Movements in Plants – Definition, Classification, Mechanism, Examples

What is plant Movement?

• Plant movements, although often overlooked due to their subtlety, are an essential aspect of plant biology. All parts of a plant, including roots, leaves, and stems, are capable of movement. These movements usually manifest as bending, turning, twisting, or elongating, rather than the more obvious forms of locomotion seen in animals. Unlike animals, most complex plants are stationary, anchored to the ground by their roots. However, some lower forms of plant life, such as unicellular algae (like Chlamydomonas), as well as spores and gametes of bryophytes and pteridophytes, do exhibit locomotion.
• The movements in plants can be triggered by both external and internal stimuli, a property referred to as irritability. For example, some unicellular algae can move towards light, a phenomenon known as phototaxis. Similarly, sperm cells of some plants move towards egg cells using flagella, enabling fertilization. In contrast, some forms of movement in plants, like those observed in Myxomycetes (slime molds), are driven by internal factors and are independent of external stimuli. These amoeboid or creeping movements are slow but critical for the plant’s survival and function.
• Although plant movements may appear less dramatic than animal movements, they play an important role in helping plants adapt to their environment. Responses to external stimuli, like light, gravity, and touch, allow plants to maximize their growth potential and reproductive success. This ability to respond and move based on stimuli ensures their survival in varying conditions, despite their relatively static existence.

Classification of plants Movements

Most of movements exhibited by plant can be classified as –

1. Movement of locomotion
2. Movement of Curvature

A. Movement of locomotion

The concept of locomotion encompasses the various ways in which organisms, including plants, move from one location to another. In plants, locomotion can occur at different levels—ranging from the movement of the entire plant body to the movement of organelles within individual cells. Such movements can be categorized into two primary types: autonomic (spontaneous) movements and paratonic (induced by external stimuli) movements.

1. Autonomic Movements of Locomotion:
   • These movements occur independently of external stimuli and arise from internal factors. The key types of autonomic movements include:
     • Ciliary and Flagellar Movements:
       • Certain motile algae, such as Chlamydomonas and Volvox, along with zoospores and gametes of lower plants, employ cilia or flagella as locomotory organs. These structures facilitate movement through fluid environments.
     • Amoeboid Movements:
       • Exhibited by organisms like Myxomycetes (slime fungi), these movements involve the formation of pseudopodia, which are temporary projections of cytoplasm. This allows the organism to crawl and navigate its surroundings. Additionally, tapetal cells in some plants also demonstrate amoeboid behavior.
     • Cyclosis:
       • Cyclosis, or protoplasmic streaming, refers to the movement of cytoplasm within plant cells. There are two distinct types:
         • Rotation: In certain plants, like Hydrilla and Vallisneria, the cytoplasm moves around a central vacuole in either a clockwise or counterclockwise direction.
         • Circulation: This involves the movement of protoplasm around multiple vacuoles in different directions, as observed in the staminal hairs of Tradescantia.
     • Oscillatory Movements:
       • An example of oscillatory movement can be seen in the apical part of Oscillatoria, which moves similarly to a pendulum.
2. Paratonic Movements of Locomotion (Tactic Movements):
   • Paratonic movements, also known as tactic movements, occur in response to external stimuli such as light, temperature, or chemicals. The primary types include:
     • Phototactic Movements (Phototaxis):
       • These movements occur as organisms respond to light. For example, Chlamydomonas, Volvox, Ulothrix, and Cladophora exhibit positive phototaxis by moving toward suitable light intensity and negative phototaxis by moving away from intense light.
     • Chemotactic Movements (Chemotaxis):
       • In this type of movement, plants or plant parts move toward or away from chemical substances. A notable example is seen in bryophytes and pteridophytes, where male gametes are attracted to archegonia that secrete sugars and malic acid to lure them.
     • Thermotactic Movements (Thermotaxis):
       • This movement is influenced by temperature changes. For instance, Chlamydomonas cells migrate from colder water to medium-warm conditions, while they will retreat from excessively hot environments.

