Plant movements and types

• Simple unicellular motile green alga, Chlamydomonas and its flagellated zoospores move from place to place but the movement of firmly rooted higher plants is of a different nature.
• The movement is not of the entire plant but only a curvature of some of the organs or twining of a part, sleep movements of the leaves and opening and closing of floral leaves.
• The basis of all such movements is the irritability of the sensitive protoplasm in response to a variety of stimuli.
• Plant movements can be classified into two major types. They are:

A) Growth movements

• This type of movement is accompanied by growth and the curvature is rather permanent.
• These movements are associated with the unequal growth of the parts of the plant.
• Some regions grow faster while others are slower.
• The growth movements could be self-regulated or controlled by external stimuli.
• Growth movement can thus be grouped under two types. They are:

i) Autonomic movements (spontaneous movement)

• The movements which occur without the effect of external stimuli are called autonomic or spontaneous movements.
• Thus autonomic movements are brought by definite internal stimulus.
• The autonomic growth movement may be of following types:

a) Epinasty: more growth on the upper surface.

b) Hyponasty: more growth on the lower surface.

c) Nutation

• They are autonomic movements which occur in the apical region of some organs like shoot apex and tendril.
• The shoot apex bends in the direction where it is to form a leaf primordium.
• Since successive leaf primordial are formed in different direction, the shoot apex rotates along with.
• Similarly in climbing stems and tendrils the area of differential growth moves successively to all directions.
• It causes the organ to bend spirally.
• Coiling of tendril after coming in contact with a support is a thigmotropic movement of growth.

d) Circum nutation

• It is a type of nutation in which the stem apex circumscribes its long axis for describing a circular oath in space which helps in twining around a support.

ii) Paratonic movements (induced movement)

• Growth movements which are induced by external stimuli are called paratonic movements.
• These movements are again divided into two types– tropic movements and nastic movements.

a) Tropic movements or tropism

• These movements are induced by unilateral external stimuli i.e. the movement of a part of the plant generally in one direction in response to an external stimulus.
• The movements could be positive or negative depending upon the growth towards or away from the stimulus.
• Tropic movements may be of following types:

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1.Phototropism(due to light)

• It is the response of the plant to light.
• Shoots and coleoptiles are positively phototrophic as they bend towards light while the roots are negatively phototrophic as they grow away from the light.

2.Geotropism (due to gravitational force)

• It refers to the response induced by gravity.
• Negative geotropism is shown by coleoptiles, stems or pneumatic roots that grow away from the gravity.
• Positive geotropism is shown by roots and rhizomes that grow towards gravity.

3.Chemotropism (due to chemicals)

• It is the response of a plant or a part of it to chemicals like the pollen tube is positively chemotropic as it moves towards the ovules due to chemicals produced over there.

4.Hydrotropism (due to water)

• It is the response of a part of the plant to water like roots and pollen tubes grow towards water and hence are positively hydrotropic.

5.Thigmotropism (due to contact)

• It is the response of a plant to a solid surface or touch like the tendrils of the pea plant coil around a support and hence is positively thigmotropic.

6.Thermotropism (due to temperature)

• It is the response of a plant to temperature.

b) Nastic movements

• These movements are non-directional unlike the tropic movements.
• Nastic movements are governed by the positions and the structures of the responding cells or organs.
• The following types of nastic movements are common.

1.Photonasty (response to light intensity)

• It is a type of movement induced by variation of light intensity.
• Many flowers open when there is strong light and close up on darkening or when artificially scudded, e.g., noon flower (pentapetes).

2.Nyctinasty (sleep movements)

• It is movement induced by the combined stimuli of light and temperature.
• The vegetative and floral leaves in certain plants belonging to the families Leguminosae and Oxialidaceae like Siris exhibit different position during day and night.
• They remain open during the day and close down by the evening.
• Such movements are also called sleep movements.

3.Thermonasty

• It is induced by variation in the temperature.
• E.g., the flowers of Crocus, Tulip, etc. open on the rise of temperature and close when the temperature is lowered.
• The flowers of Crocus and tulip are highly sensitive.

4.Thigmonasty (haptonastic)

• Bending of tentacles in Sundew or Drosera after coming in contact with an insect is called thigmonasty or chemonastic movements.
• In Mimosa pudica, the leaflets are sensitive to touch and folds upwards.
• This is due to the loss of turgor pressure in the pulvini.
• A pulvinus is a swelling at the base of the petiole or leaflet that acts as a hinge during movement.

B) Turgor movements

• These are known as curvature movements.
• The movements are caused by turgor changes (swelling or shrinkage of living cells due to change in osmotic potential).
• In the region of bending or curvature, the cells lose their turgidity and shrink on one side.

References:

i) https://www.britannica.com/science/tropism

ii) https://extension.psu.edu/how-plants-move

Plant movements and types