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Plants respond to a variety of stimuli in their environment. Unlike animals, plants cannot move from one place to another. However, they can move by forces of wind or water. Movement in plants in response to a stimulus is continuous and very slow. Movement of plants can be grouped into two:
  • Growth movements
  • Turgor movement
  1. Growth Movement
These are the movements that take place in the meristematic regions due to unequal permanent growth. Growth movements can be classified into two categories namely:
  • Autonomic movements
  • Paratonic movements.
(a) Autonomic Movements
These are self-controlled movements for instance growth in the meristematic regions i.e. tips of stems and roots.
(b) Paratonic Movements
These are the plant movements induced by external stimuli. These stimuli include:
  • Light
  • Moisture
  • Gravity
  • Chemicals
  • Touch
Paratonic movements include tropic and nastic movements
Tropic Responses
These are growth movements that are caused by a wide range of stimuli. In this case the plant grows either towards or away from the stimulus. If the response is towards the stimulus it is referred to as positive. If the response is away from the stimulus it is referred to as negative. Tropic movements are mediated through plant hormones. Tropisms are growth movements by plant organs in response to a unilateral stimulus, in which the direction of the movement is related to the direction of the stimulus.
Plant Hormones
The first plant hormones were discovered by a Dutch botanist Fritz Went in the year 1928. Fritz Went called these hormones auxin or more accurately, indoleacetic acid (IAA). This hormone has an extremely powerful effect on growth. Like the animal hormones, plant hormones act in very low concentrations. A solution of 0.001 milligram in a litre of water applied to the side of a shoot is enough to cause bending.
A part from auxins, plant hormones also include gibberellins and cytokinins. Auxins are synthesized from amino acid tryptophan in meristematic tissues such as the shoot tips, buds, young leaves and germinating seeds. Auxins increase cellwall elasticity by losing the bond between the cellulose fibres. Auxins promote cell division, cell elongation and cell differentiation.
Effects of Auxins Concentration on Growth
Experiments have revealed that higher concentrations of auxins stimulate growth in shoots while lower concentrations stimulate growth in roots. Amount of auxins which stimulate shoot growth, normally inhibit root growth
Experiments demonstrating that a hormone regulates growth in shoots and roots
A tropism is a movement by a plant organ in response to a unilateral stimulus, in which the direction of the movement is related to the direction of the stimulus. Tropisms are named according to the nature of the stimulus.
Types of Tropisms
(i) Geotropism
Geotropism is also known as gravitropism. This is the growth movement of plant parts in response to the direction of the force of gravity. The roots grow towards the direction of the force of gravity which means are positively geotropic (gravitropic). The shoot grows away from the force of gravity which means are negatively geotropic (gravitropic).
If a seedling is placed horizontally, the plumule will eventually grow vertically upwards while the radicle will grow vertically downwards. The above observation 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 shoots due to the pull of gravity.
  • Shoots respond to a higher concentration of auxin than roots. In this case the lower side of shoot grows faster than the upper side, resulting in a growth curvature that makes the shoot grow vertically.
  • Root growth is inhibited by high concentrations of auxins. Thus, the lower side of the root grows at a slower rate than the upper side where there is less auxin concentration. Consequently, this results in a growth curvature that makes the root grow vertically downwards
The effect of gravity on the growth of roots and root
(ii) Phototropism
This is the growth movement of plant organs in response to a unilateral source of light. In an experiment it was revealed that auxins are directly involved in phototropism. If a shoot is exposed to light from one direction only, the shoot bends towards the source of light. Light causes an unequal distribution of the hormone (auxin). Light causes auxins to migrate to the darker side. In this case the auxins are more concentrated on the darker side than on the side where the light is coming from. The cells on the dark side grow faster and elongate than the ones on the side where the light is coming from. As a result, the shoot bends towards light. Shoots are positively phototropism because they grow toward the light. Some roots grow away from light, which means they are negatively phototropic. However many roots are not sensitive to light.

