GROWTH AND DEVELOPMENT - Form 3 Biology Notes

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Concept of Growth and Development

  • Growth is a characteristic feature of all living organisms.
  • Most multicellular organisms start life as a single cell and gradually grow into complex organisms with many cells.
  • This involves multiplication of cells through the process of cell division.
  • This quantitative permanent increase in size of an organism is referred to as growth.

    For growth to take place the following aspects occur:
  • Cells of organisms assimilate nutrients hence increase in mass.
  • Cell division (mitosis) that lead to increase in the number of cells.
  • Cell expansion that leads to enlargement an increase in the volume and size of the organism. It is therefore possible to measure growth using such parameters as mass, volume, length, height, surface area.
  • On the other hand development is the qualitative aspect of growth which involves differentiation of cells and formation of various tissues in the body of the organism in order for tissues to be able to perform special functions.
  • It is not possible to measure aspects of development quantitatively.
  • Therefore development can be assessed in terms of increase in complexity of organism e.g. development of leaves, flowers and roots.
  • A mature human being has millions of cells in the body yet he or she started from; single cell, that is, a fertilised egg.
  • During sexual reproduction mammals an ovum fuses with a sperm form a zygote.
  • The zygote divides rapidly without increasing in size, first into 2, 4, 8, 16,32, 64 and so on, till it forms a mass cells called morula.
  • This first cell division is called cleavages.
  • The morula develops a hollow part, resulting into a structure known as a blastula (blastocyst).
  • Later, blastocyst cells differentiate into an inner layer (endoderm) and the outer layer (ectoderm).
  • The two-layered embryo implants into the uterine wall and, by obtaining nutrients from the maternal blood, starts to grow and develop.
    blastocoel
  • As the embryo grows and develops, changes occur in cell sizes and cell-types.
  • Such changes are referred to as growth and development respectively.
  • These processes lead to morphological and physiological changes in the developing young organism resulting into an adult that is more complex and efficient.
  • In the early stages, all the cells of the embryo look alike, but as the development process continues the cells begin to differentiate and become specialised into different tissues to perform different functions.
  • Growth involves the synthesis of new material and protoplasm.
  • This requires a continuous supply of food, oxygen, water, warmth and means of removing waste products.
  • In animals, growth takes place all over the body but the rates differ in the various parts of the body and at different times.
  • In plants however, growth and cell division mostly take place at the root tip just behind the root cap and stem apex.
  •  This is referred to as apical growth which leads to the lengthening of the plant.
  • However, plants do not only grow upwards and downwards but sideways as well.
  • This growth leads to an increase in width (girth) by the activity of cambium cells.
  • The increase in girth is termed as secondary growth.

Study Question 1 

State two major differences between growth and development



Measurement of Growth

  • Growth can be estimated by measuring some aspect of the organism such as height, weight, volume and length over a specified period of time.
  • The measurements so obtained if plotted against time result into a growth curve.

Study Question 2

The following results were obtained from a study of germination and early growth of maize. The grains were sown in soil in a greenhouse and.at two-day intervals. Samples were taken, oven dried and
weighed. See table .

 Time after sowing (days)  Dry mass of embryo (g) 
 0  0.002
 2  0.002
 4  0.008
 6  0.016
 8  0.024
 10  0.034
 12  0.034

Plot a graph of dry mass of embryo against time after sowing
Describe the shape of the graph

  • For most organisms when the measurements are plotted they give an S-shaped graph called a sigmoid curve such as in the figure below
    growth curve1
  • This pattern is due to the fact that growth tends to be slow at first and then speeds up and finally slows down as adult size is reached.

A sigmoid curve may therefore be divided into four parts.

Lag phase (slow growth)

  • This is the initial phase during which little growth occurs.
  • The growth rate is slow due to various factors namely:
    1. The number of cells dividing are few.
    2. The cells have not yet adjusted to the surrounding environmental factors.

