Hunan Education Edition Sixth Grade Science Volume 1

Hunan Education Edition fifth grade science volume 2, a tenacious little tree PPT

Part One: Geotropism

1. Geotropism--(root)

1. Explanation of geotropism

Geotropism is the growth response of certain parts of a plant to the force of gravity. Roots grow in the direction of gravity, so roots have positive geotropism; stems grow upward, so stems have negative geotropism.

2. Importance of geotropism

The roots of plants are positively geotropic, growing in the direction of gravity. They can penetrate deep into the soil, consolidate the plants on the ground, and absorb water and mineral salts from the soil. The stems of plants are negatively geotropic, growing upward against gravity, allowing the leaves to absorb sunlight for photosynthesis.

3. Internal causes of geotropism

Gravity can also change the distribution of auxin in the plant, making it more distributed on the side of the plant facing the ground and less on the side facing away from it. Because roots are sensitive to auxin, roots grow slowly toward the ground and grow quickly behind the ground, so the roots show gravitropism. .

A tenacious little tree PPT, part two: phototropism

2. Phototropism-(stems, leaves)

1. Definition

Phototropism is a type of tropism, which refers to the property that the growth of living things is affected by the direction of the light source. The phenomenon that plant growth organs are exposed to unidirectional light and cause growth to bend is called phototropism. Commonly found in plants. Plants grow towards the light, which is conducive to obtaining a larger area and more light, which is beneficial to photosynthesis and maintains better growth of plants.

2. Classification

1) Positive phototropism

The upper part of the plant is affected by light, causing growth to tend to the side of the light.

2) Negative phototropism

Experiments and research have shown that roots have negative phototropism, and the control mechanisms of negative phototropism and gravitropism exist independently of each other.

3) Lateral optical properties

The perpendicularity of plant organs to incoming light.

3. Examples

The phototropism of plants is most sensitive in the tips of young shoots, coleoptiles and seedlings growing in the dark. The stem ends of vigorously growing sunflowers, cotton and other plants can also rotate with the sun. Yellow seedlings of gramineous plants such as oats, wheat, and corn, as well as upper hypocotyls of peas and sunflowers, are often used as research materials for phototropism. Phototropism is an ecological response of plants, such as the phototropism of stems and leaves, which can keep the leaves in the optimal position to absorb light energy for photosynthesis.

A tenacious little tree PPT, the third part: summary

Summarize:

1. Why do plant roots grow downward?

geotropism

2. Why do plant roots have denser root systems near water?

hydrotropism

3. Why do the stems and leaves of plants grow upward?

Phototropism

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