Let's learn about tropism, tropic movement, and control and coordination in plants. You all must have seen the touch-me-not-plant, a very interesting thing about this plant is that when its leaves are touched, the leaves begin to fold up and drop. Also, when a seed germinates, the root so formed gets embedded in the soil away from the light and the stem part comes up in the air towards the light

 

Unlike animals, plants do not have a nervous system, muscles, and sense organs. But they still can respond and react to various environmental stimuli such as light, gravity, water, touch, and chemicals.

Plants, in fact, show two different types of movements in response to various stimuli
� one type of movement is independent of growth
� and other types of movements are dependent on growth

 

Both these types of movements are affected by the action of plant hormones (phytohormones). Plant co-ordinate their responses against environmental stimuli by using hormones. Plants slowly respond to various stimuli through revealing growth. As a result, the response of plants to various stimuli cannot be observed immediately

 

According to the nature of the stimulus, the movement may be spontaneous (autonomic) or induced (paratonic).

 

Induced plant movements are of two types:

 

1. Nastic movements
2. Tropic movements

 

1. Nastic movements- These are non�directional induced variation movements that occur due to turgor changes as in the case of the� touch-me-not� plant. These movements reveal the immediate response to stimulus however these do not involve growth
2. Tropic Movement or tropism- Tropic movements are the induced growth movements of curvature that occur due to differential growth. These are directional movements or orientations that occur in response to external stimuli such as light, the force of gravity, chemicals, water etc.

 

Let us understand tropism in detail.

 

Positive and Negative Tropism: If the movement of the plant�s part is towards the stimulus, it is termed as positive tropism. If the movement of the plant�s part is away from the stimulus, it is termed negative tropism.

 

Types of Tropism

 

There are four different types of tropism:

 

1. Phototropism: It is the directional movement or orientation of the plant�s part in response to light stimulus.

 

If the plant�s part moves towards the light, it is called positive phototropism. If the plant�s part moves away from light, it is called negative phototropism.

 

2. Geotropism: It is the directional movement or orientation of the plant part in response to gravity.

 

If the plant�s part moves in the direction of gravity, it is called positive geotropism.
Alternatively, if the plant�s part moves against the direction of gravity, it is termed negative geotropism.

 

3. Chemotropism: It is the directional movement or orientation of the plant�s part in response to a chemical stimulus.

 

If the plant�s part moves towards the chemical stimulus, it is called positive chemotropism.
On the other hand, if the plant�s part moves away from the chemical stimulus, it is called negative chemotropism.

 

4. Hydrotropism: It is the directional movement or orientation of the plant�s part in response to water stimulus. If the plant�s part moves towards the water stimulus, it is called positive hydrotropism.
On the other hand, if the plant�s part moves away from the water stimulus, it is called negative hydrotropism.

 

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