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Photo by: Andrey ArmyagovTropismsAndTurgor pressure7th Grade ScienceMs. DeFreeseSeptember 24, 2012

Plants can react and adjust

Since plants must make their own food to survive the ability to capture as much of this light energy as possible is crucial to its survival.

Thus, plants have developed a chemical response to light or the lack of it that causes their stems to bend toward the stronger light.

Understanding what plant tropism is and, specifically, what happens during phototropism makes us realize that plants, as living things, necessarily demonstrate the several characteristics of life. Specifically, this includes growth, response to stimuli, and adaptation. It is because of its hormones that a plant's stem always grows upwards and its roots always grown downward. 2

TropismsTropisms are directional movement responses in plants that occur in response to a directional stimulus.

Tropisms are controlled by a plant hormone called auxin.

http://editor.wn.com/page/cdefreese/phototropism/#

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A Tropism is a growth movementDirection is determined by the direction from which the stimulus strikes the plant.

Positive = the plant, or a part of it, grows in the direction from which the stimulus originates.

Negative = growth away from the stimulus.

PhototropismThe way a plant grows or bends in response to light.

Gravitropism The way a plant grows or bends in response to gravity or earth.Tropisms are namedbased upon the stimulus.

There are more types of tropisms:Chemotropism The way a plant grows or bends in response to a chemical.Hydrotropism The way a plant grows or bends in response to water.Thigmotropism The way a plant grows or bends in response to touch.

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Phototropism = Light ResponseStems are positively phototropic.

Roots are negatively phototropic.

Plants will grow in the direction of the strongest light source. This is an evolutionary adaptation developed by plants to maximize the amount of sunlight received by the plant.

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Stems are negatively gravitropic while

roots are positively gravitropic.

Gravitopism = Gravity Response

The adaptive value of these tropisms is clear.

Roots growing down and/or away from light are more likely to find the soil, water, and minerals they need.

Stems growing up and toward the light will be able to expose their leaves so that photosynthesis can occur.

Roots growing down and/or away from light are more likely to find the soil, water, and minerals they need.

Stems growing up and toward the light will be able to expose their leaves so that photosynthesis can occur.

The adaptive value of these tropisms is clear.

How Phototropism WorksReceptors made of chemical complexes known as phytochromes absorb visible wavelengths of light. When they absorb this light, they emit a chemical signal that produces a hormone known as auxin. Auxin promotes the growth of cells located in the stem of the plant, specifically the darker side of the sunflower. The growth of cells on the light-receiving side of the plant is inhibited. As the shaded side of the plant extends and the lighted side remains the same in length, the new growth of the plant will eventually be "turned" towards the light source.

Read more: Phototropism in Sunflowers | eHow.com http://www.ehow.com/info_8482804_phototropism-sunflowers.html#ixzz27L8ouYOS

Seven-day-old, dark-grown sunflower seedlings were exposed to white light from the left side of the screen for 10.4 hours. The movie starts at the time the unilateral light was turned on. Successive images were captured every 5 min and playback is at 12 frames per sec.In addition to the phototropic bending response, note that the light is also causing photomorphogenic changes such as the opening and expansion of the cotyledons and the production of chlorophyll.

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Turgor Pressure

The pressure exerted by waterinside the cell against the cell wall.

Journal Assignment

Use the folding Venn to compare and contrast phototropism and gravitropism.

Be prepared to share