Exponential phase (log phase)

  • This is the second phase during which growth is rapid or proceeds exponentially.
  • During this phase the rate of growth is at its maximum and at any point, the rate of growth is proportional to the amount of material or numbers of cells of the organism already present.
  • This rapid growth is due to:
    1. An increase in number of cells dividing,2-4-8-16-32-64 following a geometric progression,
    2. Cells having adjusted to the new environment,
    3. Food and other factors are not limiting hence cells are not competing for resources,
    4. The rate of cell increase being higher than the rate of cell death.

Decelerating Phase

  • This is the third phase during which time growth becomes limited as a result of the effect of some
    internal or external factors, or the interaction of both.
  • The slow growth is due to: 
    1. The fact that most cells are fully differentiated.
    2. Fewer ceils still dividing,
    3. Environmental factors (external and internal) such as:
      • shortage of oxygen and nutrients due to high demand by the increased number of cells.
      • space is limited due to high number of cells.
      • accumulation of metabolic waste products inhibits growth. 
      • limited acquisition of carbon (IV) oxide as in the case of plants.

Plateau (stationary) phase

  • This is the phase which marks the period where overall growth has ceased and the
    parameters under consideration remain constant.
  • This is due to the fact that:
    1. The rate of cell division equals the rate of cell death.
    2. Nearly all cells and tissues are fully differentiated, therefore there is no further increase in the number of cells.
  • The nature of the curve during this phase may vary depending on the nature of the parameter, the species and the interns! factors.
  • In some cases, the curve continue to increase slightly until organism dies as is the case monocotyledonous plants, man invertebrates, fish and certain reptiles. indicates positive growth.
  • In some of cases the curve flattens out indicating change in growth while other growth curv may tail off indicating a period of negat growth rate.
  • This negative pattern characteristic of many mammals including humans and is a sign of physical senesee associated with increasing age.

However, the sigmoid curve does not to all organisms, for example, arthropods.

  • In insects, growth takes place at intervals - volume changes are plotted against time and a different curve is obtained.
  • This is called intermittent growth curve which is shown in the figure below,
  • The intermittent growth in insects is due to the fact that they have an exoskeleton and hence growth is possible only when it is shed.
  • This shedding process is known as moulting or ecdysis.
  • However, cell division continues to take place during the inter-moult phase but the expansion of tissues is limited by the unshed exoskeleton.

    intermittent growth curve1

Study Question 3

What happens during the following; log and stationary phases of growth?

Practical Activity I: Project

To measure the growth of a plant

Requirements

Small plots/boxes, meter rule and seeds of beans (or green grams, peas, maize),

 

Procedure

  1. Place some soil in the box or prepare a small plot outside the laboratory.
  2. Plant some seeds in the box and place it in a suitable place outside the laboratory (or plant the seeds in your plot).
  3. Water the seeds daily.
  4. Observe the box/plot daily and note the day the seedlings emerge out of the soil.
  5. Measure the height of the shoot from the soil level up to the tip of the shoot. Repeat this with four other seedlings. Work out the average height of the shoots for this day.
  6. Repeat procedure 5 every three days for at least three weeks.
  7. Record the results in a table form.
  8. On the same seedlings measure the length of one leaf from each of the five seedlings (from leaf apex to itsattachment on the stem).
  9. Calculate the average length of the leaves and record in the table.
  10. Plot a graph of the height of the shoot against time. On the same axes plot length of leaf against time.
  11. Compare the two graphs drawn.


Growth and Development in Plants

  • The main growth and development phase in plants begins with the germination of the mature seed.
  • Seeds are of two kinds depending on the number of cotyledons or embryo leaves 

Practical Activity 2

To investigate structural differences between monocotyledonous and dicotyledonous seeds

Requirements

  • Bean seeds and maize grains which have been soaked overnight. Scalpel or razor blades, iodine solution, Petri-dish and hand lens.

