RECEPTION, RESPONSE AND CO-ORDINATION IN PLANTS AND ANIMALS

Introduction

• The structures involved in detecting the changes may be located far away from the ones that respond.
• There is need for a communication system within the body.
• The nervous system and the endocrine system perform this function,
• i.e. linking the parts of the body that detect changes to those that respond to them.

Irritability

• Living organisms are capable of detecting changes in their internal and external environments and responding to these changes in appropriate ways.
• This characteristic is called irritability, and is of great survival value to the organism.

Stimuli

• A stimulus is a change in the internal or external environment to which an organism responds.
• Examples of stimuli include light, heat, sound, chemicals, pH, water, food, oxygen and other organisms.

Response

• A response is any change shown by an organism in reaction to a stimulus.
• The response involves movements of the whole or part of the body either towards the stimulus or away from it.
•  It also results in secretion of substances e.g. hormones or enzymes by glands.

 

Co-ordination

• Co-ordination is the working together of all the parts of the body to bring about appropriate responses to change in the environment.

Reception

• Reception is the detection of changes in the environment through receptors.

 

Irritability in Plants

• Response in plants is not as pronounced as in animals.
• This does not in anyway diminish the importance of irritability in plants.
• It is as important to their survival as it is in animals.
• Plants respond to a variety of stimuli in their environment.
• These stimuli include light, moisture, gravity and chemicals.
• Some plants also show response to touch.

Tropisms

• Plants often respond by growing in a particular direction.
• Such growth movements are called tropisms.
• They are the result of unequal growth in the part of the plant that responds.
• The stimulus cause unequal distribution of growth hormones (auxins) produced in the plant.
• One side grows more than the other resulting in a bend either towards the stimulus (positive tropism) or away from the stimulus (negative tropism).

 

Phototropism

• If seedlings are exposed to light from one direction, their shoots grow towards the light.
• This response is called phototropism.
• Shoots are said to be positively phototropic because they grow towards the light.
• The tip of the shoot receives the light stimulus from one direction (unilateral stimulus) but the response occurs below the tip.
• The response of the shoot is due to a hormone called auxin produced at the tip.
• It diffuses down the shoot to this zone of cell elongation where it causes the cells to elongate.
• Light causes auxin to migrate to the darker side.
• The auxin is more concentrated in the dark side than on the light side.
• The cells on the dark side grow faster than the ones on the light side.
• A growth curvature is therefore produced.

     Survival value:

• Positive phototropism by shoots ensure that sufficient light is absorbed by leaves for photosynthesis.

Geotropism

• Geotropism is a growth response to gravity.
• Roots are positively geotropic because they grow down towards the direction of the force of gravity;
• shoots are negatively geotropic because they grow away from direction of force of gravity.
• If a seedling is kept in the dark with its plumule and radicle in a horizontal position, the plumule will eventually grow vertically upwards while the radicle will grow vertically downwards.
• The effect of gravity on roots and shoots can be explained as follows:
• When the seedling is placed in a horizontal position, more auxin settles on the lower side of the root and shoot due to the effect of gravity.
• Shoots respond to a higher concentration of auxin than roots.
• The lower side of the shoot grows faster than the upper side.
• Resulting in a growth curvature that makes the shoot grow vertically upwards.
• Root growth is inhibited by high concentrations of auxin.
• Therefore, the lower side of the root grows at a slower rate than the upper side where there is less auxin concentration.
• This results in a growth curvature that makes the root grow vertically downwards.

Survival Value:

• Roots in response to gravity grow downwards where they absorb water and get anchored in the soil.
• This results in absorption of nutrients needed for growth.

Hydrotropism

• Hydrotropism is the growth of roots towards water (moisture) .

      Survival Value

• It ensures that plant roots grow towards moisture to obtain water needed for photosynthesis and transport of mineral salts.

Chemotropism

• Chemotropism is the response of parts of a plant towards chemical substances,
• e.g. the growth of the pollen tube towards the ovule in flowering plants is a chemotropic response.

