Organisms have the ability to respond, to stimuli (external or internal). Animals change their position in response to stimuli. Plants also show movements, but higher plants do not change their location as a whole i.e. they lack locomotion, only they change their growth pattern.
Generally there are three types of movements in plants regarding growth sequence described below:
- Movements of Locomotion
- Movements of curvature
- Hygroscopic movements
First of all we shall discuss Movements in Locomotion in plants.
- Movements of Locomotion:
These are the movements in which the cell or cytoplasm of a plant cell moves from one place to another. Such types of movements may produce due to two reasons, 1 either spontaneously or without any external agent or 2 in response to an external stimulus. There are further two types of movements of locomotion. (a) Autonomic and (b) Paratonic movements.
- Autonomic movements; In living cells of some plants, the cytoplasm as well as some cells move from one place to another around the vacuoles. Such a movement of cytoplasm is known as Cyclosis. This movement assists the chloroplast to absorb maximum light for carbon dioxide fixation. Another type of autonomic movement is ciliary movements, which occur due to the presence of cilia or flagella e.g. Chlamydomonas or ciliated reproductive cells.
- Paratonic movements: These movements are further divided into three types of movements (a) Photo-tactic (b) chemotactic (c) thermo-tactic.
- Photo-tactic movements are the result of an external stimulus. e.g. light, (photo meaning light). Such movements are shown by the gametes of some algae. The movement may be towards the source of light (positive) or away from the source of light (negative). The most common example of negative photo-tactic movement is exhibited by an alga, Chlamydomonas. This alga goes away from the light, so that dehydration of water may not occur showing negative photo-tactic movement.
- Chemotactic movements are due to the stimulus of chemicals. The movement of sperms of mosses toward archegonia in response to the stimulus of nucleic acid released by the ovum is a common example of chemotactic movement.
- Thermo-tactic movements; Such type of movement exists in response to an external stimulus heat, (Thermo meaning Heat). For example, if a large vessel containing Chlamydomonas in cold water is warmed in one side, they will move towards the warmer side and start to collect there, indicating positive thermo-tactic movement.
Now let’s discuss the movement of curvature in plants;
- Movements of curvature: These movements occur in higher plants. Such movements are confined to the bending or curvature parts of plants and may be either spontaneous
(autonomic) or (paratonic) induced. Autonomic movement is further classified into two categories. (1) Autonomic movement of variation, (2) Autonomic movement of growth.
Autonomic movement of variation: This type of movement is indicated by the telegraph plant (Desmodium gyrans). In this plant the compound leaf consists of a larger terminal and smaller lateral leaflets. The two lateral leaflets show very interesting movement during day time. Sometimes they move upward at an angle of ninety degrees and come to lie to the rachis. Again they may move downward at an angle of one hundred eighty degrees so that they may parallel to rachis. They may again move upward at an angle of ninety degrees in order to come into their original position.
Autonomic movement of growth: This type of movement may be found in ferns, where the leaves become circinately coiled in young condition (hyponasty) or erect in older condition (epinasty) or in the opening or closing of the flower of many plants e.g. Crocus.
There is an additional kind of autonomic movement of growth i.e. Nutational movement. In nutational movement, the growth may occur in zig-zag pattern of the apices of some stems. The reason is that the two sides of the stem apex alternatively grow more. This movement is exhibited by flattened stem apices.
Paratonic or Induced movements: These movements are due to external causes. These are of the following types,
- Tropic movements
- Nastic movements
- Tropic movements: The word tropic is obtained from the Greek word ‘Tropos’ meaning turn. It is the movement in the curvature of the whole organ toward or away from the stimuli e.g. water, light, chemicals, gravity, or touch. Following are the types of tropic movements.
- Phototropism: Such movements are in response to the stimulus of light and are caused due to differential growth of parts of plants like stems or roots.
- Chemotropism: This movement occurs in response to the stimulus of some chemicals. Such movement is found in fungi.
- Thigmotropism: It is the movement in response to the stimulus of touch. Some plants use their roots to climb up and cling to trees. The Hedera genus of woody plants is a common example of thigmotropism.
- Hydrotropism: The movement of parts of plants in response to the stimulus of water. e.g. roots grow toward water showing positive hydrotropism and shoots grow away from water indicating negative hydrotropism.
- Geotropism: It is due to the response of gravity. Roots display positive geotropism and shoot negative geotropism.
- Nastic movements: These are non-directional movements of parts of plants in response to external stimuli. These are of two types : (1) Nyctinastic (2) Haponastic.
Nyctinastic: These movements occur in response to external stimuli leading to differential growth. These are due to growth and turgor changes.
Haponastic: Movements occur due to contact are called haponastic movements. e.g. the action of the venus fly trap.
- Hygroscopic movements: These are a very common type of movements employed in some plants in seed dispersal. It was first suggested by Zimmerman (Jost, in 1907) in the Geraniaceae family. Hygroscopic movements were also examined in Daucus carota, when humidity increases, umbels having mature fruits close; while relative humidity decreases umbels open. As humidity increases, the number of seeds drops near zero. Response to changes in relative humidity within an umbel increases from late August when dispersal starts. Umbels gradually increase their response and retain some seeds that may disperse over snow in winter. Change in response to relative humidity is high among plants starting dispersal at the same time. Such variability is maintained by the variable consequences of dispersing seeds at different times.