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Unit 1Ecology theory5 Topics|1 Quiz
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Nutrient recycling4 Topics|1 Quiz
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Ecological relationships and population dynamics2 Topics
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Human impact on the ecosystem2 Topics
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Study of a grassland ecosystem3 Topics
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Responses in the Flowering Plant4 Topics|1 Quiz
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Unit 3Sexual reproduction in the flowering plant1 Topic
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The human reproductive system3 Topics
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The menstrual cycle2 Topics
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Pregnancy2 Topics
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The circulatory system4 Topics
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Blood5 Topics
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The heart5 Topics
Lesson 6, Topic 1
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Topic 1: Plant tropisms
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What are plant tropisms ?
- A tropism is the growth response of a plant to an external stimulus
- Tropisms help plants to achieve the best growing conditions
Types of tropism
PHOTOTROPISM
The growth response of a plant to light
- Stems are positively phototropic (i.e. grow towards them light)
- Roots are negatively phototropic (i.e. grow downwards towards water ), e.g. ivy, where the roots grow away from the light, towards the wall
GEOTROPISM
The growth response of a plant to gravity
- Roots are positively geototropic (they grow in the direction of gravitational pull, i.e. downwards)
- Stems are negatively geototropic (they grow in against the direction of the gravitational pull, i.e. upwards )
THIGMOTROPISM
The growth response of a plant to touch
Here are some examples of thigmotropism in plants:
Mimosa pudica or “sensitive” plant responds to touch by completely folding its leaf when touched. Scientists think that this response is to help protect the plant from insects.

An example of this tropism is the curling of a vine tendril around objects that it touches. This helps the plant securely position itself and keep growing, as these types of plants do not usually have a strong stem to keep themselves upright.

The Venus fly trap is another plant that responds to the external touch stimulus. When the hairs on the inside of the leaf are triggered, usually when an insect has landed, the leaf closes shut. Why? Insects provide nutrients to the plant. Over time, the insect is dissolved and the nutrients will be absorbed by the plant.
