to fill the balloon with water. As more waterenters the balloon, it begins to take a distinct shape. The waterpushes against the inside, stretching the balloon tightly andmaking its shape more rigid. If you take several filled balloonsand stack them in a container, the resulting structure will betaller than if you were to stack the same number of emptyballoons.

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Plant cells are similar to water balloons. If the plant does notreceive 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'slungs force air through the brass tubing of the trumpet--in this ÿ " .......

ease the pressure produces a tone. With a balloon, uwater pushes against its walls with enough pressure toinflate it and give it shape.

Pressure created by various forces affects the wayplants live, grow, and reproduce. Wind, rain, animals,and people push and pull against plants. Gravity helpsdetermine the direction in which they grow. Thechemical bonds inside the plant's cells create forcesthat keep the cells attached to each other.

Another important force is turgor pressure. Turgotpressure is the force of water pushing against theinside of cell walls. Turgor pressure enables each cell toperform its functions. The pressure makes the cell walls rigid,which supports the plant's structure and keeps it upright.

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Have you ever heard someone say, "I'm under too much

pressure"? When people use the word pressure in this way, theyare probably not talking about physical forces such as turgorpressure. When people are "under pressure," they are usuallyexperiencing stress that leaves them feeling tired and worndown. Plants are just the opposite! When plants are droopy andwilted, a little water increases the turgor pressure in their cellsand quickly helps them stand tall again.

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Yucca plants growing in the desert relyon turgor xessure to stay upright,

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Sir Isaac Newton studied the way objects move anddeveloped laws of motion that today enable us todescribe the forces at work around us. For instance,we know that inertia explains the motion of a footballlying on the ground; the ball will remain on theground until another force causes the ball to move.When the ball is thrown through the air, we know thatthe strength of the toss determines how quickly theball moves. And when two helmets collide, we knowthat 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,gravity pulls him down toward the ground. Inertia acts ...............................................................................................................

on his body, causing him to fall until his impact with the groundstops him. The larger the player is, the more force it takes tobring him down. And during a collision on the field, both playersexperience the impact.

Many forces are at work on the footballfield.

Newton also developed the concept of gravity, which is anattractive force that occurs between two objects, whether theyare alive or not. The strength of gravity's pull depends on the

mass of the objects--therefore, a large object (like Earth) exertsmore gravitational pull than a smaller object (like a ball). Thisexplains why a ball that is thrown into the air will always fallback toward Earth.

Gravity keeps living things on Earth close to the planet's surface;gravity even acts on the cells within those living things. Forexample, the grass growing on the football field always grows uptoward the sky, while the roots always grow down into the soil.This is partly because plant shoots, which need radiant energy tomake their food, grow toward light. But most scientists believethat gravity also lends a hand.

Plant cells contain a hormone called auxin that accumulates onthe downward side of the cell. This hormone seems to permitthe 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 tothis process as positive geotropism° Plant shoots and stemsgrow upward, opposite to the pull of gravity, and thereby exhibitnegative geotropism.

When grass first begins to sprout, it exerts a force of its own. Aseedling must push through its seed easing and through the soilabove it. The emergence force that a seedling uses to movetoward the surface can be surprisingly strong, allowing plants toforce their way through dense soil and sometimes even

sidewalks or rocks.

Have you noticed that healthy grass always standstall and straight? Another force within the plantensures this outcome. A flexible wall surrounds allplant cells, and water can move through this wall todeliver nutrients and remove waste. If the plant doesnot receive enough water, the cell's shape will lackdefinition, and the structure will not be supported.As water fills the cells, however, this water pressesagainst the cell walls from the inside, strengtheningthe structure of the entire plant. This force of water isreferred to as turgot pressure; this pressure iswhat enables a plant to stand upright.

Turgor pressure supports cell walls,which in turn support the structure of theentire plant.

Oeotropism, emergence force, and turgor pressure allsupport a plant's survival, but other forces can be devastating.

Tension force is applied by pulling; if a plant is pulled toostrongly, its roots will be pulled out of the ground or separatedfrom the stem, and the plant will perish. Animals or humanswalking through a field apply a force that can break or crushplant stems and leaves. The forces of wind, rain, snow, and hailcan cause similar destruction and death.

Every day, both beneficial and dangerous forces affect all livingthings. At times, harmful forces cause damage to organisms.When these forces are in balance, however, they provide anenvironment in which organisms can thrive.

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