B. Movement of Curvature

The movement of curvature in plants is an essential phenomenon that allows them to adapt to their environments without the capability for locomotion. Instead of physically relocating, plants exhibit curvature movements in response to various stimuli or internal processes. These movements facilitate organ positioning for optimal functionality and are classified into mechanical (hygroscopic) and vital movements.

1. Mechanical (Hygroscopic) Movements:
   • These movements occur in the non-living parts of plants and are primarily influenced by the presence or absence of water. They do not involve protoplasmic activity and can be categorized into two types:
     • Xerochasy: This movement is triggered by the loss of water. For instance, when water is lost from the annulus of fern sporangia, it causes the sporangia to burst at the stomium, liberating spores. Similar explosive movements occur in certain fruits, such as Lupinus perennis and Iris versicolor, which release their contents when specific tissues dry out. Additionally, the fruit of Ruellia ruptures upon absorbing moisture.
     • Hydrochasy: This type of movement occurs due to water absorption. An example includes the peristomial teeth of moss, which protrude when the capsule is dry and curve when it is wet. In the plant Equisetum, elaters coil and uncoil based on the water content.
2. Vital Movements:
   • Vital movements are driven by the irritability of protoplasm and can be further divided into autonomic movements and induced movements.
     • Autonomic Movements: These movements arise from internal processes and are classified into:
       • Growth Movements: These are irreversible movements resulting from unequal growth in different parts of an organ and are independent of external stimuli. Key types include:
         • Nutation: This occurs in the growing stems of twining plants such as peas and beans, which exhibit nodding movements in two directions. The stem apex grows more rapidly on one side, resulting in a circular movement that is consistent within the same species.
         • Circumnutation: Observed in spirally growing stems and tendrils, circumnutation involves the region of greater growth gradually passing around the growing point, leading to coiling.
         • Epinasty and Hyponasty: These non-directional movements depend on the structure of the responsive organ rather than the direction of the stimulus. Epinasty occurs when rapid growth happens on the upper side, leading to leaf opening, while hyponasty involves more growth on the lower side, such as in circinate coiling and closed floral buds.
       • Variation Movements: These autonomous curvature movements are reversible and result from changes in turgor pressure within specific sensitive cells. An example includes the lateral leaflets of Desmodium gyrans, which exhibit rapid movements due to changes in turgor.
     • Paratonic (Induced) Movements: These movements are influenced by external stimuli and can be further categorized into tropic and nastic movements:
       • Tropic Movements: These are curvature movements induced by differential growth on one side of a plant organ due to external stimuli. Tropic movements are characterized by their slow nature and can be categorized as:
         • Phototropism: Movement toward light is termed positive phototropism, while movement away from light is negative phototropism.
         • Geotropism: Growth in response to gravity, where roots typically grow downward (positive geotropism) and shoots may grow upward or at angles (diageotropic).
         • Hydrotropism: Roots bending toward water sources exhibit positive hydrotropism, often more pronounced than their response to gravity.
         • Chemotropism: This occurs when growth is directed by chemical stimuli, such as pollen tubes growing toward the embryo sac.
         • Thigmotropism: This movement, often observed in tendrils, occurs when plants respond to contact with a foreign body, aiding in climbing.
         • Thermotropism: This involves curvature towards optimal temperatures and away from extremes.
       • Nastic Movements: Unlike tropic movements, nastic movements respond to stimuli in a predetermined direction, independent of the stimulus’s direction. They are generally faster and include:
         • Nyctinastic Movements: Diurnal movements seen in some species where leaves or flowers close at night, exemplified by Maranta (the prayer plant).
         • Photonastic Movements: Leaves of Oxalis exhibit horizontal positioning in sunlight and droop at night.
         • Thermonastic Movements: Flowers like tulips open during warm temperatures and close at night.
         • Thigmonastic Movements: Plants respond to touch, as seen in Drosera, which captures insects through rapid movements triggered by contact.
         • Seismonastic Movements: Responses to shock or touch, such as those exhibited by the leaves of Mimosa pudica, which fold upon stimulation.

