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Plants move in response to a stimulus. The movement of plants can be divided into:

• Tropic Movements
• Nastic Movement

Tropic Movements

Growth movements which occur in the direction of the stimulus are known as tropic movements. In this, the response acts on the protoplasm from one side. A tropic movement may be towards or away from the stimulus.

Tropic movements are of various types:

Phototropism: Response to light

In this, the plant grows in the direction of light. The plants have a chemical called auxin, which reacts in response to light. This causes the cells of the plants to elongate. Growth towards light is positive phototropism, while that away from the light is negative phototropism. The stem grows towards the direction of light and is called positively phototropic. However, the root is called negatively geotropic since it grows away from the light source.

Chemotropism: Response to chemicals

It is the movement of the plant part towards a chemical stimulus. For eg., during fertilization in plants, the stigma produces a chemical in response to which the pollen tube grows towards the ovule.

Hydrotropism: Response to water

It is the growth of the plant in the direction of the water gradient. For eg., the plant (https://byjus.com/biology/the-classification-of-plants/) roots grow in the direction of higher humidity level. That is why, when we pull a plant we find its roots growing in all the directions.

Geotropism: Response to gravity

It is the movement of plant parts in response to gravity. The roots grow towards gravity and are hence called positively geotropic. The stem, on the other hand, is negatively geotropic.

Thigmotropism: Response to touch

The growth movement regulated by touch is known as thigmotropism. This is usually found in twining plants and tendrils.

Thermotropism: Response to temperature

The growth movement regulated by temperature is known as thigmotropism. The peduncles of Anemone nemerosa grows in response to the heat imparted by the sun.

Nastic Movement

These movements are non-directional responses to the stimuli. These movements are independent of the direction of the stimulus. These movements can be due to changes in turgor or growth. E.g. the response of the touch-me-not ( Mimosa pudica ) plant to touch. However, this is not a growth (https://byjus.com/biology/growth-development/) phenomenon.

Types of Nastic movement

There are four main types of nastic movements. They are as follows:

Photonasty: Response to light

In some plants, movement is induced due to changes in light intensity. E.g. flowers of Cestrum nocturnum or night-blooming jasmine, open at night and close with the start of the day.

Nyctinasty: Movements at night or in the dark

This type of nastic movement is also called “sleeping movement”. This is governed by variations in light and temperature, hence, is also sometimes categorised in photonastic or thermonastic movements. E.g. leaves of clover and oxalis close in the evening and open in the morning.

Thermonasty: Response to temperature

As the name suggests, thermonastic movements are due to variations in temperature. Flowers of many plants show this type of movement.

Seismonasty or Thigmonasty: Response to shock/contact/vibration

This type of nastic movement is due to mechanical stimuli, e.g. touch, strong wind, raindrops, foreign body, etc. Examples include movement in stigma on receiving pollen grains, movement of stamen in response to insect, Mimosa pudica or touch-me-not, movement in insectivorous plants, etc.

Some other types of nastic movements are epinasty (downward-bending), hyponasty, geonasty or gravinasty (response to gravity), chemonasty (response to chemicals), hydronasty (response to water), etc.

Thus, we see that the tropic movements are directional and growth-dependent while the nastic movements are non-directional and may or may not be growth dependent.

I hope this helps.

Good Luck.

Thank you