Movement in Plants

Plants show two different types of movement. The plants may either respond to various stimuli very slowly by growing e.g. when a seed germinates the root goes down and the stem comes up into the air or they can show rapid movements like leaves of sensitive plants move very quickly in response to touch by folding and drooping without growing. 

Classification of Plant Movements

These are of two types -

Both tropic and nastic movement can be either due to differential growth or due to change in turgidity.

Movements due to differential growth are irreversible while movements due to change in turgidity are reversible.

1.0Tropic Movement

Tropic movement is the directional movement of the part of a plant caused by its growth. The growth of a plant part in response to the stimulus can be towards the stimulus (positive tropism) or away from the stimulus (negative tropism). 

Types of Tropic Movements

(1) Phototropism (2) Geotropism (3) Chemotropism

(4) Hydrotropism (5) Thigmotropism 

(1) Phototropism:  The movement of a part of the plant in response to light is called phototropism. If the plant part moves towards light is called positive phototropism and if the plant part moves away from light, then it is called negative phototropism.

(2) Geotropism: The movement of a part of the plant in response to gravity is called geotropism. Roots of a plant move downwards in the direction of gravity it is called positive geotropism and stem of a plant moves upwards against the direction of gravity it is called negative geotropism.

Geotropism is also known as gravitropism i.e. movement of part of plant in response to gravity.

(3) Chemotropism: The movement of a part of plant in response to a chemical stimulus is called chemotropism. e.g., growth of pollen tube towards the ovule during the process of fertilization in a flower.

(4) Hydrotropism: The movement of a part of plant in response to water is called hydrotropism. Roots of seedling show positive hydrotropism.

(5) Thigmotropism: The movement of a part of plant in response to contact or support is called thigmotropism. e.g. Pea plants climb up other plants or fences by means of tendrils. Tendrils are sensitive to touch. When tendrils come in contact with any support, the part of the tendril in contact with the object does not grow as rapidly as the part of tendril away from the object. This causes the tendril to circle around the object and thus cling to it.

Tendril is a thread-like structure which can be formed from modified shoots or leaves. It is used by climbing plants for support and attachment generally by twining around whatever it touches.

2.0Nastic Movement

Movement which is neither towards nor away from the stimuli is called nastic movement. It is growth independent movement.A number of flowers close up when light fades in the evening and re-open the next day when light appears. It is an example of nastic movement. The stimulus is light in this case.

Seismonastic/ Thigmonastic Movements 

Such movements occur in response to touch (shock). These movements are very quick and are best seen in ‘touch-me-not’ plant (Mimosa pudica), also called ‘Chhui-mui’ or ‘Lajwanti’ or ‘sensitive plant’.

If we touch the leaves of the chhui-mui plant with our finger, the stimulus is transmitted to its base and then to other parts through the xylem sap, probably in the form of a chemical due to which all its leaves immediately fold up and droop.  After some time, the leaves regain their original status. Here, no growth is involved. Instead, plant cells change shape by changing the amount of water in them (turgor changes), resulting in folding up and drooping of leaves.

Differences Between Tropic and Nastic Movement