Back to: Botany 200 Level
Hello, brilliant learner! Have you ever noticed how a plant leans toward sunlight or how roots always grow downward into the soil? Or have you seen climbing plants like yam wrap around stakes for support? These movements aren’t random—plants are responding to their environment through a process called tropism!
Today, we’ll learn about phototropism, gravitropism, and thigmotropism, the three major types of tropisms that help plants adjust to their surroundings.
Phototropism, gravitropism, thigmotropism
What Is Tropism?
Tropism is the way plants grow in response to a stimulus like light, gravity, or touch. Depending on the direction of growth, tropisms can be:
✔ Positive (towards the stimulus).
✔ Negative (away from the stimulus).
Let’s look at the different types.
1. Phototropism – Response to Light
Phototropism is when plants grow towards or away from light. This helps them get enough sunlight for photosynthesis.
How Does Phototropism Work?
The plant hormone auxin gathers on the shaded side of the stem.
This causes cells on that side to grow longer, bending the plant towards the light.
✅ Example: Ever placed a plant near a window and noticed it lean towards the sunlight? That’s positive phototropism!
✅ Negative phototropism: Roots grow away from light to stay in the soil.
2. Gravitropism – Response to Gravity
Gravitropism (also called geotropism) is how plants respond to gravity. This helps roots grow deep into the soil for water and nutrients, while stems grow upwards for sunlight.
How Does Gravitropism Work?
Positive gravitropism: Roots grow downward into the soil.
Negative gravitropism: Stems grow upward against gravity.
✅ Example: If you plant maize seeds, no matter which way they land, roots will always grow downwards and stems upwards.
3. Thigmotropism – Response to Touch
Thigmotropism is how plants respond to physical contact. It allows climbing plants to find support by wrapping around objects.
How Does Thigmotropism Work?
When a plant’s tendrils touch an object, cells on the opposite side grow faster, causing the tendrils to curl around the object.
✅ Example: Climbing plants like yam, cucumber, and passionfruit use thigmotropism to hold onto fences or stakes for support.
✅ Another example: The Mimosa pudica (touch-me-not) plant folds its leaves when touched—this is also a type of thigmotropic response!
Why Are Tropisms Important?
✔ Help plants get enough sunlight for photosynthesis.
✔ Ensure roots find water and nutrients underground.
✔ Allow climbing plants to support themselves.
✔ Help plants survive in different environments.
Summary
Phototropism – Plants grow towards light (stems positive, roots negative).
Gravitropism – Roots grow down (positive), stems grow up (negative).
Thigmotropism – Plants respond to touch, helping climbing plants attach to support.
Tropisms help plants adjust to their surroundings and grow in the right direction.
Let’s Test Your Understanding:
- Why do stems grow towards light?
- What is the difference between positive and negative gravitropism?
- How does thigmotropism help climbing plants?
Fantastic work, superstar! Now you understand how plants move without muscles! Keep learning, and see you in the next lesson!
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