CONTROL AND COORDINATION

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CONTROL AND COORDINATION

‘The nervous system, in coordination with the endocrine system communicates, integrates and coordinate the various organs and organ system in the body’

 

INTRODUCTION

There are various organ systems in all living organisms carrying out various physiological processes. These organ systems cannot work independently. They are linked in one way or the other. Working together of all these systems is called coordination. Coordination is mainly of two types :

1. Chemical coordination in both plants and animals,
2. Nervous coordination in animals.

 

COORDINATION IN ANIMALS

Life itself is a dynamic state where every organism must continually spend energy and obtain it from environment. So, the life of an organism is dependent upon its surrounding environment. All organisms are capable of sensing environmental changes and respond according to them. This character of living organisms is known as irritability, reactivity, behaviour or responsiveness. This character is less developed in plants due to their static nature. Irritability helps organisms to interact with day-to-day changing environmental circumstances. Organisms give responses to these changing conditions and remain adjustable in their habitat. This capacity of adjustment in of organisms is called adaptability. In higher animals, along with nervous system an additional control system is present which is called endocrine (or hormonal) system.

The evolution of complexity in multicellular animals required the development of a system for the control and coordination of the activities of various cells of the body. The control and coordination in a cell requires—
(i) information of changes in the external environment
(ii) transmission of changes in the external environment
(iii) exchange of information between concerned cells

 

(A) Nervous system
We know that nervous system is composed of some specialised cells called neurons or nerve cells. These cells produce electrical signals called nerve impulses and chemical substances called neurotransmitters. The control of nervous system is speedy and flexible but is localised and occurs for a short period of time.

(B) Endocrine system (Hormonal system)
This system is made up of endocrine glands which coordinate other system by sending chemical messengers termed as hormones (primary messengers). The hormonal control is generally slow but is effect is diffused.

NERVOUS SYSTEM IN ANIMALS
Nervous system in animals is the system which coordinates all the other system within the organisms as well as external environment of the organisms. All the multicellular organisms (except sponges) posses simple or complex nervous system. The structural and functional unit of nervous system is specialised cell called neuron or nerve cell.

 

Neurons

Neurons or nerve cells are specialised cells of body capable to receive, conduct and transmit excitations or impulses throughout the body. Nerve cells are specialised for receiving stimuli (as sensory or receptor cells) and transferring excitations from one to the other. The neurons are the largest cells present in the human body, sometimes reaching 90-100 cm in length. A neuron is typically divided into three parts—
(i) Cell body (cyton)
(ii) Dendrites
(iii) Axon

A Nerve cell

(i) Cyton or cell body

It is the main part of neuron. It is a nucleated body, also called soma or perikaryon. Cell body is of various shapes (rounded, pyriform or stellate) consisting of abundant granular cytoplasm called neuroplasm and large spherical nucleus. The cytoplasm contains mitochondria, Golgi apparatus, neurofibrils, neurotubules and special ribosome containing granules called Nissl’s granules. The Nissl’s granules are characteristic feature of nerve cell. Centrioles are absent in cell body.
Function: It accepts nerve impulses from dendrites and transfer them to axon (nerve fibres).

 

(ii) Dendrites (dendrons)
A cyton produces five to seven short, slender (tapering) and branched structures known as dendrites. Dendrites also contain Nissl’s granules and neurofibrils. Function: These cytoplasmic processes receive impulses and transmit them towards the cell body.

 

(iii) Axon 
It is a single, relatively thicker, long, unbraIiched cytoplasmic extension arising from the cell body. The cell membrane of axon is called axolemma and cytoplasm is called axoplasm. Nissl’s granules are not found in axon but neurofibrils are present.Axon is branched at terminal ends called telodendria Each telodendria bears a terminal knob. Telodendria of an axon make contact with other neurons at synapse.  Knobs of one neuron lie upon dendrites of adjacent and protective sheath around it, called myelin sheath. Myelin sheath itself is made up of individual cells (Schwann cells) with abundant fatty materials. The sheathed axon is called nerve fibre and a number of nerve fibres are joined to form a nerve. The nerve fibres having myelin sheath are called myelinated nerve fibres while without myelin sheath are called non-myelinated nerve fibres. At certain places in myelinated nerve fibre, sheath is not present. These areas are called nodes of Ranvier. Function : Axon transmits impulses away from the cell body to another neuron or target cell.