 Survival Value

• This ensures that fertilisation take place and the perpetuation of the species continues.

Thigmotropism

• Thigmotropism is a growth response to touch.
• e.g. tendrils of climbing plant bend around objects that they come in contact with.

      Survival Value

• This provides support and the leaves stay in a position suitable for absorption of light and gaseous exchange for photosynthesis.

Tactic Movements in Plants and other Organisms

• A tactic movement is one made by a whole organism or a motile part of an organisms (e.g. a gamete) in response to a stimulus.
• Tactic movements are named according to the nature of the stimulus that brings about the response.
• Phototaxis is movement in response to direction and intensity of light.
• Free-swimming algae such as Chlamydomonas usually tend to concentrate where light intensity is optimum and will respond to light by swimming towards it. This is an example of phototactic response.
• Osmotaxis is movement in response to changes in osmotic conditions e.g. freshwater amoeba.

Survival Value

• Ensures favourable conditions for existence.
• Chemotaxis is movement in response to concentration of chemical substances. 

Survival  Value

•  In bryophytes, antherozoids move towards archegonia to effect fertilisation

Survival Value of taxis:

• These ensure conditions favourable for life bring maximum benefit to the organism.

Nastic Movements

• A nastic movement is one made by part of a plant in response to stimulus which is not coming from any particular direction.
• Nastic movements are also named according to the nature of the stimulus.
• Seismonasty/haptonasty – response to shock.
• The ‘sensitive plant’ Mimosa pudica responds to touch by folding up its leaves.
• This is an example of a seismonastic response.

Production of auxins and their effects on plant growth

• Auxins are produced by plant apices, i.e. root apex and shoot apex.
• They bring about cell elongation resulting in growth.
• They are diffusible substances which effect growth when in very small amounts.
• Roots require lower concentrations than shoots.
• The effect of auxins on the growth of roots and shoots has already been discussed.
• Auxins also exert other effects on plant growth and development.
• There  are various other chemical substances which have been shown to influence plant growth and development.

Effects of Auxin on Plant Growth

Apical Dominance

• Auxins inhibit the growth of side branches.
• This is referred to as apical dominance.
• If the terminal bud is removed, side branches develop from the lateral buds.
• This knowledge is applied in pruning.
• As long as the main stem is allowed to remain intact, the development of side branches is suppressed.
• Pruning the terminal bud removes the main sources of auxin, thus allowing side branches to sprout.

 Growth of adventitious roots

• Adventitious roots develop from the stem.
• Auxins stimulate the growth of such roots.

Parthenocarpy

• This refers to the formation of fruits without fertilisation.
• This can be induced by treating unpollinated flowers with auxin.
• This phenomenon is applied in the development of seedless fruit varieties.
• Auxins, together with other plant hormones, are involved in secondary growth, falling of leaves and ripening of fruits.

Reception, Responses and Co­ordination in Animals

• The nervous and endocrine systems (together known as the neuro-endocrine system) act as a co-ordinating system.
• They linking the receptors to the effectors and regulating their activities.

 

Receptors

• Receptors are cells that detect or receive stimuli.
• They may be scattered more uniformly all over the body surface
• e.g. receptors for pain, touch, temperature; or they may be located in a special sense organ e.g. receptors for light, sound, taste and smell.
• Motor nerves link the Central Nervous System (CNS) to the effectors.
•  Its cell body is located at one end of the axon.
• It transmits nerve impulses from the CNS to the effectors.

Effectors

• These are the cells, organs, or organelles which enable the organism to respond.
• They include muscles, glands, cilia and flagella.

The Nervous System

Components of the nervous system in humans

• Every organ is the human body is connected to nerves.
• The nervous system is made up of nerve cells (neurons) which transmit impulses from one part of the body to another.