Procedure

  1. Using a scalpel or razor blade make longitudinal sections (LS) of both the bean seed and the maize grain.
  2. Observe the LS of the specimens using a hand lens.
  3. Note any structural difference between the specimens.
  4. Draw the LS of each specimen and label.
  5. Puta drop of iodine solution on the cut surfaces of both specimens.
  6. Note any differences in colouration with iodine on the surfaces of the two specimens.
  7. On your diagrams indicate the distribution of the stain.
  8. Account for the difference in distribution of the colouration with iodine in the two specimens.

Structure of the Seed

  •  A typical seed consists of a seed coat enclosing an embryo.
  • The seed coat is the outer covering which, in most seeds, is made up of the two layers, an outer testa and inner one, the legmen.
  • The testa is thick; the tegmen is a transparent membrane tissue.
  • The two layers protect the seed from bacteria, fungi and other organisms which may damage it.
  • There is a scar called hilum on one part of the seed. 
  • This is point where the seed had been attached the seed stalk or funicle.
  • Near one end of 1 hilum is a tiny pore, the micropyle
  • This allows water and air into the embryo, embryo is made up of one or two seed leavi or cotyledons, a plumule (embryonic sh( and a radicle (the embryonic root).
  • The radicle is opposite the micropyle.
  • In some seeds the cotyledons are swollen as they contain stored food for growing plumule and radicle. Such seeds are called non-endospermic seeds.
  • In other cases, the seeds have their food stored in: endosperm.
  • Such seeds are call endospermic seeds. Seeds with one cotyledon are referred to as monocotyledonous while those with two are referred to dicotyledonous.
  • This is the major basis in differentiation between the two large classes of plants, the monocotyledonae and dicotyledonae.
    structure of a maize seed 

    structure of a bean seed

Dormancy in Seeds

  • The embryo of a dry, fully developed seed usually passes through a period of rest after ripening period.
  • During this time the seed performs all its life (physiological) processes very slowly and uses up little food. This is a period of dormancy.
  • Even if all the favourable environmental conditions for germination are provided to the seed during this period of dormancy, the seed will not germinate.
  • This is due to the fact that the seed embryo may need to undergo further development before germination.
  • Some seeds can germinate immediately after being shed from the parent plant (e.g. most tropical plants) while others must pass through dormancy period, lasting for weeks, months or even years before the seed can germinate.
  • Dormancy provides the seeds with enough time for dispersal so that they can germinate in a suitable environment.
  • It also enables seeds to survive during adverse environmental conditions without depleting their food reserves.
  • The embryo has time to develop until favourable conditions are available e.g. availability of water.

Factors that Cause Dormancy

  • Embryo may not yet be fully developed.
  • Presence of chemical inhibitors that inhibit germination in seeds e.g.abscisic acid.
  • Very low concentrations of hormones e.g. gibberellins and enzymes reduces the ability of seeds to germinate.
  • Hard and impermeable seed coats prevent entry of air and water in some seeds e.g. wattle.
  • In some seeds the absence of certain wavelengths of light make them remain dormant e.g. in some lettuce plants.
  • Freezing of seeds during winter lowers their enzymatic activities rendering them dormant.

Ways of Breaking Dormancy

  • When the seed embryos are mature then the seed embryos can break dormancy and germinate.
  • Increase in concentration of hormones e.g. cytokinins and gibberellins stimulate germination.
  • Favourable environmental factors such as water, oxygen and suitable temperature.
  • Some wavelengths of light trigger the production of hormones like gibberellins leading to breaking of dormancy.
  • Scarification i.e. weakening of the testa is needed before seeds with hard impermeable seed coats can germinate. This is achieved naturally by saprophytic bacteria and fungi or by passing through the gut of animals. In agriculture the seeds of some plants are weakened by boiling, roasting and cracking e.g. wattle.

Seed Germination

  • The process by which the seed develops into a seedling is known as germination.
  • It refers to all the changes that take place when a seed becomes a seedling.
  • At the beginning of germination water is absorbed into the seed through the micropyle in a process known as imbibition and causes the seed to swell.
  • The cells of the cotyledons become turgid and active.
  • They begin to make use of the water to dissolve and break down the food substances stored in the cotyledons.
  • The soluble food is transported to the growing plumule and radicle.
  • The plumule grows into a shoot while the radicle grows into a root.
  • The radical e merges from the seed through micropyle, bursting the seed coat as it does so.

