An Introduction
A biological phenomenon that indicates growth or turning movements in plants due to the influence of the environment is termed tropic movement in plants or tropism in plants.
In English, the word tropism means an action done in a very unthoughtful manner. But here tropism is a very directional process. In general, tropism is described as 3 step process which includes
sensation to a stimulus, as a living being, it becomes a beneficiary factor for the plants.
Signal transduction occurs( in simple words, the environment reaction towards the plant)
And finally, the directional growth response takes place.
These are the following steps that give rise to tropism or what we call "tropic movements in plants".
Tropism is seen not only in plants but also in viruses, pathogens and other biological organisms also. This phenomenon is direction-dependent and reacts according to the direction of the stimulus. Other types of tropism in different organisms are also called host tropism, tissue tropism or cell tropism.
Types of Movements in Plants
In plants, there are five types of tropic movements. These tropic movements can be explained as described below:
Phototropism
sunlight is a major source that helps plants to sustain life. This type of tropism in plants occurs due to the response to lights. In general, the plants will grow in the direction of light. It can be positive if the reaction is towards the light and it can be negative if the reaction is away from light.
Geotropism
When plants show movement in the direction of gravity the roots will grow downwards and hence shows positive geotropism. Similarly, if its shoots will grow upwards, then the process can be defined as negative geotropism.
Chemotropism
Apart from the environmental factors, there are some chemical factors also which involve the tropic movements. The movement in seed plants while transferring pollen grains to the female gamete releases some chemicals; this process leads to positive chemotropism.
Hydrotropism
When the movement of plants occurs due to the stimulus of water then the phenomenon is termed hydrotropism. This process leads to roots growing towards the area where water is present.
Thigmotropism
The movement of reflex of a plant takes place in response to touch gives rise to thigmotropism. A very common example of this kind of tropism occurs in the touch me not plant. As soon as one touches the leaves of this particular plant they droop.
In the case of phototropism or geotropism (also known as gravitropism), directional growth occurs due to the asymmetric distribution of the chemical substance known as "Auxin".
Tropism in plants is a process of growth of the plant shoots and roots in accordance with the natural environment surrounding it.
FAQs on Tropic Movement in Plants (Tropism)
1. What is tropic movement in plants, also known as tropism?
Tropic movement, or tropism, is a directional growth movement of a part of a plant in response to an external stimulus. The direction of the stimulus determines the direction of the plant's response. For instance, a plant's stem growing towards a light source is a classic example of a tropic movement.
2. What are the main types of tropic movements in plants based on the stimulus?
Plants exhibit several types of tropism, each corresponding to a specific environmental stimulus. The primary types as per the CBSE syllabus for the 2025-26 session are:
- Phototropism: Growth in response to light.
- Geotropism (or Gravitropism): Growth in response to gravity.
- Hydrotropism: Growth in response to water.
- Chemotropism: Growth in response to chemical signals.
- Thigmotropism: Growth in response to touch or contact.
3. What is the difference between positive and negative tropism?
The difference lies in the direction of growth relative to the stimulus. Positive tropism is when a plant part grows towards the stimulus. A common example is the shoot growing towards sunlight (positive phototropism). Negative tropism is when a plant part grows away from the stimulus, such as the shoot growing away from gravity (negative geotropism).
4. How is tropic movement different from nastic movement in plants?
Tropic and nastic movements are two distinct types of plant responses. The key differences are:
- Direction: Tropic movements are directional and depend on the direction of the stimulus. Nastic movements are non-directional; the response is the same regardless of the stimulus's direction.
- Growth-Dependence: Tropic movements are slow, growth-dependent movements. Nastic movements are often rapid and are caused by changes in turgor pressure, not growth.
- Example: A sunflower tracking the sun is an example of phototropism (tropic). The folding of a 'touch-me-not' plant's leaves upon touch is seismonasty (nastic).
5. What is the significance of tropic movements for a plant's survival?
Tropic movements are crucial for a plant's survival and optimal functioning. They help the plant adapt to its environment by:
- Ensuring shoots grow towards light for maximum photosynthesis (phototropism).
- Guiding roots to grow downwards to anchor the plant and find water and nutrients (geotropism and hydrotropism).
- Enabling climbing plants to find support to grow taller and access more light (thigmotropism).
- Directing pollen tubes towards ovules for successful fertilisation (chemotropism).
6. How does the plant hormone auxin cause the shoot to bend towards light in phototropism?
The bending of a plant shoot towards light is mediated by the hormone auxin. When light shines on one side of the shoot tip, auxin migrates to the shaded side. This higher concentration of auxin on the shaded side stimulates the cells there to elongate more rapidly than the cells on the illuminated side. This differential growth results in the shoot curving and bending towards the light source.
7. Why do plant roots show positive geotropism while the shoot shows negative geotropism?
This is a vital adaptation for survival. Plant roots exhibit positive geotropism (growing downwards, with gravity) to securely anchor the plant in the soil and to efficiently absorb water and mineral nutrients. In contrast, the shoot exhibits negative geotropism (growing upwards, against gravity) to reach sunlight, which is essential for photosynthesis. This opposing response to the same stimulus (gravity) ensures that each part of the plant grows in the most advantageous direction for its function.
8. Can you provide a real-world example for each main type of tropism?
Certainly. Here are some common examples:
- Phototropism: A houseplant on a windowsill bending its stem and leaves towards the window to get more light.
- Geotropism: No matter how you plant a seed, its roots will always grow downwards into the earth.
- Hydrotropism: The roots of a plant growing sideways towards a leaking water pipe in the soil.
- Chemotropism: The growth of a pollen tube down the style towards the ovule during plant reproduction, guided by chemical signals.
- Thigmotropism: The tendrils of a pea plant or a grapevine coiling around a fence or a stick for support.
9. What would happen if a potted plant is placed horizontally in a dark room with only a single light source from one side?
In this scenario, the plant would display two different tropic movements simultaneously. The shoot would bend upwards, growing away from the pull of gravity (negative geotropism), and it would also bend towards the light source (positive phototropism). Meanwhile, the roots would bend downwards, growing in the direction of gravity (positive geotropism) to seek soil and anchor the plant. This demonstrates how plants integrate responses to multiple stimuli to optimise their growth and survival.