Effects of light on shoots
(iii) Hydrotropism
That is growth movement of plant organs in response to unilateral source of water or moisture. In hydrotropism the root grows toward the source of water means the root are positively hydrotropic. On the other hand the shoot either grow away from the source of water meaning are negatively hydrotropic or show no response meaning are neutral.
Root is positively hydrotropic
The term thigmo comes from a Greek word thigma meaning touch. Thigmotropism is also referred to as haptotropism. In plants such as possiflora and gloriosa with tendrils which curl around and cling to stems, auxins also play a major role. When climbing stems or tendrils come into contact with a hard object, the contact causes them to curve and coil round the hard object.
This is caused by the migration of the auxins from the point of plant contact and the hard object. In this case the part in contact with the hard object has a lower auxin concentration than the outer part. Higher auxin concentration promotes faster growth in shoots. Therefore, greater auxin concentration in the outer part causes faster growth than the part in contact with the object, hence the shoot continues to round the object.

(v) Chemotropism
This is the growth movement of plant organs in response to a unilateral source of chemicals. For instance, pollen tubes grow through the style towards the ovary and finally towards the ovules.
(vi) Thermotropism
This is the growth movement of plant organs in response to a unilateral source of heat as shown by movement of sunflower orienting itself towards the sun. However, there is an overlap between thermotropism and phototropism and sometimes a combination of both tropisms.
(vii) Rheotropism
This is the growth movement of plant organs in response to a unilateral source of air currents.
Importance of Tropisms
  1. Phototropism: exposes the leaves of the plant to trap maximum sunlight for photosynthesis.
  2. Haptotropism: enables plants with weak stems to obtain mechanical support.
  3. Geotropism: enables the roots of the plant to grow deep in the ground to provide anchorage.
  4. Chemotropism: enables the growth of the pollen tube in flowering plants to facilitate fertilization.
  5. Hydrotropism: enables roots of the plant to obtain water.
Nastic Responses
These are non-directional movements of plant organs in response to diffuse stimuli, such as folding of leaves in warm weather, opening and closing of flowers in response to intensity of light and the closing of leaves when touched. Such movements occur as a result of changes in turgor pressure in certain cells.
Types of Nastic Responses
(a) Nyctinasty
This is a plant movement in response to temperature changes. This is a thermostatic movement; therefore nyctinasty is referred to as thermonasty.
(b) Photonasty
This is a plant movement in response to a change in light intensity. Some flowers in certain plants open in presence of light and close in its absence.
(c) Seismonasty
This is plant movement in response to shock or vibration.
(d) Hydronasty
This is plant movement in response to changes in atmospheric humidity
(e) Haptonasty
This is plant movement in response to contact. The sensitive plant Mimosa pudica response to touch by folding up its leaves.
(f) Chemonasty
This is a plant movement in response to chemical stimuli.
Tactic Movement
This is the movement of whole organism in response to an external stimulus. If the movement toward stimulus the tactic is positive, when the movement is away from the stimulus, the tactic is negative. Tactic movement is known as taxis.
Types of Tactic Movement
  1. Phototaxis – locomotary response to light
  2. Chemotaxis – locomotary response to chemical
  3. Aerotaxis – locomotion response to variation in oxygen concentration
  4. Rheotaxis – locomotary in response to direction of water current
  5. Magnetotaxis – locomotary in response to magnetic field.
  6. Osmotaxis – locomotary response to variations in osmotic pressure.
  7. Thermotaxis – locomotary response to temperature changes.
Other Effects of Auxins
(a) Apical Dominance