Other Types of plants Movements

A. Tropic Movements

Tropic movements in plants are essential responses initiated by external directional signals, which are perceived by sensory receptors. These movements can either be toward (positive tropism) or away from (negative tropism) the signal. Here is a detailed breakdown of various types of tropic movements:

1. Phototropism (Light-Directed Movement):
   • Plants respond to light stimuli to maximize sunlight capture for photosynthesis. This movement, known as phototropism, can be either positive or negative.
   • Aerial parts of plants, such as coleoptiles and hypocotyls, are positively phototropic, meaning they grow toward light. Conversely, the tendrils of climbing plants are negatively phototropic, growing away from the light.
   • Leaves are plagiotropic, orienting at angles to light, while roots are mostly non-phototropic and do not respond significantly to light.
   • Phototropism is influenced by light’s quality (wavelength), quantity (fluence), direction, and duration (photoperiod).
   • Light-driven cellular processes include the movement of chloroplasts within cells to maximize or prevent light absorption, depending on intensity. In high light, chloroplasts align to avoid damage, while in low light, they position to absorb more.
   • Phototropins, blue light receptors, mediate phototropic responses and other light-induced movements such as stomatal opening and chloroplast redistribution.
2. Gravitropism (Gravity-Directed Movement):
   • Gravitropism, or geotropism, refers to plant movement in response to gravity, allowing roots to grow downward (positive geotropism) and shoots to grow upward (negative geotropism).
   • Growth in specific directions, whether vertical (orthogravitropic), diagonal (plagiogravitropic), or horizontal (diagravitropic), ensures efficient resource capture.
   • Statocytes, specialized cells containing statoliths (sedimentable starch grains), perceive gravity in roots and shoots. The position of statoliths, not their mass, plays a critical role in sensing gravity.
   • The redistribution of auxin, a growth hormone, facilitates differential growth in response to gravity. In roots, higher auxin concentrations on the lower side inhibit growth, causing downward movement, while in shoots, it promotes upward growth.
3. Thigmotropism (Touch-Directed Movement):
   • Thigmotropism occurs when plants respond to mechanical stimuli like touch or friction. Tendrils of climbing plants exhibit this by wrapping around supports when they encounter resistance.
   • This allows plants to secure themselves and grow in environments where mechanical support is necessary for stability and growth.
4. Chemotropism (Chemical-Directed Movement):
   • Chemotropism is the movement of plants in response to chemical signals. An example includes the growth of the pollen tube toward the egg cell in response to chemical signals released by synergids.
   • Chemicals secreted by the cells lining the style and ovary guide the pollen tube toward the egg for fertilization, ensuring reproductive success.
5. Hydrotropism (Water-Directed Movement):
   • Hydrotropism is the response of plant roots to water. Roots grow toward areas of higher water concentration, allowing plants to efficiently access the resources they need for survival.

B. Nastic Movements

Nastic movements are non-directional plant movements that occur in response to various stimuli. Unlike tropic movements, which depend on the direction of the stimulus, nastic movements are genetically predetermined and can be caused by internal or external factors. These movements can be broadly classified based on their triggering stimuli, such as light, temperature, touch, and other environmental conditions.