Transmission of Nerve Impulses (ELECTROCHEMICAL MECHANISM)


Neurons are situated end to end in bodies of animals and transmit nerve impulses throughout the body. Nerve impulse is a self-propagated electric current which travels from one end (dendrites) to another end (axon end) of a neuron for the conduction of a message. Each neuron receives an impulse through dendrites and passes it to the next neuron in a channel and the information finally reaches to the effector organ. The neurons are not connected. There is a small gap between the terminal portion of axon of a neuron and the dendron of the other neuron. This is called synapse. These occurs a presynaptic knob formed by the axon terminal. The dendrite terminal is slightly bordered and depressed to form postsynaptic depression. The narrow space between the presynaptic knob and postsynaptic depression is called synaptic cleft and is filled with a fluid containing neurotransmitter. We know that any two neurons are not in direct contact with each other. When an electrical impulse reaches the end of axon, it stimulates the release of neurotransmitter (for example, acetylcholine within the gap (or synapse). The neurotransmitter molecules come in contact with the membrane of postsynaptic depression. It acts as a stimulus and produce an impulse to be carried on further. In this way, nerve impulses are transmitted.

Neuromusular junction

 

REFLEX ACTION AND REFLEX ARC

These are nerve-mediated, automatic, involuntary and spontaneous actions that occur without the will of an animal. These responses are natural and automatic and occur suddenly on receiving a stimulus. A reflex action is an immediate involuntary response which do not require any thinking by the brain. A reflex action may be defined as a spontaneous, automatic and mechanical response towards stimulus without the will of an animal. Reflex actions (reactions) are the simplest responses neurologically. Functionally, these are the basic units of nervous coordination.  Examples of reflex actions are watering of mouth at the sight of delicious food, blinking of eyes due to sudden appearance of some objects in front of the eyes, coughing, yawning, sneezing and sudden withdrawal of hands or feet with a jerk on sudden contact with hot, cold or sharp objects.

 T.S. of spinal cord and arrows are showing reflex arc 

Reflex responses occur immediately and very fast. These fast reflex reactions protect the body against injurious effects of sudden stimuli. A number of reflex responses occur in the daily life of animals. The reason of fastness of the reflex action is that these actions occur without the sensory impulse being carried to the brain for analyses. It is an automatic response and is controlled. by the spinal cord.

On receiving a stimulus, the dendrites of sensory neurons, i.e., receptor, pass the message in the form of electric impulse to the spinal cord. The relay neurons in the spinal cord pass the message to motor neurons. The spinal cord in turn sends information via motor neurons through electrical impulse to the effectors, i.e., muscles or glands which show responses. For quick response, reflex action involves the spinal cord but the information goes to the brain also where the thinking process takes place.

    •  Reflex Arc
      A reflex action involves coordination between receptor organ, sensory neurons, a part of CNS, motor neurons and effector organ. The path taken by nerve impulse in a reflex action is called reflex arc. The entire impulse circuit of a reflex response is as follows :Stimulus → Receptor → Sensory neurons → CNS → Motor neurons → Effector

This route or sequence by which a nerve impulse acts to be effective is known as reflex arc. The nerve fibres carry the impulses to their cell bodies (cyton) located in the dorsal root ganglion of the nerve. Axons of these neurons then carry the impulses into the grey matter of the spinal cord.

Reflex arc

Reflex arc involves five parts

(i) Receptor organ (sensory organ)
It is the organ which receives the stimulus and initiates a sensory nerve impulse.

(ii) Sensory nerve (afferent nerve)
it conducts sensory nerve impulse from the receptor organ to the part of CNS (brain or spinal cord).

(iii) Part of CNS (spinal cord or brain)
The neurons in spinal cords or brain analyse and interprets the sensory impulse and sets upon appropriate motor impulse. The reflex formed from the spinal cord are called spinal reflexes and the reflexes formed by the brain are called cerebral reflexes.

(iv) Motor nerve (efferent nerve)
It conducts motor nerve impulse from the CNS to the specific effector organ (muscle or gland).

(v) Effector organ
Impulse terminates and response occurs as per the instructions given by the CNS to the effector organs, i.e., muscles or glands.