This refers to the inhibition of lateral bud development by the terminal bud. If the terminal bud is removed, lateral buds develop into side branches. This is because when the apical bud is cut and removed, the apical dominance is reduced. However, if the apical bud is cut and then a substance containing auxin is applied to the cut end, lateral buds do not sprout or develop. This experiment clearly indicates that apical dominance is brought by auxins. The principle of apical dominancy is applied in pruning. Removal of the terminal bud encourages the sprouting of side branches causing the plant to grow sideways instead of upwards.
(b) Development of Adventitious Roots
Adventitious roots are the roots that develop from a stem cutting. Plant cuttings which do not develop roots readily may be dipped in rooting auxins e.g. Indole Butyric Acid (IBA) and Naphthalene Acetic Acid (NAA).
(c) Storage
NAA is used to increase the period of dormancy in tubers and bulbs so that they can be stored for a longer period of ti
(d) Parthenocarpy
This is the formation of fruits without fertilization. Parthenocarpy can be inducedby treating unpollinated flowers with auxin. This phenomenon is applied in the development of seedless fruit varieties.
(e) Falling of Leaves and Fruits
Falling of leaves and fruits is brought about by a reduction in the concentration of auxins. Premature falling of fruits occurs due to the failure of the plant to produce adequate amount of auxins. This situation can be reversed by application of auxins.
(f) Weed Killer
In higher concentrations, auxins interfere with normal plant growth and can cause death. In this case auxins are used as herbicides or selective weed killers. For instance 2,4-dechlorophenoxyacetic acid (2,4-C) can be used as a weed killer (herbicide) killing broad-leaved plants.
Other Plant Hormones
(a) Gibberellins
These are a mixture of chemical compounds which have an effect on plant growth. A common example of gibberellins is Gibberillic acid. Gibberillic acid causes stem elongation in plants. It stimulates rapid growth in dwarf varieties of certain plants by increasing the length of the internodes. Also used in breaking seed dormancy and inducing parthenocarpy.
(b) Ethylene
Speeds up ripening of fruits such as citrus
(c) Abscisic Acid (ABA)
Regulates fruit drop at the end of the season.
(d) Cytokinins
These are active growth substances which promote growth in plants in the presence of auxins. Cytokinins promote cell division by inducing growth of roots, leaves, callus tissue and repair or wounds in plants.
These are pale blue-green compounds consisting of a pigment, which absorbs light energy. Phytochrome exists in two interconvertible forms. One absorbs red light at a wavelength of 665mm while the other one absorbs far red light at a wavelength of 725mm. These two forms of phytochrome are designated as Pr and Pfr respectively. When Pr absorbs red light it is rapidly converted into Pfr and when Pfr absorbs far-red light it is rapidly converted into Pr.
The two phytocromes, that is Pr and Pf have the following effects:
  1. Elongation of the stem is stimulated by far-red light but inhibited by the red light,
  2. Leaf expansion is stimulated by the red light but inhibited by far-red.
  3. Lateral roots growth is stimulated by far-red and inhibited by the red light.
  4. Seed germination is stimulated by the red light but inhibited by the far-red light.
This is a flowering response in plants relative to lengths of day and night. When a plant is exposed to light, phytochrome absorbs light energy and P725 accumulates. P725 initiates the formation of a flowering hormone known as florigen, which is transported to the stem apices to promote flowering.
With reference to photoperiodism, plants can be classified into three groups:
  • Short day plants
  • Long day plants
  • Day neutral plats
These are the plants that require short-length illumination but shorter night periods to flower. Examples include chrysanthemum andpoinsettias.
Long-day Plants
These are the plants that require longer day-length illumination but shorter night periods in order to flower. Examples include wheat and lettuce.
Day – neutral plant
These are the plants that flower irrespective of day – length or right periods. Examples of day-neutral plants include cotton andtomatoes.