Conditions Necessary for Germination

  • Seeds can easily be destroyed by unfavourable conditions such as excessive heat, cold or animals.
  •  Seeds need certain conditions to germinate and grow.
  • Some of these conditions are external, for example water, oxygen and suitable temperature while others are internal such as enzymes, hormones and viability of the seeds themselves.
Water
  • A non-germinating seed contains very little water.
  • Without water a seed cannot germinate.
  • Water activates the enzymes and provides the medium for enzymes to act and break down the stored food into soluble form.
  • Water hydrolyses and dissolves the food materials and is also the medium of transport of dissolved food substances through the various cells to the growing region of the radical and plumule.
  • Besides, water softens the seed coat which can subsequently burst and facilitate the emergence of the radicle.
Oxygen
  • Germinating seeds require energy for cell division and growth.
  • This energy is obtained from the oxidation of food substances stored in the seed through respiration thus making oxygen an important factor in seed germination.
  • Seed in water logged soil or seed buried deep into the soil will not germinate due to lack of oxygen.
Temperature
  • Most seeds require suitable temperature before they can germinate.
  • Seeds will not germinate below 0°C or above 47° C.
  • The optimum temperature for seeds to germinate is 30°C.
  • At higher temperature the protoplasm is killed and the enzymes in the seed are denatured.
  • At very low temperatures the enzymes become inactive.
  • Therefore, the protoplasm and the enzymes work best within the optimum temperature range.
  • The rate of germination increases with temperature until it reaches an optimum.
  • This varies from plant to plant.
Enzymes
  • Enzymes play a vital role during germination in the breakdown and subsequent oxidation of food.
  • Food is stored in seeds in form of carbohydrates, fats and proteins which are in insoluble form.
  • The insoluble food is converted into a soluble form by the enzymes.
  • Carbohydrates are broken down into glucose by the diastase enzyme, fats into fatty acids and glycerol by lipase, and proteins into amino acids by protease.
  • Enzymes are also necessary for the conversion of hydrolysed products to new plant tissues.
Hormones
  • Several hormones play a vital role in germination since they act as growth stimulators.
  • These include gibberellins and cytokinins.
  • These hormones also counteract the effect of germination inhibitors.
Viability
  • Only seeds whose embryos are alive and healthy will be able to germinate and grow.
  • Seeds stored for long periods usually lose their viability due to depletion of their food reserves and destruction of their embryo by pests and diseases.

Study Question 4

In an experiment to investigate the effect of neat on germination of seeds, ten bags each containing 60 pea seeds were placed in a water-bath maintained at 85°C . After every two minutes a bag was removed and seeds contained in it planted. The number that germinated was recorded. The procedure used for pea seeds was repeated for wattle seeds. The results obtained were as shown in the table below

 Time (min)  Number of seeds that germinated  
   Pea seeds    Wattle seeds 
 0  60  0
 2  60  0
 4  48  0
 6  42  2
 8  34  28
 10  10  36
 12  2  40
 14  0  44
 16  0  46
 18  0  48
 20  0  49
 22  0  49
  1. Using a suitable scale and on the same axes, draw graphs of time in hot water against number of seeds that germinated for each plant. Use horizontal axis for time and the vertical axis for the seeds that germinated
  2. Explain why the ability of pea seeds to germinate declined with time of exposure to heat.
  3. Explain why the ability of the wattle seeds to germinate improved with time of exposure to heat.

Practical Activity 3

To investigate conditions necessary for seed germination

Requirements

  • Cotton wool, seeds, water, six fiat bottomed flasks, 2 corks, 2 test-tubes, blotting paper, incubator, refrigerator, thermometer, pyrogallic acid and sodium hydroxide.