 

Parts of nervous System

Nervous system can be divided into two main parts-
Central Nervous System (CNS) (consisting of brain and spinal cord)  and Peripheral Nervous System (PNS) (consisting of all the nerves arising from brain and spinal cord).
PNS is further divided into two parts, Voluntary Nervous System (VNS) (It is under the control of our will.) and Involuntary Nervous System (INS) or Autonomic Nervous System (ANS) (It is not under our control and controls the activity of internal body organs.) Involuntary nervous system is again of two types— Sympathetic Nervous System (SNS) and Parasympathetic Nervous System (PNS).  These two systems control the functioning of various body parts. The classification of nervous system is outline below :

Main parts of human nervous system

 

[I]Central Nervous System (CNS)

Central Nervous System (CNS) is considered as the supreme power of controlling all the body responses. It consists of a brain and spinal cord. Both of them are protected by hard skeletal structures. Brain is protected by skull and spinal cord is protected by vertebral column.

 

(1) Brain

 External view of human brain

It is the widest and uppermost part of CNS. It is present in the cranial cavity of cranium (brain box) of skull (head). Brain box protects brain from mechanical injury. Brain is a soft organ which weighs 1.2-1.4 kg. Brain constitutes approximately 98% of the whole nervous system. Brain contains about 100 billion neurons.  Human brain is the most advanced and developed among all the animals found on the earth. It is the centre of thinking and main coordinating centre of the body.

Brain is covered by three meninges or membranes, i.e.,
(i) Piamater
(ii) Arachnoid membrane
(iii) Duramater

The spaces between the meninges and the brain cavities are filled with a clear, slightly-alkaline fluid called cerebrospinal fluid. This fluid supplies useful materials to the brain cells and collects metabolic wastes from these cells. The meninges and the cerebrospinal fluid give support to the brain and protect it from external pressure, shocks and other hazards. The blood vessels of pia-arachnoid mater supply oxygen and nutrients to the brain by cerebrospinal fluid of brain cavity which acts as the tissue fluid of brain. Venous drainage of CO2 and other metabolic wastes is done by veins of epidural space.

Different parts of the human brain

Functions of brain

(i) It receives information from the sensory receptors, process the information, generate the appropriate responses and send the instructions to effectors.
(ii) It controls, regulates and coordinates the overall functions of the body.
(iii) It is the site of intelligence, memory, reasoning, learning, and emotions.

Parts of brain

Lateral view of human brain

The brain is divided into three main regions—
(a) Fore brain
(b) Mid brain
(c) Hind brain

 

Location of various sensory areas of cerebral hemisphere

(a) Fore brain
It is made up of cerebrum, hypothalamus and thalamus.

    • Cerebrum
      It is the largest·and main thinking part of the brain and is made up of two hemispheres-called the cerebral hemispheres. The cerebrum has sensory areas, association areas and motor areas.
      (i) The sensory areas receive the messages.
      (ii) The association areas associate this information with the previous.
      (iii) Other sensory informations.
      (iv) The motor areas are responsible of the action of the voluntary muscles.
      Cerebrum is responsible for the intelligence, memory, consciousness and will power.
    • Thalamus
      It is an area which coordinates the sensory impulses from the various sense organs -eyes, ears and skin and then relays it to the cerebrum.
    • Hypothalamus
      Hypothalamus, though a small region situated below the thalamus, is an important region of the brain. It receives the taste and smell impulses, coordinates message from the autonomous nervous system, controls the heart rate, blood pressure, body temperature and peristalsis. It also forms an axis with the pituitary which is the main link between the nervous and the endocrine systems. It also has centres that control mood and emotions.

 

(b) Mid brain
It is a small portion of the brain that serves as a relay centre for sensory information from the ears to the cerebrum. It also controls the reflex movements of the head,neck and eye muscles. It provides a passage for the different neurons going in and coming out of the cerebrum.

Sagital (median) section of human brain

(c) Hind brain
It consists of cerebellum, pons and medulla oblongata.