It consists of the following:

• The Central Nervous System (CNS) is a concentrated mass of interconnected nerve cells which make up the brain and the spinal cord.
• The peripheral nervous system is made up of nerves which link the CNS to the receptors and the effectors.
• Sensory nerves link the sensory cells (receptors) to the central nervous system and transmit nerve impulses from a sense organ to the CNS.

 

Structure and Functions of Neurons

• A nerve cell consists of a cell body (centron) where the nucleus is located, and projections called dendrites arise.
• One of the projections is drawn out into an axon i.e. the longest process.
• Each axon contains axoplasm which is continuous with the cytoplasm in the cell body.
• The axon is enclosed in a fatty myelin sheath which is secreted by Schwarm cell.
• The myelin sheath is interrupted at approximately 1 mm intervals by constrictions known as nodes of Ranvier.
• The myelin sheath is enclosed by a thin membrane called the neurilemma, which is part of the Schwann cell in contact with axon.
• The myelin sheath and nodes of Ranvier enhance transmission of the impulse.

 There are three types of neurons:

Sensory neurone

• Also known as afferent neurone.
• Transmits impulses from sensory cells to the CNS.
• The cell body of a sensory nerve cell is located at some distance along the length of the axon outside the CNS.

Motor neurone

• Known as efferent or effector neurone
• Transmit impulses from the CNS to the effectors(muscles and glands)
• Its cell body is located inside the CNS.

 Intermediate or connector neurone

• Also called relay neurone
• Found inside the CNS.
• The connect sensory and motor neurons with each other and with other nerve cells in the CNS.

Functions of the neurone

• The nerve impulse is electrical in nature.
• Its transmission depends on differences in electrical potential between the inside and the outside of the axion.
• The outside is positive while the inside is negative.
• The stimulus triggers a change that affects the permeability of neurone membrane.
• The result is a change in the composition of ions on either side of the membrane.
• The outside becomes negative as the inside becomes positive due to sodium ions rushing in.
• The above constitutes a nervous impulse which is transmitted along the sensory neurone to the CNS.
• The speed of transmission is very high.
• Certain mammalian axions transmit impulses at the rate of 100m/s.
• The dendrites of neurons do not connect directly to each other, but they leave a small gap called synapse.
• The transmission of an impulse from one cell to the next takes place through synapse.
• Synaptic knobs are structures found at the ends of dendrites.
• Thus the dendrites of one nerve cell make contact with the dendrites of the adjacent nerve cell through the synapses.
• Impulses are transmitted in the form of a chemical transmitter substance which crosses the gap between one dendrite and the next.
• The transmitter substance is found within synaptic vesicles.
• The chemical substance is either acetylcholine or
• The synaptic vesicles burst and release the transmitter substance when an impulse arrives at the synaptic knob.
• Impulses in motor neurone s are trans mitted to effectors.
• The space between motor end dendrite and muscle is known as neuro-muscular Junction.
• Synaptic vesicles in the ends of the dendrites release the transmitter substance across the neural muscular junction.

Functions of Major Parts of the Human Brain

• The Central Nervous System (CNS) consists of the brain and the spinal cord.
• The CNS co-ordinates body activities by receiving impulses from sensory cells from different parts of the body.
• It then sends the impulses to the appropriate effectors.
• The brain is enclosed within the cranium or brain­case.
• It is covered and protected by membranes known as meninges.
• When meninges are infected by bacterial or fungi they cause meningitis.

The brain consist of the following parts:

Cerebrum.

• This is the largest part of the brain.
• It consists of two cerebral hemispheres.
• It is highly folded in order to increase the surface area.
• The cerebrum controls learning, intelligence, thought, imagination and reasoning.

The medulla oblongata (brain stem).

• The medulla oblongata has centres which control breathing (ventilation) rate,
• heart beat rate (cardiac frequency),
• swallowing, salivation, blood pressure
• temperature regulation, hearing, taste and touch.

The cerebellum

• Is located in front of the medulla and is a folded dorsal expansion of the hindbrain.
• It controls posture movement and balance. 