1. Autonomic Nastic Movements: These movements are internally controlled and include the actions of developing buds as they swell and open. They are not influenced by external stimuli but are rather dictated by the plant’s internal processes.
2. Paratonic Nastic Movements: These movements are directed by external environmental factors, such as light and temperature. Leaves and flowers may react to changes in these conditions, making paratonic movements vital for plant survival and adaptation.
3. Nyctinastic Movements: These are rhythmic movements that involve the folding of flowers and leaves at night. Plants like tobacco and clover display nyctinastic movements, which are driven by reversible turgor pressure changes. During the day, leaves remain open, but as night falls, they fold or close, presumably as a protective mechanism.
4. Seismonastic and Thigmonastic Movements: These are rapid movements triggered by mechanical stimuli, such as touch. For example, in Mimosa pudica (the touch-me-not plant), touch causes a rapid loss of turgor pressure in the pulvini at the base of the leaflets, resulting in the folding of the leaves. The closing movement is facilitated by the efflux of potassium ions (K⁺) and water from the cells, while reopening is achieved through active pumping of K⁺ back into the cells. Such responses are believed to be energy-dependent, involving ATP, calcium ions (Ca²⁺), and other molecular signaling pathways.
5. Photonastic Movements: These movements occur in response to the transition from darkness to light. Examples include the folding and unfolding of leaves and the opening and closing of flowers. Photonastic movements help plants optimize their exposure to light for photosynthesis and protect delicate parts during night or low-light conditions.
6. Skotonastic Movements: These movements are synchronized by the transition from light to darkness. While photonastic movements occur during the transition to light, skotonastic movements take place when light diminishes.
7. Thermonastic Movements: Temperature fluctuations drive thermonastic movements. In plants such as Crocus and Tulipa species, flowers open when temperatures rise and close when temperatures drop, even by a small margin (1-3°C). These temperature-sensitive movements ensure that reproductive organs are protected from adverse weather conditions.
8. Chemonastic Movements: Chemical changes, particularly the concentration of carbon dioxide (CO₂) in the substomatal cavity, can drive chemonastic movements. For example, the opening and closing of guard cells in response to CO₂ are critical for regulating gas exchange and water loss in plants. The changes in guard cell turgor are controlled by the osmotic potential in response to these chemical cues.
9. Epinasty and Hyponasty: Developing plant organs, such as leaves, petals, and sepals, often exhibit unequal growth between their upper (adaxial) and lower (abaxial) sides. This unequal growth leads to bending movements, known as epinasty (downward growth) or hyponasty (upward growth). For instance, tomato plants experiencing waterlogged soil conditions often show epinastic leaf movements. The growth on either side of the organ is irreversible, but the bending can be reversed. These movements are controlled by plant hormones like auxin and ethylene.
   • In developing buds, epinastic and hyponastic movements contribute to the formation of compact bud structures. Leaf primordia initially grow more rapidly on the lower side, creating a convex shape, while the upper side remains concave. This differential growth leads to the formation of a protective sheath around the young shoot.
   • As growth progresses, the shift from hyponastic to epinastic movements results in the characteristic flat leaf lamina, enabling leaves to spread out for photosynthesis. Similarly, during reproductive development, epinastic movements help expose floral parts like stamens and ovaries, ensuring efficient pollination.
10. Vegetative and Floral Bud Development: Nastic movements are also crucial in the transition from vegetative to reproductive growth. The shoot apex changes from producing leaf primordia to forming floral buds, which then produce sepals, petals, stamens, and carpels. The epinastic growth of flower stalks, such as in the poppy plant, is an example of how nastic movements facilitate the unfolding of flowers. As the flower opens, the growth reverses, resulting in hyponastic movement and the straightening of the flower. In the case of rain, poppy flowers exhibit a downward bending of the peduncle, which, along with the loss of petal turgor, causes the flower to close and protect its pollen.

C. Autonomous Movements

Autonomous movements in plants refer to internal mechanisms that enable movement without direct dependence on external stimuli. These movements are often guided by endogenous biological rhythms and processes such as the circadian clock. They play a vital role in synchronizing plant behavior with environmental cycles, including light and dark periods, thereby optimizing processes such as pollination, growth, and survival. Autonomous movements are crucial for the physiological coordination of plants with their surroundings.