Procedure

  • Prepare three set-ups as shown in figure below
    SETUP I
    setup II
    setup III
  • Leave the set-ups to stand for five days.
  • Record all the observable changes that have taken place in the flasks hi each set up in a table form as shown
     Setup  Observations  
       In flask A  In flask B

     I

    II

    III

       

Study Question 5

  1. Which condition was being investigated in set-up I, II and III?
  2. For each set-up explain the results obtained.
  3. What was the role of flask B in each set-up?

Types of Germination

  • The nature of germination varies in different seeds.
  • During germination the cotyledons may be brought above the soil surface.
  • This type of germination is called epigeal germination.
  • If during germination the cotyledons remain underground the type of germination is known as hypogeal.
Epigeal Germination
  • During the germination of a bean seed, the radicle grows out through the micropyle.
  • It grows downwards into the soil as a primary root from which other roots arise.
  • The part of the embryo between the cotyledon and the radicle is called the hypocotyl.
  • This part curves and pushes upwards through the soil protecting the delicate shoot tip.
  • The hypocotyls then straightens and elongates carrying with it the two cotyledons which turn green and leafy.
  • They start manufacturing food for the growing seedling.
  • The plumule which is lying between two cotyledons, begins to grow into first foliage leaves which start manufacturing food.
 

 

Hypogeal Germination
  • In maize, the endosperm provides food to the embryo which begins to grow.
  • The radicle along with a protective covering(coleorhiza) grows out of the seed.
  • The epicotyl is the part of the embryo between the cotyledon and the plumule.
  • The epicotyl elongates and the plumule grows out of the coleoptile and forms the first foliage leaves.
  • The seedling now begins to produce its own food and the endosperm soon shrivels.
  • This type of germination in which the cotyledon remains below the ground is known as hypogeal germination.
    epigeal

    hypogeal

Practical Activity 4

To investigate epigeal and hypogeal germination

Requirements

  • Tin or box, soil, water, maize grains and bean seeds.

Procedure

  • Place equal amounts of soil into two containers labelled A and B.
  • In A, plant a few maize grains.In B, plant a few bean seeds.
  • Water the seeds and continue watering daily until they germinate.
  • Place your set-ups on the laboratory bench.
  • Observe daily for germination.
  • On the first day the seedlings emerge from the soil, observe them carefully with regard to the soil level
  • Carefully uproot one or two seedlings from each set.

Study Question 6

  1. Observe and draw the seedlings from each set Label the parts and indicate the soil level on your diagram.
  2. On the fifth day since emergence, again uproot another seedling. Observe and draw.
  3. Indicate the soil level on your diagram..
  4. Tabulate the differences between the two types of germination studied.

Primary and Secondary Growth

  • The region of growth in plants is found in localised areas called meristems as shown .
  • A meristem is a group of undifferentiated cells in plants which are capable of continuous mitotic cell division.
  • The main meristems in flowering plants are found at the tips of shoots and roots, in young leaves, at the bases of the inter-nodes, and in vascular cambium and cork cambium.
  • The meristems at the tips of the shoots and the roots are known as apical meristems and are responsible for primary growth. The cambium meristems are responsible for secondary growth.
    meristem
    dicot stem and root
    monocot stem

Primary Growth

  • Primary growth occurs at the tips of roots and shoots due to the activity of apical meristems. These meristems originate from the embryonic tissues. In this growth there are three distinctive regions, the region of cell division, cell elongation and cell differentiation. See figure above.
  • The regipn of cell division is an area of actively dividing meristematic cells. These cells have thin cell walls, dense cytoplasm and no vacuoles. In the region of cell elongation, the cells become enlarged to their maximum size by the stretching of their walls. Vacuoles start forming and enlarging. In the region of cell differentiation the cells attain their permanent size, have large vacuoles and thickened wall cells. The cells also differentiate into tissues specialised for specific functions.
  • Primary growth results into an increase in the length of shoots and roots.
Region of Growth in a root
  • This is determined by taking a young germinating seedling whose radicle is then marked with the Indian ink at intervals of 2 mm. The seedling is left to grow for sometime (about 24 hours or overnight) and then the ink marks are examined. When the distance between successive ink marks are measured.
  • it is found that the first few ink marks, especially between the 2  and 3rd mark above tip of root have increased significantly. This shows that growth has occurred in the region just behind the tip of the root.
  • The difference between the length of each new interval and the initial interval of 2 mm gives the increase in the length of that interval during that period of time. From this the rate of growth of the root region can be calculated.
    Growth = Increase in length/Original length × 100