The hypothalamus

• Controls functions such as body temperature and osmoregulation.

The pituitary gland

• Is an endocrine organ that secretes a number of hormones which control osmoregulation, growth, metabolism and sexual development.

Optic lobes -control the sense of sight.

Olfactory lobes -control the sense of smell.

Spinal Cord

• The spinal cord is located within the vertebral column and consist of the following:
• The grey matter forms the central part of the spinal cord.
• It consists of nerve­cell bodies and intermediate nerve fibres.
• The white matter of the spinal cord carries sensory nerve fibers while the ventral root carries motor nerve fibers.

 

Simple And Conditioned Reflex Actions

Simple Reflex Action

• A simple reflex action is an automatic response to a stimulus.
• The route that is followed by impulses during a reflex action is called a reflex arc.

A reflex action follows the following sequence:

• A receptor is stimulated and an impulse is transmitted along a sensory nerve fibre to the spinal cord.
• The impulse is picked up by an intermediate neurone within the CNS.
• The intermediate nerve fibre transmit the impulse to a motor nerve fibre which is connected to an effector.
• The effector responds.

Examples of reflex action include:

• Pulling the hand away from a hot object.
• The knee jerk.

Conditioned Reflexes

• These are learned responses.
• When two or more stimuli are presented to an animal at the same time and repeatedly, the animal eventually responds to either stimulus.
• For example, if a hungry animal is presented with food, it will respond by salivating.
• If a bell is rung at the same time as the food is presented to the animal, the animal will learn to associate the sound of the bell with food.
• Eventually, the animal can be made to salivate at the sound of the bell alone.
• This response is called conditioned reflex and is one of the ways by which animals learn.

The Role of Endocrine System in Human Beings

• Endocrine system consists of glands that secrete hormones.
• The glands have no ducts and are known as endocrine glands.
• Other glands are known as exocrine glands because they have ducts.
• The pancreas has an outer exocrine portion and an inner endocrine portion.
• Hormones are chemical substances, protein in nature which are secreted at one part of the body and have effects on other parts not necessarily near the point of secretion.
• They are secreted directly into blood and transported by blood.
• Each hormone either has a generalised co-ordinating effect on the body or brings about a specific response in a particular target organ.

Adrenaline

• Enhance activity of sympathetic nervous system.

Over secretion

• Increased heartbeat
• High blood pressure
• Thin toneless muscles. 

Under secretion

• Low blood pressure
• Inability to withstand stress
• Muscular weakness

Thyroxine

Over secretion is termed hyperthyroidism this causes:

• Increased Basal Metabolic Rate (BMR) hence increased temperature.
• Person becomes very angry, nervous and hands may shake.
• Increased heartbeat which lead to cardiac failure.

Under secretion is termed hypothyroidism:

• Poor growth and mental retardation (cretinism).
• Reduced metabolic rate hence decreased temperature.
• Person becomes inactive and slothful.
• Eyes and face become puffy as fluid gets stored under skin.
• In extreme cases the tongue is swollen and skin becomes rough.
• Enlarged thyroid gland.

Similarities

• Both endocrine and nervous system are involved in the coordination of body functions.
• Both have target organs.
• Both are controlled via a negative feedback mechanism, i.e too high production results in a reduced production.

Effects of drugs abuse on the human health.

• Drug abuse can be defined as misuse of drugs.
• Drugs are chemical compounds that affect the working of body or kill disease causing microorganisms.

Prescription drugs

• Are drugs prescribed by a doctor.
• Prescribed drugs can be abused through taking overdose which may cause death.

Over the counter drugs(OCD)

• Are self prescribed drugs.
• These have harmful effects and may lead to tolerance such that higher doses are needed.