• Diurnal Movements and Circadian Rhythms:
  • Diurnal movements are plant responses to the daily cycles of light and dark. Examples include rhythmic movements of leaves and petals, as well as the opening and closing of stomata. These movements are closely associated with metabolic processes like photosynthesis and respiration.
  • Flowers of plants in families such as Onagraceae, Cactaceae, Convolvulaceae, and Oxalidaceae exhibit prominent diurnal movements, where petals close and open depending on the time of day. These movements are primarily mediated by growth and occur as the flower or inflorescence develops.
  • These rhythms, known as circadian rhythms, occur with a periodicity of around 24 hours, regardless of external conditions. They are governed by an internal biological clock, an oscillator, that operates independently of environmental factors like temperature. This phenomenon is known as temperature compensation, ensuring that the circadian clock functions across various climatic conditions.
  • Several components define circadian rhythms:
    • The period refers to the time between repeating points in the cycle, such as consecutive peaks or troughs.
    • The phase represents any identifiable point on the cycle in relation to the whole cycle.
    • Amplitude is the magnitude of the observed response, which reflects the variation from the mean.
  • Under constant conditions, the cycle’s length does not adhere strictly to 24 hours but can vary slightly. This variation is called the free-running period. However, under natural conditions, circadian rhythms match the 24-hour period because they are entrained to environmental changes, such as light. Environmental signals that synchronize the rhythm are called zeitgebers (meaning “time givers”). If rhythms fade over time, exposure to zeitgebers can reinitiate them.
  • The endogenous oscillator driving circadian rhythms can be connected to multiple physiological responses, and these responses can occur out of phase with one another. Moreover, a single oscillator can regulate different circadian rhythms within a plant.
• Photoperiodism:
  • Plants use their internal circadian clock to measure the length of the day, allowing them to respond to seasonal changes. This response is known as photoperiodism and is essential for processes such as flowering. Photoperiodism is modulated by the length of day and night, which varies with geographical location.
  • Historical studies by Garner and Allard in the 1920s demonstrated the significance of day length in flowering in plants like the Maryland Mammoth variety of tobacco, which only flowered under short-day conditions.
  • Plants can be classified into categories based on their flowering responses to day length:
    • Short-day plants (SDPs) flower when day length is shorter than a critical duration. If flowering is strictly tied to short days, they are called qualitative SDPs. If flowering is only accelerated by short days, they are termed quantitative SDPs.
    • Long-day plants (LDPs) flower when day length exceeds a critical duration. Qualitative LDPs flower only under long-day conditions, while quantitative LDPs flower faster under such conditions.
  • The critical day length varies among species and determines whether a plant will flower. For instance, long-day plants like wheat bloom as day length increases in the spring, while short-day plants like Chrysanthemum flower in the fall when the days shorten.

D. Prey-Driven Movements

Prey-driven movements in plants refer to specialized adaptations enabling certain species to trap, immobilize, and digest prey, typically insects, for nutrient acquisition. These mechanisms are crucial for plants living in nutrient-poor environments, particularly those deficient in nitrogen. Prey-driven movements can be categorized into those utilized by parasitic plants and carnivorous plants, with each employing distinct strategies to locate and capture prey.