Practical Activity 5

To determine the region of growth in roots

Requirements

  • Germinating bean seeds with radicle of about 1cm in length, cork, pin, beaker or gas jar, water, Indian ink, blotting paper or filter paper, marker and ruler marked in mm.

Procedure

  1. Take the germinating been seed, andvusing a blotting paper, dry the radical taking care not to damage the root.
  2. Using a marker and ruler make light ink marks 2mm apart along the length of the root. 
  3. Make a drawing of the marked root. Pin the seedling onto the cork and place it in the beaker containing a little water. See figure 4.10(b). Leave it overnight. Take out the seedling and examine the ink marks.
  4. Measure the distances between the successive ink marks and record. Make a well labelled drawing of the seedling at the end of the experiment and compare with the drawing of the seedling at the start of the experiment.

Study Question 7

  1. What part of the radicle has the ink marks moved further apart?
  2. Give an explanation for your answers in (a) above.
  3. What is the increase in length within each interval?
  4. Work out the rate of growth for the root

Secondary Growth

  • Secondary growth results in an increase in width or girth due to activity of the cambium. In secondary growth new tissues are formed by vascular cambium and cork cambium. In monocotyledons plants there are no cambium cell in the vascular bundles.The growth in diameter is due to the enlargement of the primary cells.
  • Secondary growth in dicotyledonous pjants begins with the division of vascular cambium to produce new cambium cells between the vascular bundles. This forms a continuous cambium ring. These cambium cells divide to form the new cells that are added to the older ones. The cambium cells have now become meristematic.
  • The new cells produced to the outer side of cambium differentiate to become secondary phloem and those to the inner side differentiate to become the secondary xylem. More secondary xylem is formed than secondary phloem. The intervascuiar cambium also cuts parenchymous cells which form secondary medullary rays.
  • As a result of the increase in the volume of the secondary tissues, pressure is exerted on the outer cells of the stem. This results in stretching and rupturing of the epidermal cells. In order to replace the protective outer layer of the stem, a new band of cambium cells are formed in the cortex. These cells, called cork cambium  or phellogen originate from the cortical cells. The cork cambium divides to produce new cells on either side. The cells on the inner side of the cork cambium differentiate into secondary cortex and those produced on the outer side become cork cells. Cork cells are dead with thickened walls. Their walls become coated with a waterproof substance called suberin. The cork cells increase in number and become the bark of the stem. This prevents loss of water, infection from fungi and damage from insects. The corky bark is also resistant to fire and thus acts as an insulatory layer.
  • The bark is normally impermeable to water and respiratory gases. Periodically the cork cells, instead of being tightly packed, they form a loose mass. This mass is known as Lenticel. The lenticles make it possible for gaseous exchange between air and inner tissues.
    lenticels


    sec and pri growth

  • The rate of secondary growth in a stem varies with seasonal changes. During rainy season, xylem vessels and tracheids are formed In large numbers. These cells are large, have thin walls and the wood has a light texture. In the dry season, the xylem and trancheids formed are few in number. They are small, thick-walled and their wood has a dark texture. This leads to the development of two distinctive layers within the secondary xylem formed m a year, called annual rings. See figure 4.13. It is possible to determine the age of a tree by counting the number of annual rings. Furthermore climatic changes of the past years can be infered from the size of the ring.