  • Cerebellum 
    Cerebellum is like cerebrum. It consists of outer grey cortex and inner white medulla. It is responsible for maintaining the balance while walking, swimming, riding, etc. It is also responsible for precision and the fine control of the voluntary movements.  For example, we can do actions like eating while talking or listening. One has to concentrate for talking sensibly. However the action of eating , while talking is done automatically. This is controlled by the cerebellum.
  • Pons 
    Pons literally means bridge. It is hidden as it is well protected because of its importance. It has the cardiovascular centre and the breathing centre.
  • Medulla
    It is a somewhat triangular part between pons varolii and spinal cord. It is the posterior part of brain which lies below the cerebellum. It froms most of the ventral part of the hindbrain. Medulla oblongata contains vital reflex centres which control the rate of heartbeat, breathing movements, blood pressure (B.P.) by expansion and contration of blood vessels, peristaltic movement of alimentary canal, swallowing, coughing, sneezing, vomiting and salivation.

 

(2) Spinal cord

It is a collection of nervous tissue running along the back bone. It is, in fact, protected by the vertebral column. It is a continuation of the brain.
The functions of the spinal cord are :
1. Coordinating simple spinal reflexes
2. Coordinating autonomic reflexes like the contraction of the bladder
3. Conducting messages from muscles and skin to the brain,
4. Conducting messages from brain to the trunk and limbs.

 

Cross section of spinal cord

Connection of spinal cord with brain

 

[II] Peripheral Nervous System (PNS)

All the nerves connecting the CNS (brain +spinal cord) with receptors and effectors (muscles and glands constitute the pheripheral nervous system.  Peripheral nervous system connects CNS with different parts of the body with the help of nerves arising from the brain and the spinal cord.  In humans, there are 12 pairs of nerves which arise from the brain (cranial nerves) and 31 pairs of nerves which arises from the spinal cord (spinal nerves).  In addition to these nerves, there are some special kinds of nerves, mostly arising from the spinal cord which connect internal organs such as heart, kidney, lungs, blood vessels, glands, etc. These nerves are called visceral nerves.

 

Difference between cranial nerves and spinal nerves

Thus, PNS consists of all three type of nerves.
(i) cranial nerves
(ii) spinal nerves
(iii) visceral nerves

(i) Cranial nerves
Cranial nerves arise from the brain and extend to various parts of the head. They are 12 pairs in number, of which 3(I, II, VIII) are sensory, 5 (III, IV, VI, XI, XII) are motor and 4(V, VII, IX and X) are mixed nerves.

(ii) Spinal nerves
Spinal nerves arise from the spinal cord and extends throughout the body except head. They are 31 pairs in number. They are mixed type of nerves.

(iii) Visceral nerves
Visceral nerves are a particular set of nerves that control many activities of the internal organs of the body like kidneys, lungs, heart, blood vessels and urinary bladder inspite of regulating normal functions of the body. Most of these nerves arise from the spinal cord and a few from the brain. The visceral nerves constitute the autonomic nervous system.

 

Endrocrine system

A group of endocrine gland which produce various hormones is called an endocrine system. In addition to nervous system, the endocrine system also helps coordinating the acitivites of our body.

Endocrine glands – The glands which pour their secretions directly in the blood are called endocrine glands.

Hormones – are secretions of the endocrine glands and one of the most important substance that controls the body chemistry. Also known as “Chemical messengers.”

 Endocrine glands in human beings

Physical and chemical properties hormones

  • These are secreted by endocrine glands.
  • Hormones are secreted only when required.
  • Their secretion is regulated by feedback mechanisms.
  • These are generally released in the blood stream.
  • The molecules of most of the hormones are small.
  • The secretion of hormone is always in very small quantity.
  • Hormones are destroyed after use i.e. hormones can not be stored in the body.
  • hyroxine is an exception.

 

Endocrine glands in human body

 

FEEDBACK MECHANISM TO CONTROL THE SECRETION OF HORMONE

Usually glands secrete hormones continuously or at intervals. The quantity of secretion of hormone depends on many factors like age, health state of individual, biological cycle and the body requirement  in particular circumstances. Therefore, regulation of hormones in the body is a well-defined and regulatory process. Living organisms maintain homoestasis, i.e., maintenance of favourable internal environment. Hormones play a significant role in maintaining homeostasis. To maintain homeostasis, it is necessary to keep the level ofhormones at an optimum level. This is achieved through the feedback mechanism.  Feedback mechanism refers to a regulatory mechanism where presence of a substance at certain level promotes or inhibits its further formation. The feedback control mechanism is mostly negative but in rare cases, it is positive.

