Category : 11th Class
Plants show movements in response to a variety of stimuli. Stimulus can be defined “as a change in external or internal environment of an organism that elicits response in the organism”. The reaction of plant to a stimulus is known as response. The power or ability of a plant to respond to a stimulus is called sensitivity or reactivity or irritability.
The movements which occur without the effect of external stimulus are called autonomic or spontaneous movements. Thus spontaneous movements are brought by definite internal stimulus, and if the movements are produced in response to external stimulus, they are known as paratonic or induced movements.
The area which perceives a stimulus is called perceptive region, while the plants part showing the response is known as responsive region. The minimum duration or time required for a stimulus to be applied continuously on the perceptive region to produce visible response is called presentation time. The duration between the application of stimulus and production of visible response is called latent time or reaction time.
Classification of plant movements
Plants movements are broadly classified into two types:
(1) Movements of locomotion: In this case, plant moves physically from one place to another. The movements of locomotion are of two type-autonomic (occurs spontaneously) or paratonic (induced by external stimuli).
(i) Autonomic movement of locomotion : These movement of locomotion are due to internal stimuli they are of following types :
(a) Ciliary movements : Certain motile algae (e.g., Chlamydomonas, Volvox, etc). Zoospores and gametes of lower plants move from one place to another by means of cilia or flagella.
(b) Amoeboid movements : It is the movement of naked mass of protoplasm by means of producing pseudopodia like process e.g., members of Myxomycetes (slime fungi).
(c) Cyclosis : These are movements of cytoplasm with in a cell (also called protoplasmic streaming). These are of two types :
(d) Excretory movements : Apical part of Oscillatoria is like a pendulum. It is considered that such movements are due to excretion of substances by the plants. (movements opposite to the side of excretion).
(ii) Paratonic movement of locomotion (Tactic movement) : These movements take place in whole small plants. e.g., chlamydomonas or small free ciliated organs e.g., gametes. These movements are due to external factors like light, temperature or chemicals and are of following types :
(a) Phototactic movements or phototaxisms : It is the movement of free living organism towards or away from light. e.g., movement of Chlamydomonas, Ulothrix, Cladophora, Volvox etc. towards suitable light intensity. Three types of arrangement present in columular cells in chloroplast of dorsiventral leaves.
(b) Chemotactic movements or chemotaxisms : It is the movement of plant or plant parts from one place to another towards or away from chemical substance. e.g., male gametes (antherozoids) of bryophyta move towards archegonia under the influence of sugars produced by neck canal cells and also in pteridophyta male gametes move towards archegonia due to the malic acid produced by disintegration of neck canal cells and ventral canal cells.
(c) Thermotactic movements or thermotaxism : It is the movement of free living organism in response to external stimuli of temperature. e.g., Chlamydomonas move from cold water to medium warm water and from very hot water to medium temperature.
(2) Movement of curvature : In these cases, plants are fixed, thus they fail to move from one place to another. Somehow, movement is noticed in the form of bend or curvature on any part of the plant. Movement of curvature can be classified into.
(i) Mechanical or Hygroscopic movements : These movements depend upon the presence or absence of water and occurs in non-living parts of plants. It is of two types.
(a) Hydrochasy : This movement occurs due to the absorption of water.
Example : Peristomial teeth of moss protrude out when the capsule is dry and curve when capsule is wet.
Spores of the Equisetum coil and uncoil in the presence and absence of water respectively.
(b) Xerochasy : This movement occurs due to the loss of water.
Example : When water is lost from the annulus of the sporangia of fern, it burst from stomium and spores are thus liberated out.
(ii) Vital movement : These movement are of two types :
(a) Growth movements : These movements are due to unequal growth in different parts of an organ and are irreversible. They are further divided into two types :
Autonomic growth movements
(i) Nutation (Nutatory movements) : These movements occur in the growing stem of twiners and tendrils. The stem exhibits a kind of nodding movements in two directions. This is because the stem apex shows more growth on one side at one time and a little later there is a greater growth on the opposite side. It is called nutation. In spirally growing stems the region of greater growth passes gradually around the growing point resulting in the spiral coiling of stem and tendrils. Such a movement is called circumnutation. Coiling of a tendril after coming in contact with a support is a thigmotropic movement.
(ii) Nasty movements : They are non-directional movements in which the response is determined by the structure of the responsive organ and not the direction of the stimulus. The responsive organ has an asymmetrical or dorsiventral structure. Greater growth on one side causes the organ to bend to the opposite side. Greater growth on the adaxial side is called hyponasty. e.g., circinate coiling and closed sepals and petals in a floral bud. Whereas more growth on abaxial side is called epinasty. e.g., opening of fern leaf and spreading of sepals and petals during opening of the floral bud.
Paratonic growth movement (Tropic and nastic movements) : These are movements of curvature brought about by more growth on one side and less growth on the opposite side of plant organ induced by some external stimuli. Depending upon the nature of stimuli these movements are of the following type :
(i) Phototropism (Heliotropism) : When a plant organ curves due to unilateral light stimulus it is called phototropism. Some parts of the plant e.g., stem moves towards light. These organs are called positively phototropic. Some other organs e.g., roots move away from light and they are called negatively phototropic. If we keep a plant in a dark chamber (Heliotropic chamber) with an opening on one lateral side the stem tip moves towards light i.e., towards opening. Phototropism of stem and root are due to differential hormonal effect. Violet blue light is most effective. Photoreceptor seems to be a carotenoid. Young stems are positively phototropic, leaves diaphototropic, shoots of Ivy plagio-phototropic, roots either non phototropic or negatively phototropic (e.g., white mustard, Sunflower). Mechanism is believed to be Cholodny-Went theory which states that unilateral light produces more auxin (IAA) and hence more growth on the shaded side resulting in bending.