This is a process of getting rid of waste products from the body of living organism formed during metabolic process. Metabolic process includes all chemical reactions taking place inside living system. Example respiration
During the process of respiration, carbon dioxide is one of the products. Therefore carbon dioxide is known as excretory product and the organs that get rid of them are called excretory organs.
1. It is important that all unwanted products be removed from the body of a living organism, because if they are allowed to remain in the body, they would soon become harmful and poisonous to the living.
2. Also sometimes materials that are taken into the body from outside maybe in excess of what is required. If so they will have to be removed as waste. E.g. proteins
3. In some cases excretory product undergo detoxification in order to make them less toxic to the organism before they are moved from the body

Unicellular or single celled organism such as amoeba and paramecium get rid of their waste product simply by diffusion through the surface of their bodies
-These waste substances diffuse from cytoplasm where they are at high concentration to outside of the body where concentration is low. Another method of excretion is by use of contractile vacuole
Excretion in higher animal is carried out by elaborate system made up of specialized tissue and organ. This is because their bodies are complex and have greatest number of cell such that simple diffusion will not suffice.
Excess amino acid/ protein cannot be stored in the body instead they are broken down to form ammonia .Nitrogenous waste product can be removed in 3 forms.

Ammonia is high poisonous and dissolve in water it is removed in soluble form. It can be rapidly and safely removed if diluted in a sufficient volume of water. eg fish
Ammonia with carbon dioxide to form less toxic form of waste product .Urea is formed in the liver and insoluble in water. Urea is formed in the liver and insoluble in water. Urea is excreted by aquatic mammal and terrestrial animal E.g. man

Ammonia is also excreted as uric acid; uric acid is unstable and non toxic. This is excreted by animals living in shortage of much water E.g. insects, birds and reptile’s uric acid is shortage and excreted in form of crystals.
Carbondioxide is produced during respiration. It is excreted through gaseous exchange.

Excess metabolic water form chemical breakdown of glucose lost either as water vapour sweat or urine.
These are dark red bean shaped organ located at the back of abdominal cavity. There are two kidneys in human body the right kidney and the left kidney. Above each kidney are adrenal glands which secrete hormones which stimulate reabsorption of sodium ions. There are two blood vessels connected to the kidney one of them is RENAL ARTERY, supplies blood to the kidney. The other is the RENAL VEIN, takes blood away from the kidney.
A tube called URETER runs from each kidney to the bladder. Urine passes through ureter from the kidney to store in the bladder. From there, it is released periodically through a tube called URETHRA.
When the bladder is nearly full the stretching stimulates sensory nerve ending in its wall so that nerve impulse are relayed to the brain and argue to urinate develop. The sphincter muscle located at the base of the bladder relaxes and the urine is released via urethra. This tube is contained within the penis in mammals
The kidney is composed of three regions namely
Is the outer zone which is dark in colour contains a dense network of blood capillaries that form the glomerali of the nephrone, which are the functional unit of the kidney.
This part lies between the cortex and the pelvis. The surface of the medulla facing the pelvis is folded to form projection called PYRAMIDS
Pelvis narrow to form ureter. Pelvis is a collecting space leading to the ureter which takes the urine to the bladder.
Nephrone is a function unit of the kidney. The nephrone performs both function of OSMOREGULATION and EXCRETION (Osmoregulation – maintains constant osmotic pressure of body fluids)
Each nephrone consist of a long tubule closed at one end and open at the other
The Nephrone is divided into four parts
  1. Bowman’s capsule
  2. Proximal convoluted tubule
  3. Loop of henle
  4. Distal convolute
    d tubule.
This is a round – cup shaped part of the closed end of the tubule and encloses the glomerulus, which is a network of blood capillaries the glomerulus formed from the afferent blood vessels, a branch from the renal artery.
Is the coiled part of the tubule next to the Bowman’s capsule. It lies in the cortex.
The portion of the tubule which extends from the approximation convoluted tubule and dips into the medulla, from the medulla it bends back into the cortex to form a u – shaped loop.
This is coiled part next to the open end of the tubule which joins with a collecting duct (ureter). The whole length of the nephrone is surrounded by network of capillaries.
Excretion takes place in three types
(i) Filtration
(ii) Reabsorption
(iii) Removal