(A) Negative Feedback Control
In this type of control, when the level of a certain hormone rises in the blood above normal its synthesis slows or stops. Let us see following example

  • Blood glucose homeostasis When you eat a meal which is rich in carbohydrate the sugar (glucose) level in the blood increases. This increased glucose level stimulates the b cells of islet of Langerhans of pancreas to secrete insulin. Insulin directs the target cells to utilise glucose in respiration or to store as glycogen and thereby bring blood glucose level to normal. On decreasing of blood glucose level, the serection of insulin by pancreas decreases. In this way, through direct negative feedback the blood glucose homeostasis is maintained by insulin.

    Blood glucose homeostasis by direct negative feedback 

  • Blood thyroxine homeostasis
    On receiving an external stimulus, the hypothalamus in brain produces T-RH which stimulates the anterior pituitary to secrete TSH. TSH stimulates thyroid gland to produce thyroxine. If thyroxine concentration increases in the blood, it causes negative feedback on hypothalamus to secrete less T-SHRH and subsequently TSH secretion from anterior pituitary also lessens. When TSH level increases in blood, it causes hypothalamus to secrete lesser T-RH.

Blood thyroxine homeostasis by indirect negative feedback

If thyroxine hormone level decreases below normal, it generates negative feedback on hypothalamus and anterior pituitary. These glands then secrete more T-RH and TSH respectively for increased secretion of thyroxine. Thus, by an indirect negative feedback mechanism the blood thyroxine homeostasis is maintained.

(B) Positive Feedback Control
In this type of control, a hormone increases its own production. This could be understood by the example of oxytocin. Oxytocin is relased from posterior lobe of pituitary on receiving stimulus by uterine contractions during the onset of labour pain in females before the child birth. Oxytocin increases the intensity of uterine contractions. The increased contractions stimulate the production of oxytocin. This cycle stops on the birth of the child.

 

COORDINATION IN PLANTS

Plants are less complex organisms as compared to animals. They lack nervous system, so, they do not respond guickly to stimulus unlike animals. When we touch (stimulus is provided) Chhui-mui plant (Mimosa pudica), then it frequently folds its leaves. A question comes to our mind, “If plants lack nervous system, how can Mimosa respond so quickly?” “How do plants respond and react to various environmental stimuli like light, gravity, water, touch and chemicals?” The answer of these questions lies in the fact that although nature has not provided plants with brain and nervous system, but they have hormones which help them to respond. These plant hormones are called phytohormones. Phytohormones affect the plant growth as well as the movement of plant parts like leaves, stem and roots. Plants control various movements only through chemical coordination. Generally, responses of plants cannot be observed immediately, as they take considerable time to respond.

Mimosa pudica (Chhui-mui plant)

Mimosa is a sensitive plant. it shows quick response but other plants may take considerable time for responding in most cases

Phytohormones act differently as compared to animal hormones. They coordinate in two ways-
(i) They control movements by affecting the growth.
(ii) They affect the shape of cells by bringing changes in the amount of water (turgor changes) and plasmolysis.

Chemical coordination in plant

It takes place by the plant hormones or phyto hormones. They help to coordinate growth, developement and response to the environment. They synthesis in minute quantity in one part of the plant body and simply diffuse to another part. Where they influence specific physiological processes.

1. Auxin
(i) Promote cell enlargement and differentiation in plants.
(ii) Cause initiation of root of formation on stem cuttings or callus.
(iii) Promote growth of fruits.
(iv) Show promotive effect on growth of stem and slows down the growth of roots.
(v) Cause growth of apical buds (apical dominance) and prevent growth of lateral buds.
(vi) Prevent premature leaf and fruit fall (reduction in abscission).
(vii) Regulate tropic (geotropic and phototropic) movements in plants.
(viii) Use as weedicides [chemicals that kills the unwanted plants (weeds)].
(ix) Control of lodging.
(x) Synthetic auxins are used in agriculture and horticulture as weedicides.
(xi) Induce formation of seedless fruits without involving fertilisation, i.e., parthenocarpy.