• Parasitic Plants:
  • Parasitic plants locate their hosts using a variety of cues. Mistletoes (Viscum album, V. cruciatum) from the Santalaceae family and plants from Loranthaceae (Loranthus acacia, L. europaeus) rely on animal vectors to disperse their seeds. Birds are drawn to their brightly colored fruits, ingesting them and later depositing the seeds onto tree branches. The seeds, upon germination, attach to the host plant by forming structures called haustoria. Haustoria are specialized organs that invade the host’s tissues, absorbing nutrients.
  • A more advanced example of parasitic plant behavior is seen in the dodder plant (Cuscuta, family Convolvulaceae). Dodder is a holoparasite, completely dependent on its host for survival. It uses chemical signals from its host, such as volatile terpenoids (α-pinene, β-myrcene, β-phellandrene), to locate and twine around the host. As it makes contact, dodder forms a prehaustoria structure that adheres to the host using sticky secretions like pectins. The prehaustoria later differentiate into functional haustoria, penetrating the host’s tissues by enzymatic degradation of cell walls. Once established, the haustoria connect with the host’s vascular system, allowing an exchange of nutrients, solutes, and even macromolecules between the parasite and the host.
• Carnivorous Plants:
  • Carnivorous plants use modified leaves to trap and digest prey. These traps can be categorized as either passive or active, depending on their mode of action.
    • Passive traps: Passive traps do not involve movement but rely on physical structures to ensnare prey. Pitcher plants (Nepenthes species) exemplify this strategy. Their leaves are modified into pitcher-like containers filled with digestive enzymes. Insects are attracted by the plant’s nectar and fall into the slippery neck of the pitcher, where they drown and are digested by the enzymes secreted from specialized glands.
    • Active traps: Active traps involve mechanical movements triggered by prey interaction. The Venus flytrap (Dionaea muscipula) has snap traps composed of two lobes that close together when triggered. The surface of each lobe has stiff bristles that function as mechanical sensors. When prey touches these bristles, action potentials are generated, leading to rapid closure of the trap within milliseconds. The lobes close tightly, interlocking the bristles, and the plant begins secreting digestive enzymes to break down the prey.
    • Another example of active traps is found in the sundew (Drosera species), which uses tentacle-like structures to ensnare prey. The tentacles are covered in sticky mucilage and arranged in concentric circles on the leaf surface. When an insect is captured, the tentacles curl inward, bringing the prey to the center of the leaf where digestive enzymes are secreted, further immobilizing and digesting the prey.
  • In aquatic environments, the bladderwort (Utricularia) employs suction traps to capture prey. The plant forms specialized leaves into bladder-like traps with hinged doors. When small aquatic organisms touch trigger hairs near the trap, the door opens, and water rushes in, sucking the prey into the bladder where they are digested.

E. Movements for Dispersal

The ability to effectively disperse seeds and spores has been a crucial factor in the dominance of angiosperms on Earth. Plants have evolved various mechanisms for seed dispersal, ensuring the survival and spread of their offspring. Seed dispersal can occur through passive or active means, with some plants utilizing explosive mechanisms to propel their seeds. Below are different methods that plants use for dispersal, along with the biological mechanisms that drive these processes.

• Cohesion-Mediated Seed Propulsion:
  • In plants with dry dehiscent fruits, seeds are liberated passively through the action of gravity, wind, rain, or animals. Examples include Lilium (Liliaceae) and Iris (Iridaceae), where seeds are released as the fruit dries and splits open.
  • Several plants exhibit a more dynamic method of dispersal through explosive movements of the fruit coat. This is common in families like Acanthaceae, Balsaminaceae, and Euphorbiaceae, where fruits undergo ballistic dehiscence. For example, plants such as Alstroemeria (Alstroemeriaceae), Lotus (Papilionaceae), and Cytisus scoparius (Fabaceae) display this behavior.
  • A more specific case is seen in Ricinus communis (Euphorbiaceae), where the fleshy outer layer of the capsule (exocarp) dries out and the hard inner layer (endocarp) splits open. This sudden dehiscence catapults the seeds to distances of around 3 to 3.5 meters.
  • Similarly, plants from the Acanthaceae family exhibit explosive dehiscence in their capsules. The two-valved capsules, encased by hardened sepals, dry up, and the structural differences between the capsule’s inner and outer layers cause increasing tension. After absorbing moisture, the capsules explosively split open, launching the seeds into the surrounding environment.
• Turgor-Mediated Seed Propulsion:
  • In some plants, seed dispersal is driven by changes in turgor pressure within the fruit. These plants maintain fleshy and turgid fruits until they dehisce explosively. Species such as Ecballium (Cucurbitaceae), Impatiens parviflora (Balsaminaceae), Cardamine impatiens (Brassicaceae), and Oxalis acetosella (Oxalidaceae) exhibit this form of seed propulsion.
  • A notable example is Lathraea clandestina (Orobanchaceae), a parasitic plant capable of catapulting its seeds to distances of 12 to 20 meters. In these plants, the fruit remains under pressure as it matures, and when the pressure is released, the seeds are forcefully ejected.
  • In Impatiens, the elongated fruit capsules have a fleshy pericarp composed of several valves. The subepidermal cells in these capsules are large and parenchymatous, aligned in an anticlinal orientation. During maturation, turgor pressure builds up in these cells, increasing the strain along fault lines in the sutures where the walls are weaker. Once the tension reaches a critical point, the sutures split, causing the valves to curve inward explosively, launching the seeds far from the parent plant.

