Movements in Plants

Tropic movements are directional growth responses of plants in response to external stimuli such as light, gravity, touch, and moisture. These movements allow plants to adapt to their environment and optimize their growth and survival. There are several types of tropic movements based on the nature of the stimulus. Let’s explore each type with examples:

1. Phototropism: Phototropism is the growth or movement of a plant in response to light. It allows plants to maximize their exposure to light for photosynthesis. Typically, plant stems show positive phototropism, i.e., they grow towards the light source, while roots display negative phototropism, growing away from light.

   Example: Sunflowers exhibit positive phototropism. During the day, the sunflower heads track the movement of the sun, following it from east to west.

2. Geotropism (Gravitropism): Geotropism is the growth or movement of a plant in response to gravity. Roots show positive geotropism, growing towards the force of gravity and anchoring the plant into the soil, while stems exhibit negative geotropism, growing against gravity, towards the light.

   Example: When you place a potted plant on its side, the roots will exhibit positive geotropism by growing downwards, and the stems will show negative geotropism by growing upwards.

3. Thigmotropism: Thigmotropism is the growth or movement of a plant in response to touch or mechanical stimuli. It allows plants to respond to physical support or attachment and aids in climbing or grasping structures for support.

   Example: The tendrils of climbing plants like peas and grapevines exhibit thigmotropism. When they come in contact with a support, they coil around it, helping the plant to climb and grow vertically.

4. Hydrotropism: Hydrotropism is the growth or movement of a plant in response to moisture or water. Roots display positive hydrotropism, growing towards a source of moisture to enhance water uptake.

   Example: When a plant’s root system detects a moist area in the soil, it will show positive hydrotropism by growing towards that region, where water is more available.

5. Chemotropism: Chemotropism is the growth or movement of a plant in response to specific chemicals. It allows plants to detect and respond to chemical cues in their environment.

   Example: Pollen tubes of flowering plants exhibit positive chemotropism. They grow towards the ovules guided by the chemical signals released by the ovules to facilitate fertilization.

Tropic movements are essential for the survival and successful growth of plants, as they enable plants to optimize their growth and respond to environmental cues effectively. By employing these tropic movements, plants can adapt to varying conditions, find necessary resources, and ensure their reproductive success.

 

NASTIC MOVEMENT

Nastic movements are non-directional, reversible, and rapid movements of plant organs in response to environmental stimuli, such as touch, light, temperature, or humidity. Unlike tropic movements, nastic movements do not involve the plant’s growth towards or away from the stimulus. Instead, they are more like a temporary, instantaneous response of the plant to the specific stimulus. Nastic movements are not dependent on the direction of the stimulus; they occur irrespective of the source’s position. Here are some examples of nastic movements in plants:

Photonasty is a type of nastic movement in plants, but in this case, the movement is in response to changes in light intensity rather than the direction of the light source. The term “photonasty” is derived from “photo” (light) and “nasty” (movement), indicating the light-induced nature of the response. Photonastic movements are rapid, reversible, and often occur in flower petals or leaves. The movements are not directed towards or away from the light source; instead, they are triggered by the amount of light the plant receives. Here are some examples of photonastic movements in plants:

1. Oxalis triangularis (Purple Shamrock): The leaves of Oxalis triangularis, commonly known as the Purple Shamrock or False Shamrock, display photonastic movements. During the day, when exposed to bright light, the leaves open up, spreading wide to maximize light absorption for photosynthesis. In the evening or under low light conditions, the leaves close and fold down, reducing their exposed surface area.
2. Dandelion (Taraxacum officinale): Dandelions are known for their photonastic movements in response to light changes. During the day, when the sun is shining, the bright yellow flowers of dandelions fully open, presenting their vibrant petals to attract pollinators. As the evening approaches or under cloudy conditions, the flowers close up, protecting their reproductive structures from potential harm or moisture loss.
3. Tulips (Tulipa spp.): Some species of tulips exhibit photonastic movements. When exposed to light during the day, the flowers open up, allowing pollinators to access their nectar and facilitate pollination. During the night or under low light conditions, the flowers close up, conserving energy and protecting their reproductive organs.

Photonastic movements in plants are often controlled by light-sensitive pigments or photoreceptors that trigger changes in turgor pressure within specific cells, causing the movements. These responses help the plants optimize their exposure to light, which is crucial for their photosynthetic processes and reproductive success. Additionally, photonasty may have protective functions, safeguarding delicate flower structures from unfavorable conditions such as excessive heat or strong winds.

Thigmonasty is a specific type of nastic movement in plants that occurs in response to touch or mechanical stimuli. Unlike tropic movements, thigmonastic movements are not directed towards or away from the stimulus source; instead, they are triggered simply by physical contact with an object or an organism. Thigmonastic movements are usually rapid and reversible and serve various adaptive purposes for the plant. Here are some examples of thigmonastic movements in plants:

1. Mimosa Pudica (Sensitive Plant): The sensitive plant, also known as Mimosa pudica, is perhaps the most well-known example of thigmonasty. When the leaves of the sensitive plant are touched or disturbed, they respond by rapidly folding and drooping. This reaction is a defensive mechanism to deter herbivores or protect the plant from potential harm. The movement occurs due to changes in turgor pressure within specialized cells at the base of the leaflets called pulvini. Once the stimulus is removed, the leaves gradually reopen.
2. Venus Flytrap (Dionaea muscipula): The Venus flytrap is a carnivorous plant known for its thigmonastic movement in catching and trapping insects. The modified leaves of the plant have sensitive trigger hairs on their inner surfaces. When an insect touches these trigger hairs, it stimulates the plant to close its leaf lobes rapidly, trapping the prey inside. The trapped insect then serves as a source of nutrients for the plant.
3. Drosera (Sundew): Sundews are another group of carnivorous plants that exhibit thigmonastic movement. The leaves of sundews are covered with sticky glandular hairs that secrete a sugary substance to attract and capture insects. When an insect lands on the sticky surface, the leaf edges slowly curl inward, wrapping around the prey and trapping it. The plant then releases enzymes to digest the captured insect and absorb nutrients.

Thigmonastic movements in plants play a crucial role in their survival and adaptation. By responding to touch or mechanical stimuli, plants can avoid potential threats, capture prey for additional nutrients, or enhance their chances of successful reproduction. These movements are fascinating examples of how plants have evolved to interact with their environment in dynamic and complex ways.