Plant movement refers to the change in position of plant parts due to change in the environment or due to some internal factors. Such movements are seen due to the irritability of the sensitive protoplasm in response to a variety of stimuli. Stimulus is defined as any change in the external environment which produces a reaction(response) in the plant or plant parts. Response refers to the visible change or reaction of a plant or plant part as a result of stimuli.

Plant movement are of two major types:

A) Movement of growth

B) Movement of variation

A) Movement of Growth

It is accompanied by growth of the plant and the curvature is rather permanent. These are associated with the unequal growth of plant parts. Growth movement are of two types:

1) Autonomic or Spontaneous Movement

2) Paratonic or Induced Movement

1) Autonomic Growth Movement

It occurs automatically due to internal factors. It is of two main types:

a) Nastic movement

These movement are shown by bi facial organs like bud scales, sepals, petals, etc. These movement are caused due to differences in the rate of growth on the two sides of bi facial organs. Opening of petals and sepals are food examples of it.

Nastic movements are of two types:

I) Epinastic movement

When the movement occurs due to more growth on the upper side of the organ than on the lower side, it is called epinasty. Example: movement of sepals and petals during opening of flower.

II) Hyponastic movement

When the movement occurs due to more growth on the lower side of the organ than on the upper side, it is called hyponasty. Example: in Poppy plant.

b) Nutation

They occur in the apical region of some organs like growing stem of twiners and tendril. The stem exhibits a kind of nodding in two directions. This is because the stem apex shows more growth on one side at one time and a little later there is greater growth on the opposite side.

In spirally growing stems, the region of greater growth passes gradually around the growing point instead of alternating in two opposite directions. It results in the spiral coiling of stem and tendrils around the support. Such movement is called circum-nutation.

2) Paratonic Growth Movement

They are the growth movements which are induced by external stimuli. They are also called tropic movement. On the basis of external factors, they are of following types:

a. Phototropism

It is the response of a plant to light. Roots are negatively phototropic and shoots are positively phototropic.

b. Geotropism

It is the response to the force of gravity. Roots are positively geotropic and shoots are negatively geotropic.

c. Chemotropism

It is the response to chemicals. Example: growth of pollen tube from stigmatic surface into the style and ovary.

d. Hydrotropism

It is the response to water. Roots are positively hydrotropic.

e. Thigmotropism (Haptotropism)

It is the response to touch or contact. Example: tendrils bending on a support.

B) Movement of variation

It involves change in the position of plant parts without growth. They are the result of changes in the turgidity of cells. These are temporary as the organs return to their normal position after sometimes.

They are of two types:

1) Autonomic movement of variation

They are a result of internal factors.

Example: Leaves of Desmodium gyrans ( Indian telegraph plant)

2. Paratonic movement of variation

They are in response to external stimuli. They are independent of the direction of stimulus. They are also known as nastic movement.

Some of its types are:

a. Photonasty

It is induced by variation of light intensity.

Example: Head of sunflower

b. Nyctinasty (Sleep movement)

It is induced by the combined stimuli of light and temperature.

Example: Oxalis and Albizzia flowers open in day and close at night.

c. Thermonasty

It is induced by variation of temperature.

Example: Tulip, Crocus open on the rise of temperature and close when the temperature is lowered.

d. Thigmonasty

It occurs in response to touch.

Example: insectivorous plants

e. Seismonasty

In Mimosa pudica (Touch me not plant), the leaflets are sensitive to touch and fold upwards. Its leaves drop immediately when touched. This is due to loss of turgor pressure in the cells of base called pulvini. A pulvinus is a swelling at the base of the petiole or leaflet that acts as a hinge during movement.