 

(2) Gibberellins
(i) Promote stem elongation (dwarf varieties grow to normal height because the hormone promote internodal growth) and cell differentiation in the presence of auxin.
(ii) Break dormancy of buds and seeds.
(iii) Stimulate growth of leaf, stem and increase size and number of fruits.
(iv) Induce parthenocarpy (formation of fruits without seeds) in many plants.
(v) Can replace requirements of photoperiodic length for flowering in certain plants.
(vi) Neutralise the effects of growth inhibitor (ABA)

(3) Cytokinins
(i) Promote cell division and are also required for differentiation of cells and tissues.
(ii) Delay ageing (senescence) in plant organs like cut flowers, vegetables and fruits.
(iii) Promote the opening of stomata in leaves.
(iv) Break dormancy in buds and seeds.
(v) Promote growth of fruits.
(vi) Play vital role in morphogenesis in plants.
(vii) Inhibit apical dominance and allow growth of lateral buds.
(viii) Regulate transport of nutrients.
(ix) Increase resistance to diseases and temperature stress.

(4) Abscisic Acid (ABA)
(i) Controls plant growth by inhibiting the activity of growth promoters.
(ii) Induces dormancy of buds and seeds.
(iii) Controls the closing of stomata during stress conditions in order to prevent water loss through transpiration.
(iv) Causes abscission (falling) of flowers, fruits and leaves.
(v) Promotes wilting and senescence of leaves.
(vi) Affects synthesis of a-amylase enzyme.
(vii) Causes tuberisation in potato.
(viii) Promotes flowering in short day plants.

(5) Ethylene (Volatile/gaseous plant hormone)
(i) Promotes ripening of fruits and dehiscence of dry fruits
(ii) Promotes senescence of leaves
(iii) Stimulates abscission of flowers, fruits and leaves
(iv) Causes breaking of dormancy in buds and seeds
(v) ‘Triple response’ is an important character of ethylene. This includes swelling of nodes, stimulation of lateral growth and inhibition of elongation.
(vi) Sex modification

 

Plant movement

The plants respond to various stimuli very slowly by growing. E.g. when a seed germinates the root goes down and the stem comes up into the air. But the leaves of sensitive plant move very quickly in response to touch by folding and drooping without growing. So plant show two different types of movement

Classification of plant movements : These are of two types


(A) Tropic movement

Tropic movement is the directional movement of the part of a plant caused by its growth. The growth of a plant part in response to the stimulus can be towards the stimulus (positive tropism) or away from the stimulus (negative tropism).

Types of tropic movements
(1) Phototropism
(2) Geotropism
(3) Chemotropism
(4) Hydrotropism
(5) Thigmotroprism

(1) Phototropism
The movement of a part of the plant in response to light is called phototropism. The plant part moves towards light is called positive phototropism and the plant part moves away from light then it is called negative phototropism.

(2) Geotropism
The movement of a part of the plant in response to gravity is called geotropism. Roots of a plant move downwards in the direction of gravity it is called positive geotropism and stem of a plant moves upwards against the direction of gravity it is called negative geotropism.

Plant showing geotropism

 

(3) Chemotropism
The movement of a part of plant in response to a chemical stimulus is called chemotropism. E.g. Growth of pollen tube towards the ovule during the process of fertilisation in a flower.

(4) Hydrotropism 
The movement of a part of plant in response to water is called Hydrotropism. Roots of seedling show positive hydrotropism.

(5) Thigmotropism
The movement of a part of plant in response to contact or support is called thigmotropism.  E.g. Pea plant climb up other plants or fences by mean of tendrils. Tendrils of are sensitive to touch. When tendrils come in contact with any support, the part of the tendril in contact with the object does not grow as rapidly as the part of tendril away from the object. This causes the tendril to circle around the object and thus cling to it.

Tendrils wrap around a support

(B) Nastic movement
Movement which is neither towards nor away from the stimuli. It is growth independent movement.

Seismonasty
Such movements occur in response to touch (shock). These movements are very quick and are best seen in ‘touch-me-not’ plant (Mimosa pudica), also called ‘Chhui-mui’ or ‘Lajwanti’ or ‘sensitive plant’. If we touch the leaves of the chhui-mui plant with our finger, we find that all its leaves immediately fold up ane droop by using electrical chemical means to convey this information from cell to cell. After sometime, the leaves regain their original status. Here, no grmvth is involved. Instead, plant ceil change shape by changing the amount of water in them (turgor changes), resulting in folding up and drooping of leaves.

 Mimosa pudica leaves exhibit nastic movement

 

Difference between tropic and nastic movement

 

 


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