IntroductionLocomotion is a common characteristic among living beings which differentiates them from non-living organisms. Plants also have life. Thus, they too move. However, their movement differs from other living organisms. Their movements are not too obvious. Also, they do not have legs or limbs like other living organisms, Thus, they seem non-motile. Roots of plants and trees are fixed in the soil. It no way means that they do not move. Movement is taking place inside the structure of plants and trees which is constant. They also move when there is an external stimulus. Plants do not show obvious movements such as animals. They cannot walk or run from one place to another. Yet, they move and the reason for their movement is same as that of other living organisms. It is in the search for food and water. They move from harmful stimuli which help in their survival.
Plants respond to external stimuliAs described earlier, plants display movement whenever there is some form of external stimulus in the environment. It can be anything from a change in light to temperature and as simple as touch to flow of heavy chemicals. They also respond to water and gravity. The whole of plant or tree structure does not respond to stimulus and move. Certain parts do that and show movement as a response. Also, certain parts receive the stimuli and are referred to as perception site. Usually, the site of perception and site of response is different in the majority of plants and trees. Any stimulus such as light, temperature, water should first pass through the plasma membrane of cells of the site of response. When the plasma membrane receives the stimulus and transmits further, it will lead to a response.
Types of plants movementsMovement of plants can be generally classified into two major types.
It is also known as a spontaneous movement. It is because plants movement takes place very naturally under the absence of an external stimulus.
It is also known as an induced movement. In it, plants show movements in response to various external stimuli.
Types of autonomic movement of plantsPlants show three different types of autonomic movements. They are-
Movements of locomotionAny movement of plant that is displayed by the whole structure of a plant or by a certain part or even by a cell material in absence of external stimuli, but in presence of some internal factor is termed as movements of locomotion. These movements are autonomic and spontaneous. Some examples of movements of locomotion are a movement of zoospores and cyclosis.
Variation movementsThese movements are also spontaneous and autonomic in nature. When the plants witness changes in turgidity of cells of a certain part, they display variation movements. The plant such as Desmodium gyrans display variation movement by allowing its lateral leaflets to move up and down in a rhythm.
Growth and curvature movementWhenever plants witness unevenness in growth on two different sides of a part, they display this movement as a response. Growth and curvature movement is further divided into two different types. One in nutation and the other one is nastic movement.
Since the stimulus is internal, there is no such response as growing towards or against the direction of the stimulus. Plants either show permanent response in the form of growth or reversible response by displaying turgor movements. For example, the growth displayed by the side of the supporting part of the plant that is in all the time contact with the support usually shows slow growth rate than the opposite side of the same part. Thus, the supporting organ shows some curvature type movement around the support in the form of twinning or spiralling.
When the same organ of plant displays uneven growth on two different surfaces, plants display nastic movements. It is a response of the plants that occur when there is uneven growth in the same organ on different surfaces in opposite direction. Nastic movement of the plant taking place due to the rapid growth rate in the upper surface of the organ is known as the epinastic movement. When it takes place in the lower surface, it is referred to as a hyponastic movement. Coiling of leaves is one such example. Nastic movements can be permanent in the form of growth or reversible in the form of turgor movements.
Types of paratonic movement of plantsPlants also show three different types of paratonic movements. They are-
Tropic movementsThese are the movements of plants that take place in response to various external stimuli which are unidirectional in nature. Consequently, plants respond to the direction of the stimulus. External stimuli can be light, gravitational pull, water or chemicals. Below-mentioned is different types of tropic movements.
It is also known as geotropism. It is the response of plants and trees to the gravity of the earth. Every part of plants responds differently to the gravitational pull of the earth. For example, the response showed by the root system is not same as the shoot system. Roots grow in a unidirectional way towards the gravitational pull. On the other hand, the shoot system grows away from it. The difference in response is due to the difference in an inbuilt mechanism inside the tips of roots and stems. The root system and shoot system are the products of the same embryonic cell. Yet, they function differently. It is because though the genetic composition is the same, but both have different functional programming.
Plants respond to external stimuli such as light. This response of plants is known as phototropisms. When the part of the plant responds by bending towards the source of light, it is known as the positive phototropic response of plants. The negative phototropic response is when parts of the plant move away from the light source. For example, stem, branches move towards the source of light. On the other hand, root moves away from the source of light. Plants contain phototropin which are specialized receivers responsible for activating hormone auxin. This hormone is responsible for phototropisms in plants.
Structure of plant needs water. They move in the direction of availability of water which is known as hydrotropism. Though they use up a minimal amount, yet, they need it to sustain. Roots respond positively by moving towards the direction of the water source. Hence, it can be termed as positive hydrotropism.
When there is the exposure of chemicals in the external environment, plants show some movements. It is known as chemotropism. Roots elongate themselves in the direction of availability of essential minerals. Similarly, when there is plenty of oxygen, plant organs move in that direction. It is known as aerotropism or oxytropism.
Nastic movementsThese movements in plants can take place due to the change in growth and turgor. Different stimuli bring about different nastic movements in plants. There are three types of nastic movements.
They are also referred to as sleep movements. It is because they occur as a response to light or day time. Sometimes nyctinasty occurs when there is an alteration in the intensity of light and thus, it is referred to as photonastic. When there is a change in thermal intensity, it is referred to as the thermonastic movement. An example is saffron.
It is the movements of plants in response to chemical substances in the external environment. In the case of sundew leaves, peripheral tentacles bend towards the middle portion of leaf whenever there is the presence of organic nitrogen.
It is the movement of plants that occur whenever there is a touch stimulus such as shaking, burning, falling of raindrops, heating and wounding. An example is Mimosa pudica.
Tactic movementsPlants respond by moving in the direction of the external stimulus. It is known as tactic movements of plants. Different types of tactic movements are described below-
It is the movement of plants in response to light as external stimuli.
It is the movement of plants in response to certain chemicals in the external environment.
It is the movement of plants towards the direction of temperature as a stimulus. An example is whenever there is an increase in temperature, cytoplasmic movement takes place in Vallisneria leaves.