Below is a list of effects of hard drugs on human health

• Lung cancer caused by nicotine.
• Emphysema.
• Liver cirrhosis -caused by alcohol.
• Interferes with vision – alcohol.
• Sterility – khat (rniraa).
• Sleeplessness – insomnia – khat (miraa).
• Hallucinations – Canabis sativa (Bang i).
• Digestive system is upset, nausea.
• Diarrhoea and vomiting.
• Headache and double vision.
• Skin tone changes – e.g. too dark.
• Appetite is extreme – very poor or very great.
• Weight loss.
• Personality changes e.g. irritable and confused.
• Convulsions, lethargy and depressions due to inhalation of solvents e.g. glue.

 

Structure and Function of Parts of the Human Eye

Structure

• The human eye is spherical in shape and situated within a socket or orbit in the skull.
• It is attached to the skull by three pairs of muscle, which also control its movement.
• It is made up of three main layers; sclerotic layer, choroid and the light sensitive retina.

Sclerotic layer

• Outermost white part situated at the sides and back of the eye.
• Made up of collagen fibres.
• It protects the eye and gives its shape.

Cornea

• This is the transparent front part of the sclera that allows light to pass through.
• It is curved, bulging at the front. It thus reflects light rays hence helps to focus light rays onto the retina.

Choroid

• The second or middle layer.
• It has many blood vessels that supply nutrients to the eye and remove metabolic wastes from the eye.
• It has dark pigments to absorb stray light and prevent its reflection inside the eye.

Ciliary body

• Is glandular and secretes aqueous humour.
• It has blood vessels for supplying of nutrients excretion and gaseous exchange.
• It has ciliary muscles – which contract and relax to change the shape of lens during accommodation.

Suspensory ligaments

• Are inelastic and attach the lens onto the cilliary body holding it in position. 

Lens

• Biconvex in shape, to refract light.
• Crystalline and transparent to allow light to pass through and focus it on to the retina.

Aqueous humour

• Found between lens and the cornea.
• Transparent to allow light to pass through it.
• It is watery thus helping in focusing.
• Helps maintain shape of eye ball.
• To convey nutrients and oxygen to cornea, and remove waste products.

Iris

• The coloured part of the eye has an opening – the pupil at the centre.
• Iris has circular and radial muscles which controls size of the pupil, hence the amount of light entering the eye through the pupil.

Vitreous humour

•  It is a fluid.
• Found between lens and retina.
• Is viscous and gives eye the shape.
• It is transparent and refracts light.

Fovea centralis

• Fovea centralis (yellow spot) is the most sensitive part of the retina.
• Consists mainly of cones for accurate vision (visual acuity).

Retina

• Retina contains light sensitive cells and is situated at the back of the eye.
• There are two types of light sensitive cells in the retina:
• Rods – are sensitive to low-intensity light and detect black and white. Nocturnal mammals have more rods.
• Cones – are sensitive to high intensity  of light;
• They detect bright colour.
• Diurnal mammals have more cones.

Optic nerve

• Optic nerve, has neurons for transmission of impulse to the brain for interpretation.

Blind spot

• Blind spot is located at the point where the optic nerve leaves the eye on its way to the brain.
• It is not sensitive to light it has no rods or cones.

Eye lid

• Eye lid is a loose skin that covers the eye. It closes by reflex action.
• Protects it from mechanical damage and from too much light.

Eyelashes

• Prevent dust and other particles from entering eye.

Conjuctiva

• It is transparent and thin and allows light to pass through.
• It is a tough layer that is continuous with the epithelium of the eye lids.
• It  protects the cornea. 

Accommodation

• Accommodation refers to the change in the shape of the lens in order to focus images.
• Rays from a distant object would be focused at a point behind the retina if the lens were not adjusted appropriately.
• When the eye is focusing at a distant object, the cilliary muscles are relaxed and the suspensory ligament are stretched tight.
• The lens is pulled thin, thus allowing light rays from a distant object to be properly focused on to the retina.
• When the eye is looking at near object, the ciliary muscles contract and the suspensory ligament become slack.
• The lens becomes more convex.
•  This allows light rays from near object to be focused onto the retina.