Kidney receives blood at high pressure through renal artery (branch of Aorta)
  • The blood is rich in nitrogenous waste such as urea, dissolved food substance, plasma, protein, mineral ions, hormones and oxygen.
  • The afferent vessels entering the glomerulus are wider than the efferent vessels leaving the glomerulus. The narrowness of the efferent vessels produces resistance to blood flow and thus creates pressure in the glomerulus.
  • Due to high pressure in the glomerulus, the liquid of the blood dissolve substance of small molecular sizes are forced out of the glomerular in the Bowman’s capsule (urea, glucose, salt and amino acid).
  • Large sized molecule such as proteins and blood cells are not filtered because the walls of the capillaries of glomerulus and Bowman’s capsule have very small pores.
-Hence the blood which remains is rich in plasma and has very little water.
– This process is known as ultrafilitration and the filtrate formed is called glomerular filtrate
– As the glomerular filtrate moves along tubules, useful substance to the blood are selected and reabsorbed back into the blood.
– Most of the reabsorption occurs in the proximal convoluted Tubule through the process of Active transport.
– For efficient reabsorption of substance, the proximal convoluted tubule is adopted in several ways;
a) The cell the tubule has mitochondria which provide necessary energy in form of ATP.
b) The cell has microvillus to increase surface area reabsorption
c) Tubule is long and coiled to provide large surface area for reabsorption
d) Well supplied with blood capillaries.
e) The tube is coiled to slow down the speed of the flow of the filtrate.
Certain disorders of the body can be diagnosed by examining the contents and measuring the quantity of urine
1. For example, if the urine contains glucose then this indicates the disease DIABETS MELLITUS
– Diabetes Mellitus occurs because the pancreas does not produce enough of the hormone (insulin) which controls blood sugar level.
– It can be treated with injections of insulin
2. Another type of diabetes is DIABETES INSPIDUS, which results from large quantity of dilute urine being produce
-This happens because the sufferer cannot produce enough Anti – Diuretic hormone (ADH), which is responsible for regulation of the amount of water in the blood.
  • Diabetes inspidus can be treated by Nasal sprays which contains ADH.
Nephritis is the general term for any infection or inflammation of the kidney.
  • In one type the glomeruli fail to function normally and allow protein to filter through into the tubules.
  • It is diagnosed by the presence of protein in the urine
Causes of Nephritis
  1. Could be allergic reaction
  2. It may be blood vessels disorders or high blood pressure
  3. Damage of the kidney.
  • It is dependent on the cause
Small stones can sometimes be formed in the pelvic region of the kidney.
– These stones may be made of
1) Uric acid
2) Calcium oxalate or (mixture of calcium)
3) Magnesium
4) Ammonium phosphate
– They form as a result of obstruction of urine flow on excess of certain chemicals in the blood stream
– There are often no symptoms of kidney stones unless stones move from their original position
– If a stone moves into the ureter, it causes severe of pain (Renal pain) which can be felt in the lower back to the grain, accompanied by vomiting sweating.
– There may also be blood in the urine.
  1. Can be treated by x- rays some small stones may put down the ureter and out through the bladder with the need for treatment.
  2. Larger stones may have to be removed surgically actively they can be shattered into fragments by treatment.
  3. The small fragments are passed out harmless in the urine.
NB: An untreated kidney stone may obstruct urine and lead to Nephritis
Cystitis is an inflammation at the bladder caused by infection.
  1. Symptoms are frequent painful urination and blood urine.
Middle symptoms maybe
  1. Slight increase in the frequency of urination accompanies by a burning sensation.
  2. If the infection spreads to the kidney, it may cause fever, blood in urine and backache.
  1. It may be caused by bacterial infection of the bladder usually from the urethra.
Kidney failure is a condition where one or both kidney cease to function.
  • If it happens to both kidneys it is fatal it not treated
  • It can happen suddenly as a result of high blood pressure
  • It is possible to live with one kidney only, but the only treatment for failure of both kidneys is DIALYSIS or a KIDNEY TRANSPLANT.
  • In dialysis, the patient’s blood supply is linked to a kidney machine. The machine performs the functions of normal kidney by filtering the blood and removing and excess salts and water.
  • The patient’s spends several hours in a week linked to the machine.
  • A kidney transplant involves surgically inserting a health kidney from a donor to replace a diseased kidney.
  • The kidney has to be compatible to avoid problems with rejections.
  • Sometimes a health person will donate a kidney to save the life of a close relative suffering from kidney failure.
  • Plants manufacture all their organic needs according to demand.
  • They only make as much protein as they need at any one time, for example
  • Therefore they do not excrete urea from excess amino acids because they do not usually have any.
  • They do however respire and photosynthesis and as a result products.
  • However, they waste products of one process may be the raw material for the other e.g. carbon dioxide and waste produced by respiration are recycled during daylight hours is usually high enough to produce oxygen faster than its use by respiration.
  • Therefore the oxygen is not used up by respiration and the excess is excreted.
  • Some plants produce tannins and other organic acids from nitrogen and carbohydrate metabolism.
  • These are passed into leaves. Where they build up and are lost from the plant when the leaves fall off.
  • Bitter substances such as tannins and other organic acid have a protective role in deterring leaf acting animals, from feeding on the plants.
  • Trees produce various gums, resins and latexes which can be collected from the tree and have a wide range industrial uses.
  • Products such as turpentine, paints, varnishes, soap cosmetics, food, surgical items, gold balls, bubble gum and rubber are manufactured from these plants products.
Mechanism through which plant remove their waste product
  1. Diffusion
  2. Abscission
  3. Degradation

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    Namanya saul, December 9, 2023 @ 4:10 am Reply


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