Mechanisms of Movement

Plant movements arise through various mechanisms, including changes in turgor pressure, growth dynamics, contraction, and conformation shifts. These movements enable plants to adapt to their environment and optimize their growth and survival.

• Turgor-Mediated Movement
  • Turgor-driven movements rely on the physical force exerted by cells known as motor tissues. These tissues may be single-celled, like guard cells and root hairs, or multicellular, such as those that move entire leaves or flowers.
  • Multicellular motors, such as the pulvinus, operate by turgor changes in cells. The pulvinus, located at the base of leaves in Leguminosae and Oxalidaceae families, contains specialized cells that change size and shape to move the leaf.
  • The opposite sides of the pulvinus—the flexor (upper) and extensor (lower)—control leaf movement through alternating turgor pressure. At night, flexor cells gain turgor, and extensor cells lose turgor, causing the leaf to close. The reverse occurs during the day, leading to leaf opening.
  • These turgor-driven movements are modulated by environmental signals, particularly blue light and phytochrome, and involve complex redistribution of potassium ions between the cells.
  • A model suggests that light signals activate phytochrome, raising levels of secondary messengers such as inositol 1,4,5-triphosphate (IP3). This triggers the release of calcium ions into the cytosol, causing further molecular changes that drive potassium ion movement and, in turn, leaf motion.
• Growth-Mediated Movement
  • Plants, being sessile organisms, optimize their growth by directing actively growing parts, like roots and shoots, toward available resources. These movements are driven by elongation of motor tissues located just behind the growing tip.
  • Curvature in growing organs, such as shoots, occurs through differential growth rates on opposite sides of the elongating tissue. This process, which involves cell elongation acceleration on one side and inhibition on the other, helps the plant change the spatial orientation of its organs.
  • For example, as root hairs navigate through the soil, their growth shifts in response to obstacles, allowing the root to move forward by circumventing the obstruction.
• Movement by Change in Conformation
  • Conformational changes in plant cells can drive movement. The stomatal guard cells serve as an example of single-celled motors. These cells have an anisotropic structure with differentially thickened cell walls, which cause changes in volume and, consequently, stomatal opening and closing.
  • Multicellular motors, such as those in grass leaves, operate by altering the conformation of the leaf. Bulliform cells located along the midrib of grass leaves lose turgor as the leaf dries out, causing the leaf to roll. When water becomes available, these cells rehydrate, restoring the leaf’s flattened shape.
• Movement by Contraction
  • Some movements in plants result from contraction rather than conformation shifts. Specialized contractile roots found in bulbs and corms enable the movement of these storage organs through soil.
  • These roots have a thick, fleshy upper part that contracts, pulling the bulb deeper into the soil. The contraction is facilitated by the radial expansion of the parenchyma cells in the cortex, which creates space for the organ to move downward.
• Twining Plants
  • Twining plants use tendrils to locate and attach to a support, forming circumnutations. These tendrils may be modified leaves, as in Lathyrus aphaca or Pisum sativum, or modified branches, as in Bryonia and Passiflora.
  • The tendrils undergo differential contraction and expansion, creating dorsiventral asymmetry, which leads to progressive coiling around the support. Upon touching the support, the tendrils contract below the contact point, drawing the plant closer.
  • Tendril movement is highly selective and sensitive to mechanical stimuli (thigmonastic response). This response is facilitated by epidermal protuberances, which act as receptors, and the interconnected protoplasts within the tendril cells, enabling rapid transmission of the thigmonastic signal.