What is the Difference Between Tropic and Nastic Movement

The main difference between tropic and nastic movement is that the direction of the tropic movement depends on the direction of the stimulus, but in contrast, the direction of the nastic movement is independent of the direction of the stimulus. Furthermore, tropic movement is a growth movement while nastic movement can be either a growth movement or change in the turgor. For example, stimuli such as gravity, oxygen, chemicals, electric filed, etc. result in tropic movements. Meanwhile, the stimuli such as temperature, humidity, light irradiance, etc. result in nastic movement. 

In brief, tropic and nastic movement are two types of paratonic movements occurring in response to various external stimuli. Generally, both types can alter the growth pattern of the plant. 

Key Areas Covered 

1. What is Tropic Movement
     – Definition, Type of Movement, Importance
2. What is Nastic Movement
     – Definition, Type of Movement, Importance
3. What are the Similarities Between Tropic and Nastic Movement
     – Outline of Common Features
4. What is the Difference Between Tropic and Nastic Movement
     – Comparison of Key Differences

Key Terms 

Directional, Nastic Movement, Paratonic Movements, Reversible, Tropic Movement

What is Tropic Movement 

Tropic movement is the type of paratonic movement of plants, occurring in response to external stimuli. These stimuli are unidirectional, but the movement of the plant part is directional. That is; the movement can occur towards the stimulus (positive tropism) or away from the stimulus (negative tropism).

Figure 1: Gravitropism

Moreover, the movement of the plant occurs through cell division. Therefore, it occurs slowly. Furthermore, as tropic movement is a type of growth movement, it is not reversible. Some examples of tropic movements are phototropism, gravitropism, chemotropism, hydrotropism, thermotropism, etc. 

What is Nastic Movement 

Nastic movement is a type of plant movement that mainly occurs through the changes of the turgor pressure. Therefore, it is a quick movement. And, it occurs in response to an external stimulus. However, this stimulus is not unidirectional. Also, unlike the tropic movement, the nastic movement is not a directional movement.

Figure 2: Mimosa pudica

Moreover, since it occurs through the changes of the turgor pressure of plant cells, nastic movement is reversible. Some examples of nastic movement include photonasty, thermonasty, epinasty, nyctinasty, etc. In addition to these, the closing of the carnivorous Venus flytrap leaf when it captures prey, the folding of the mimosa leaf when it is disturbed, the closing and opening of stomata as a mechanism to regulate photosynthesis and transpiration under various environmental conditions, and the closing of certain plants at night in order to prevent chilling injury are some other examples of nastic movement. 

Similarities Between Tropic and Nastic Movement  

• Tropic and nastic movement are two types of paratonic movements.  
• Both types of movements occur in response to external stimuli.  
• Therefore,  they are induced movements.  
• They are responsible for curving the plant body.  
• Moreover,  they play a key role in the functioning of plants. 

Difference Between Tropic and Nastic Movement 

Definition 

Tropic movement refers to a type of plant movement in which the plant is moved depending on the stimuli. On the other hand, nastic movement refers to the movement of a plant independent of the direction of the external stimuli. 

Directionality 

The direction of the tropic movement depends on the direction of the stimulus. But, the direction of the nastic movement is independent of the direction of the stimulus. 

Diffusion of the Movement 

Moreover, tropic movement is not diffused, but the nastic movement is diffused. 

Type of Plants 

Tropic movements occur in most plants, while nastic movement occurs in specialized plants. 

Type of Plant Organs 

Plant organs with radial symmetry such as the stem and the root show the tropic movement. In contrast, the plant organs with bilateral symmetry such as leaves and stomata show nastic movement. 

Speed of the Movement 

Furthermore, tropic movement is slow while nastic movement is comparatively quick. 

Method 

In addition to the above differences, tropic movement is a growth movement occurring through cell division, while nastic movement can be either a growth movement or change in the turgor. 

Reversibility 

Also, while tropic movement is irreversible, nastic movement is reversible. 

Examples 

Some examples of tropic movement include gravitropism, chemotropism, aerotropism, hydrotropism, etc. while some examples of nastic movement include photonasty, epinasty, chemonasty, thermonasty, hydronasty, etc. 

Conclusion 

Tropic movement is a type of plant movement that occurs through cell division. Therefore, it is a slow and directional movement. Some examples of tropic movements are gravitropism, chemotropism, aerotropism, hydrotropism, etc. Meanwhile, nastic movement is another type of plant movement that occurs mainly through the changes of the turgor pressure. Therefore, it is a quick movement but, not a directional movement. For example, nastic movements include photonasty, epinasty, chemonasty, and thermonasty. Therefore, in summary, the main difference between tropic and nastic movement is the directionality, speed, and the type of movement. 

References:

1. “Tropic Movements In Plants- Different Types of Tropic Movements.” BYJUS, Byju’s, 14 Nov. 2019, Available Here.
2. Goyal, Shikha. “What Is Nastic Movement in Plants?” Jagranjosh.com, 12 Feb. 2018, Available Here.

Image Courtesy:

1. “Upsidedown-tree” By I, Kleuske (CC BY 2.5) via Commons Wikimedia   
2. “Mimosa pudica also (Thottavadi)” By Augustus Binu (CC BY-SA 3.0) via Commons Wikimedia