Control of light intensity entering the eye

• In bright light (high intensity) the circular muscles of the iris contract.
• The diameter of the pupil decreases and less light enters.
• This protects retina from damage by too much light.
• In dim light circular muscles of iris relax (radial ones contract).
• Pupil’s size (diameter) increases, more light enters the eye.

Image formation and Interpretation

• Light rays from an object enter the cornea and are directed onto the lens through the pupil.
• They are refracted by the cornea and the lens.
• The latter brings the rays into fine focus.
•  It makes the light rays converge so that an image is focused at a point on the retina.
• The image on the retina is inverted.
• This stimulate, the rods and cones on the retina and impulses generated are transmitted through the optic nerve to the brain.
• The brain interprets the image as upright.

Common Eye Defects and their Correction

Short-sightedness (Myopia)

• A shortsighted person cannot focus distant objects properly.
• Light rays from a distant object fall at a point in front of the retina.
• This may be due to the eyeball being too long.
• This defect can be corrected using spectacles with concave lenses.
•  The lenses make the light rays diverge before they reach the eye.

Long-sightedness (Hypermetropia)

• A long-sighted person cannot focus near objects properly.
• Light rays from the object are not focused on the retina.
• This may be due to the eyeball being too short.
• This defect may be corrected by using spectacles with convex lenses which make light rays converge before they reach the eye.

Astigmatism

• Astigmatism refers to a condition in which the cornea or the lens is uneven, so that images are not focused properly on the retina.
• This defect can be corrected by wearing spectacles with special cylindrical lenses.
• Presbyopia is a condition in which light rays from a near object are not focused on the retina.
• This is caused by hardening or loss of elasticity of lense due to old age.
• This defect is corrected by wearing convex (converging) lenses.

Structure and Functions of Parts of Human Ear

The Mammalian Ear

• The mammalian ear performs two major functions:
• hearing and detecting changes in the positions of the body to bring about balance and posture.

The ear is divided into three sections.

The Outer Ear

This consists of:

• An outer flap, the pinna which is made up of cartilage.
• The function of the pinna is to catch and direct sounds.
• The external auditory canal is a tube through which sound travel.
• The lining of the tube secretes wax, which traps dust particles and microorganisms.
• The tympany is a membrane stretching across the inner end of the external auditory canal.
• The tympanum vibrates when it is hit by sound waves.

The Middle Ear

• This is a chamber containing three small bones called the ear ossicles, the malleus, incus and stapes.
• The three ossic1es articulate with one another to amplify vibrations.
• The vibrations are transmitted from the tympanum to the oval window.
• At the end of the chamber is a membrane called the oval window.
• When the tympanum vibrates, it causes the ear ossic1es to move forwards and backwards.
• This causes the oval window to vibrate.
• The Eustachian tube connects the middle ear to the pharynx.
• It allows air to get in and out of the middle ear, thus equalising the pressure between the inside and the outside of the tympanum.

The Inner Ear

• This consists of a series of chambers filled with fluid.
• It comprises the cochlea and semi­circular canals.
• Cochlea is a coiled tube that occupies a small space and accommodates a large number of sensory cells.
• The cells are connected to the brain through the auditory nerve.
• They detect vibrations which lead to hearing.

Hearing

• The sound waves set the tympanum vibrating and are transformed into vibrations.
• The vibrations are transmitted to the oval window by the three ossicles.
• Vibrations of the oval window cause the fluids inside the cochlea tube to vibrate.
• The membranes inside the cochlea have sensory cells which change the sound vibrations to nerve impulses.
•  These are transmitted to the brain through the auditory nerve.
• Hearing is perceived in the brain.