(ii) Geotropism (Gravitropism) : Growth of movements induced by the stimulus of gravity are known as geotropism.
Generally, the primary root grows towards the force of gravity and hence is positively geotropic. The stem coloptile and pneumatophores grows away from the force of gravity and is negatively geotropic. The secondary roots and stem branches arise at angle less than 90o. They are thus plageotropic. Certain undergorund stems such as rhizomes, stolons of potato are oriented at right angle to the direction of force a gravity and are called diageotropic. Some of the lateral organs (e.g., corolloid roots of Cycas) possess little or no geotropic sensitivity, they are called ageotropic.
If some seedlings are kept in a dark chamber in different directions, root always move downwards and shoot away from the gravitational force.
According to Cholodny-Went theory there is more auxin on the lower side of both stems and roots. In stem higher auxin concentration increases growth while in roots it inhibits growth. Therefore, stem grow more on the lower side while roots grow more on the upper side causing the stem to bend upwardly and roots to bend downwardly. Another theory is statolith theory which states that perceptive regions contain statoliths (microscopic particles). Change in their position causes irritation and hence differential growth. Clinostat / Klinostal is a instrument which can eliminate the effect of gravity and allow a plant to grow horizontly by slowly rotating it.
The main axis of which is attached to a rod. On the top of the rod is attached a flower pot. The clinostat is kept in a horizontal position. When the clock axis rotates the flower pot also rotates. As a result of this the plant grows horizontally as the effect of gravity is nullified by clinostat. If the clock of the clinostat is stopped the rotation of the plant stops, the shoot apex moves upward (negative geotropism) and the root apex moves downwards (positive geotropism).
(iii) Hydrotropism : Growth movements in response to external stimulus of water are termed as hydrotropism. Roots are positively hydrotropic (i.e., bend towards the source of water).
Stem are either indifferent or negatively hydrotropic. Positive hydrotropic movement of the roots is stronger than their geotropic response. In case of shortage of water, roots bend towards the sewage pipes and other sources of water in disregard to the stimulus of gravity.
(iv) Thigmotropism (Haptatropism) : The movement which are due to contact with a foreign body. It is most conspicuous in tendrils which coil around support and help the plant in climbing. e.g., Tendrils of cucurbitaceae, petiole of clematis, leaf apex of Gloriosa.
(v) Chemotropism : When a curvature takes place in response to a chemical stimulus. The growth of pollen tube through stigma and style towards the embryo sac occurs with the stimulus of chemical substances present in the carpel or movement of fungal hyphae towards sugars and peptones.
(vi) Thermotropism : Curvature of plant parts towards normal temperatures from very high or very low temperatures. e.g., peduncles of Tulip, Anemone.
Variation movements (Turgor movements) : These movements are caused by turgor changes especially due to efflux and influx of K+ ions. (swelling or shrinkage of living cells due to change in osmotic potential) and are reversible. Variation movements are further divided into two types :
(1) Autonomic variation movement : These movement of variation, which occurs without the external stimulus. Rhythmic autonomic turgor changes produce jerky rising and falling of two lateral leaflets in Indian Telegraph plant (Desmodium gyrans). Here, large thin walled motor cells found at the leaflet bases regularly lose and gain water bringing about changes in turgor pressure.
Motor (Bulliform) cells present in the epidermal cells of some grasses cause their folding and unfolding movements (hydronasty).
(2) Paratonic variation movement (Nastic movements): These movements of variation are determined by some external stimuli such as light, temperature or contact but the direction of response is prefixed (not determined by the direction of stimuli). Nastic movements are of the following types :
(i) Nyctinastic (sleeping) movements : The diurnal (changes in day and night) movements of leaves and flowers of some species which take up sleeping position at night are called nyctinastic movement. Depending upon the stimulus they may be photonastic (light stimulus) or thermonastic (temperature stimulus). Maranta (Prayer plant), an ornamental house plant provides most common examples of nyctinastic response.
(ii) Photonastic movements : Leaves of Oxalis take up horizontal position in sunlight and droop down during night. Many flowers open during the day and close during night or cloudy sky e.g., Oxalis.
(iii) Thermonastic movements : Flowers of tulips and crocus open during high temperatures and close down during low temperatures.
(iv) Thigmonastic (Haptonastic) movements : When marginal glandular hair of Drosera come in contact with some foreign body e.g., body of insect, they show haptonastic movements. Due to this the insect comes in contact with the central glandular hair which after being stimulated bring the marginal glandular hair on the body of insect. These later movements are chemotropic whereas the previous movements of marginal glandular hair is chemonastic movement Drosera shows both nyctinasty and thigmonasty movements.
(v) Seismonastic movements : This type of movement is brought about in response to external stimulus of shock or touch. The best example of seismonastic movement is the leaves of sensitive plant Mimosa pudica (Touch me not). It shows both nyctinastic (Sleeping movement) and seismonastic movement (shock movements).
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