What is Plant movement?
The movement of higher plants are generally in the form of bending, twisting, and elongation of certain plant parts or organs. Some plant movements are caused in response to certain stimuli and they are said to be induced or paratonic movements. These movements depend on irritability and sensitivity of protoplasm. There are other plant movements which take place spontaneously, without any external stimuli. These movements are described as spontaneous or autonomic movements.
Autonomic or Spontaneous Plant Movement
Plant Movement of locomotion
Movements of the whole plant body or of an organ or of material within the plant cell which occur in response to inherent factors and are free and spontaneous from one place to another in some medium are said to be movements of locomotion
Growth and curvature movements
This type of plant movement brought about by unequal growth on different sides of at organ, are of two types, viz. nutation and nastic.
These movements occur due to periodic variations in the turgidity of cells of an organ.
Paratonic or Induced Plant Movement
Tropic: plant movement
Growth movements, which occur in response to unidirectional external stimuli and result in positioning of the plant part in the direction of the stimulus, are said to be tropic plant movement. Depending upon the nature of stimuli, these movements are of the following five types.
Geotropic Plant Movement or Geotropism
Growth movements induced by the stimulus of gravity are said to be geotropism. The primary roots always grow downward in the direction of gravity and are thus positively geotropic, whereas the main shoots grow upward away from the gravity and are thus negatively geotropic. The negative and the positive responses of the stem and root tips respectively to the gravity are due to increased concentrations of auxin on their lower sides. The concentration of auxin which is stimulatory to shoot growth is inhibitory to root growth. Thus it is postulated that the increased auxin concentration in the lower half of a horizontally placed shoot stimulates growth in that half, causing the shoot to curve upwards. But in the horizontally placed root auxin increase on the lower side, results in growth inhibition and a downward curvature.
Phototropic plant movement or Phototropism
These type of curvature plant movement occur when a plant is provided with artificial or natural light only from one direction. Stems which generally show a curvature towards the source of light are said to be positively phototropic. Roots which grow away from the source of light are called negatively phototropic. Leaves of some plants can keep themselves perpendicular to the source of light and they are said to be diaphototropic.
Hydrotropic Plant Movement or Hydrotropism
Growth movements in response to unilateral stimulus of water are known as hydrotropism. Roots are positively hydrotropic as they bend towards the source of water. Hydrotropic movements are thus more powerful than geotropism.
Chemotropic Plant Movement or Chemotropism
This is the plant movement caused by unilateral stimulus of some chemicals. Movement of pollen tube through the style towards the ovary or movements of fungal hyphae towards the region of culture medium where more nutrients are concentrated are the examples of chemotropism.
This is the curvature movement of plants in response to unilateral stimulus of touch. Haptotropism can best be seen in twinners and climbers. Whenever the supporting organ touches a support, it makes a curvature movement to get hold of the object and then tightens its hold. In tendril climbers, the tendril first shows nutation and as soon as its tip touches a support it makes thigmotropic movement to coil around it.
Tactic: Plant Movement
Tactic movements are movements of locomotion, which are induced by some unidirectional external stimuli. Their direction is controlled by the direction of the stimulus. Depending upon the type of stimulus, these movements are of the following three types.
i) Phototactic: These movements are in response to unidirectional light.
ii) Chemotactic: These movements are in response to certain unidirectional chemical stimuli.
iii) Thermotactic: These movements are in response to certain unidirectional temperature stimuli.
Nastic: Plant Movement
Nastic movements are also growth movements which occur in response to external stimuli but here the direction of the stimulus is not fixed, it is diffused. Depending on the type of stimulus, such movements may be of the following three types.
Nyctinastic plant movement
These movements of plant organs occur in response to day and night and thus are also known as sleep movements. If the movement is induced by the change in light intensity, it is called photonastic, and if by the change in temperature, it is said to be thermonastic. Photonastic movements are exhibited by the flowers ofOxalis, Portulaca, Nicotiana, Oenothera, etc. The flowers of OxaliswA Portulaca open in day and close at night, whereas those of Nicotiana and Oenothera close in day and open in night.
The flowers of tulip (Tulipa) and saffron (Crocus) show thermonasty. They respond to an increase in temperature by increased growth on the upper side of the perianth segments. This results in opening of flower. Decrease in temperature, however, retards the growth on the lower surface of the perianth segments, resulting in the closure of flower. Under natural conditions, this response causes flowers to open by day and to close by night.
Chemonastic plant movement
These movements occur in response to some chemical stimulus. Strong chemonasty is exhibited by long peripheral tentacles of sundew leaves (Droserd), which respond to the presence of organic nitrogenous compounds by bending towards the middle of the leaf.
Seismonastic plant movement
These movements occur in response to shock by a touch stimulus. They are best exhibited by the leaves of Mimosa pudica, the sensitive plant. It is a herbaceous plant with bipinnate compound leaves. The base of the petiole is swollen (pulvinus) and similar but smaller pulvinules are present at the base of each leaflet. If the terminal pinnule is struck a blow or touched, the stimulus is conducted to its base and then to the other pinnules