Balance and posture

The semi-circular canals

• There are three semi-circular canals in each ear.
• They are situated at right angles to each other and each one is sensitive to movement in a different plane.
• They are filled with fluid and each has a swelling called the ampulla at one end.
•  Inside the ampulla are sensory cells.
• Balance and posture are detected by these cells.
• Movement of the head in a given direction causes the fluid to move the hairs on sensory cells.
• This transmit impulses to the brain through the auditory nerve so that the movement is registered.

Defects of the ear

Acute labyrinthitis

• This is an inflammation of the middle ear and cochlea.
• It may lead to deafness.
•  It can be treated by using certain drugs but sometimes an operation may be necessary.

Tinnitis:

• This is a sensation of noises in the ear.
•  It is caused among others by accumulation of wax in the ear or use of certain drugs e.g. quinine.
• Treatment is by removal of wax, stopping use of the causative drug.

Vertigo – Giddiness

• This is disorientation of body in space – one of the causes is dilation of endolymph.
• Corrections: Use of appropriate drugs.

Deafness.

• This is inability to hear.
• It is presented in various degrees in various individuals, some have partial hearing, others are completely deaf.

  This may be as a result of:

• Chronic infection of cochlea.
• Lack of sensory cells.
• Excess wax in external auditory canal.
• Fusion of ear ossicles.

Otitis Media

• This is the inflammation of middle ear due to build-up of fluid.
• It is marked by the swelling of tissues surrounding the Eustachian tube due to infection or severe congestion.
• A strong negative pressure creates a vacuum in the middle ear.
• Treatment – use of antibiotics or surgery.

 

Practical Activities

• To investigate tactic response
• Tactic response in fly maggots are investigated using choice chambers(s).
• Responses to various stimuli are observed e.g. to chemical substances – chemotaxis.
• On one side of choice chambers is placed beef/fish that has been dried in the sun.
• On the opposite chambers is placed rotting meat/fish.
• Ten maggots are placed at the center and choice chamber is covered.
• After 10 minutes the number of maggots at each end is counted.
• Most of the maggots have moved to the chamber with rotting meat.

Tropisms

• Maize or been seeds are soaked and germinated, to the stage when radical and coleoptile/plumule just appear .
• (about 5 days for beans and seven days for maize).
• Seedlings with straight radic1es and plumules are used ..

Geotropism

• The seedlings are placed horizontally on the medium (Soil or vermiculite or saw dust or sand).
• Observations are done after three days and results recorded.

Phototropism

• A potted plant or a young seedling planted in a beaker is kept next to a window which is the only source of light in the laboratory.
• Alternatively, a dark box may be used.
• Observations are made after 3-5 days and results recorded.
• The shoots grow bending towards the same light.

Etiolation

Young seedlings are placed in a dark box.

It is kept moist but not exposed to light.

After two weeks the seedlings are removed and observations made to note the following:

Colour of leaves is yellow.

Size of leaves is small

Length of internodes is long

Length of stem elongated long and thin.

Other seedlings that were grown in light are observed (as control) and similar measurements taken.

They are green in colour with larger leaves, shorter internodes and the stem is shorter and thicker.

Those in the dark have smaller yellow leaves, long thing stems with long internodes. (etiolated).

Experiment to Determine Distance of the Blind Spot

• Students should work in pairs so that one takes measurements while the other observes.
• A cross and a dot are marked on a white paper .
• The two  points are 6-9 cm apart.
• The paper is held 50 cm away from the face.
• Closing the left eye, the paper is slowly moved towards the face as the right eye is fixed on the cross.
• At 50 cm distance the cross and the dot are seen clearly.
• As-the paper is moved closer to the face, the dot disappears.
• The distance at which the dot disappears is measured.
• This is the distance of the blind spot.
• When the light rays from the dot are focused on the blind spot it disappears hence the dot is not seen.

The Knee Jerk Experiment

• Students work in pairs, one student sits on the table, high stool or bench with one leg crossed over the other.
• The other student chops the crossed knee just below the knee cap with the edge of palm or wooden ruler.
• It is observed that the crossed knee jerks.
• This is a spinal reflex.