What are Movements in Plants? Living Organisms respond to the external as well as internal stimuli. Animals move in response to external stimuli by motion. Similarly, plants also show movements. Animals change their location in response to the stimulus. Plants are fixed therefore they change their growth pattern. Let’s discuss in detail different types of movements in plants are as under;
TYPES OF MOVEMENTS IN PLANTS
There are two types of movements in plants:
- Autonomic Movements
- Paratonic Movements
1. AUTONOMIC MOVEMENTS
Autonomic movements are spontaneous movements due to internal or external causes. Autonomic movements are of three types:
(i) Tactic movements
(ii) Turgor movements
(iii) Growth movement.
(i) Tactic Movements
These are the movements of an entire cell or organism i.e. Locomotion due to an external stimulus. The tactic movement may be positive if it is towards the stimulus or negative if it is away from the stimulus. Tactic movements are the movement of locomotion they are further classified on the basis of the nature of the stimulus as phototeotic and chemotactics.
(a) Phototactic Movement
It is a movement in response to a stimulus of light. The movement may be towards the light (positive) or away from the source of light (negative). The best examples of positive tactic movement are the passive of chloroplast due to cyclosis (cytoplasmic movement). This movement helps the chloroplast to absorb maximum light for CO2 fixation. The light intensity and direction both affect the intracellular distribution of chloroplast.
(b) Chemotactic Movement
The movement in response to a stimulus of chemicals chemotactic movements. The movements Shown by sperms of liverworts, mosses, ferns towards archegonia into stimulus of nucleic acid released by the Ovum are such examples.
2. Turgor Movements
Turgor movement is due to differential changes in turgor and size of cells as a result of gain or loss of water. Rapid movement of leaflets in “touch me not” plant and sleep movements of ‘the plants fall under this category of movements.
a) Sleep Movements
Bean plants and some members of legume family lower their leaves in the evening and raise, them in the morning, There are known as sleep These sleeping movements are due to daily changes in turgor pressure in the pulvinus, the place of attachment of leaf with the shoot. The pulvinus is a swollen portion of the petiole composed of parenchymatoué cells with relatively large intercellular spaces and a central strand of vascular tissues. When turgor pressure on the lower side of pulvinus increases the leaves rise and become horizontal, When turgor pressure decreases on the lower side of pulvinus, the leaves lower and go to ‘sleeping” position.
b) Rapid Movement of Leaflets
When the compound leaf of sensitive plant Mimosa (touch-me-not) is touched, the leaflets fold together its response takes a second or two resulting from rapid loss of turgor by the cells in pulvinus at the base of each leaflet, The investigation has shown that potassium (K+) ions move first, which causes water to -leave the cell by exosmösis. It takes about ten minutes to regain the turgor and restore the internal turgidity of a leaf.
iii) Growth Movements
Growth due to unequal growth on two sides of plant of organs like stem, root, tendrils, birds etc.There are three types of growth movements.
It is shown by leaves petals etc. The upper surface of the leaf in bud conditions shows more growth as compared with the lower surface. This Leads to an opening of buds.
If growth in the lower Surface of the leaf in bud condition to more than that of the upper surface then the bud will remain closed.
The growing tip of young stern moves in zig zag fashion due to alternate changes in growth on opposite side of the apex. This mode of growth is called nutation.
2) PARATONIC MOVEMENTS
These movements are due to external causes, These are of following types,
a) Tropic Movements or tropism
The word tropic is derived from Greek word ‘Tropos’ meaning ‘turn’. It is the movement in a curvature of the whole organ towards or away from stimuli such as light, gravity and touch, Following are common tropic movements:
It is the movement of part of the plant in response to a stimulus of light and is caused due to differential growth of a plant part like stem or root.
It is the movement in response to a stimulus of touch, for example climbing vines. When they come in contact with some solid object the growth on the opposite side of contact increases and the tendril coils around the support.
The movement in response to some chemicals is called chemotropism. The hyphae of fungi are chemotropic.
The movement of plant parts in response to a stimulus of water is called hydrotropism. The growth of root toward water is due to positive hydrotropism and shoot grow away from water is negatively hydrotropic.
v) Geotropism or Gravitropism
It is the response to gravity. Roots display positive geotropism and shoots negative geotropism.
vi) Nastic Movements
These are the non-directional movements of parts of the plant in response to external stimuli. These are of two types.
The nyctinastic movements are shown by the organs in response to external stimuli leading to differential growth. These are due to turgor and growth changes. It may be of two types:
The principal stimulus is the photoperiod. The flowers open and close due to light intensity.
ROLE OF PLANT GROWTH SUBSTANCES IN PLANT MOVEMENT
Plants do not move from one place to other like animals. However, their organs show movements; which are controlled by hormones.
1. Role of Auxins
i) Auxins play a major role in phototropism. It is believed that unequal distributions of auxins Indole Acetic Acid (IAA) in the coleoptiles stumps produce unequal cell enlargement causing a bend in the organ towards a source of light.
ii) Auxins are also responsible for positive gravitropism of roots and negative gravitropism of stem.
iii) Auxins inhibit the growth of root cells. The cells of the upper surface therefore elongate if the root curves downward.
iv) Auxins stimulate the growth of the stem cells. The cells of the lower surfaces therefore elongate if stem curves upward.
v) It has been observed that epinasty is due to auxins.
2. Role of Gibberellins
i) Nastic movements are due to some balance or ratio between growth inhibitors (abscise acid) and growth stimulators (gibberellins).
ii) It has been observed that hyponasty is due to Gibberellins.