Tropic and nastic movements are two different types of plant movements that have been overlooked by many. They are essential, and they result from stimuli provided by the environment. Stimuli can be physical or chemical, and the responses are different depending on the plant species. In this essay, I will describe and explain the difference between nastic and tropic movement.
What is Tropic Movement?
Tropic movement is the directional movement of an organ in response to a directional stimulus. The directional stimulus can be light, gravity, or touch. The movement is determined by the direction of the stimulus. When a stimulus is perceived by a plant, the cells on one side of the organ start to grow more than the other side. This unequal growth causes the organ to bend in the direction of the stimulus. There are many types of tropic movements that plants can display. They include:
Phototropism: This is a directional movement in response to light. For example, the bending of the stem towards the light is a result of phototropism. When a plant perceives the light stimulus, the cells on the shaded side grow more, which causes bending towards the light.
Gravitropism: This is a directional movement in response to gravity. When a plant is growing in a particular direction, gravity pulls the plant downwards. Due to this, the cells on the lower side of the plant grow more. This increases the weight of the lower side of the plant, which causes the plants to bend upwards. For example, the bending of roots downwards is gravitropism.
Thigmotropism: This is a directional movement in response to touch. For example, the tendrils in climbing plants wrap around objects by thigmotropism. When a plant touches a surface, the cells on that side stop growing, while the cells on the other side keep growing. This causes the tendril to curve towards the object it is touching.
Heliotropism: This is a directional movement in which the plants follow the sun's movement. For example, the sunflower plant's head follows the sun's movement during the day. When a plant perceives the sunlight, the cells on the shaded side stop growing, while the cells on the illuminated side grow. This causes the plant to turn towards the sun.
What is Nastic Movement?
Nastic movement is a non-directional movement of an organ. It occurs when a plant perceives a stimulus, but the response is not directional. This means that the organ's movement is not determined by the direction of the stimulus. It does not matter where the stimulus is coming from, the organ will move in a particular way. Nastic movements are usually rapid and reversible, and they are triggered by environmental stimuli such as touch, temperature, humidity, and light. Nastic movement does not require rapid growth or bending of the organ. Instead, the organ undergoes changes in its shape, turgor pressure, or orientation. There are many types of nastic movements that plants can display. They include:
Nyctinastic Movement: This is a movement that occurs in response to the passage of time or the day/night cycle. For example, the closing of flowers during the night is a result of nyctinastic movement. When the daylight decreases, the plant perceives the change in light intensity and triggers the movement.
Thermonastic Movement: This is a movement that occurs in response to changes in temperature. For example, the opening or closing of petals in some flowers is a result of thermonastic movement. When the temperature changes, the plant perceives the change and triggers the movement.
Hydronastic Movement: This is a movement that occurs in response to changes in humidity or water availability. For example, some plant leaves fold when they are dehydrated to reduce water loss through transpiration.
Chemonastic Movement: This is a movement that occurs in response to chemical stimuli. For example, the closing of the petals in some flowers when they are touched is a result of chemonastic movement.
Difference between Tropic and Nastic Movements
The primary difference between tropic and nastic movements is that tropic movement is a directional response while nastic movement is a non-directional response. Tropic movement occurs in response to a directional stimulus such as light or gravity, while nastic movement occurs in response to environmental stimuli such as temperature or humidity, which are not directional.
Another difference between the two is the speed at which they occur. Tropic movements are typically slow and can take days or weeks, while nastic movements are typically rapid and can occur within minutes or hours.
Tropic movements result from unequal growth of cells on the organ's sides, which leads to bending in the direction of the stimulus. In contrast, nastic movements result from changes in the organ's shape, turgor pressure, or orientation, which are not due to rapid growth or bending of the organ.
Lastly, tropic movements are irreversible, while most nastic movements are reversible. Tropic movement results from irreversible changes in the organ's growth, while nastic movement involves changes in turgor pressure that can be reversed by changes in the plant's internal water balance.
In conclusion, tropic and nastic movements are essential for plant survival and are triggered by environmental stimuli. Tropic movement is a directional response to a directional stimulus, while nastic movement is a non-directional response to non-directional stimuli. In general, tropic movements are slow and irreversible, while nastic movements are rapid and reversible. Knowing the difference between tropic and nastic movements is essential for understanding plant growth, development, and survival, and it is vital in agriculture and horticulture.