Role of Growth Hormones in Plants

  • Plant hormones are chemicals produced in very small amounts within the plant body, and play a very important part in regulating plant growth and development. Most growth hormones are produced at the tip of a shoot and transported downwards to the root. The root tip produces very small quantities of the hormones.
  • There are many different types of plant hormones and one well-known group is the auxins. Indole acetic acid (IAA) is one best known auxin. Auxins are produced at the shoot and root tips. Maximum influence on growth in plants occurs when auxins are produced simultaneously with other plant hormones e.g. gibberellins. Maximum growth response in stems requires more IAA than in the roots.
  • Auxins are known to have various effects on the growth and development in plants. They stimulate cell division and cell elongation in stems and roots leading to primary growth. Auxins cause tropic responses, which are growth responses in plants due to external stimuli acting from a given direction.
  • On the other hand IAA stimulates the growth of adventitious roots which develop from the stem rather than tbe main root. Cuttings can be encouraged to develop roots with the help of IAA. If the cut end of a stem is dipped into IAA, root sprouting is faster. IAA is also used to induce parthenocarpy. This is the growth of an ovary into a fruit without fertilisation. This is commonly used by horticulturalists to bring about a good crop of fruits particularly pineapples.
  • Auxins are known . to inhibit development of side branches from lateral buds. They therefore enhance apical dominance. During secondary growth auxins Play an important role by initiating cell division in the cambium and differentiation of these cambium cells into vascular tissues.
  • Auxins in association with other plant hormones such as the cytokinins induce the formation of callus tissue which causes the healing of wounds. When the concentration of auxins falls in the plant, it promotes formation of an abscission layer leading to leaf fall. A synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D) induces distorted growth and excessive respiration leading to death of the plant. Hence it can be used as a selective weed killer. Gibberellins are another important group of plant growth hormone. GibbereHins are a mixture of compounds and have a very high effect on growth. The most important in growth is gibberellic acid. Gibbereilins are distinguished from auxins by their stimulation of rapid cell division and cell elongation in dwarf varieties of certain plants. Dwarf conditions are thought to be caused by a shortage of gibberellins due to a genetic deficiency.
  • Gibberellins are important in fruit formation. They induce the growth of ovaries into fruits after fertilisation. They also induce parthenocarpy. Gibberellins also promote formation of side branches from lateral buds and breaks dormancy in buds. This is common in species of temperate plants whose buds become dormant in winter. In addition, this hormone also inhibits sprouting of adventitious roots from stem cuttings, it retards formation of abscission layer hence reduces leaf fall. Gibberellins also break seed dormancy by activating the enzymes involved in the breakdown of food substances during germination.
  • Cytokinins also known as kinetins, are growth substances which promote growth in plants when they interact with auxins. In the presence of auxins, they stimulate cell division thereby bringing about growth of roots, leaves and buds. They also stimulate formation of the callus tissues in plants. The callus tissue is used in the repair of wounds in damaged parts of plants.
  • Cytokinins promote flowering and breaking of seed dormancy in some plant species. They also promote formation of adventitious roots from stems and stimulate lateral bud development in shoots. When in high concentration cytokinins induce cell enlargement of leaves but in low concentration they encourage leaf senescence and hence leaf fall.
  • Ethylene is a growth substance produced in plants in gaseous form. Its major effect in plants is that it causes ripening and falling of fruits. This is widely applied in horticultural farms in ripening and harvesting of fruits. It stimulates formation of abscission layer leading to leaf fall, induces thickening of stems by promoting cell division and differentiation at the cambium meristem. But it inhibits stem elongation. Ethylene promotes breaking of seed dormancy in some seeds and flower formation mostly in pineapples.
  • Abscisic acid is a plant hormone whose effects are inhibitory in nature. It inhibits seed germination leading to seed dormancy, inhibits sprouting of buds from stems and retards stem elongation. In high concentration, abscisic acid causes closing of the stomata. This effect is important in that it enables plants to reduce water loss. It also promotes leaf and fruit fall. Another hormone, florigen is produced in plants where it promotes flowering.

Apical Dominance

  • Although auxins, particularly IAA are important stem and root elongation, they are known to exert profound effects on other aspects of plant growth and development. If an apical bud which normally contains high concentrations of auxins is removed, it is observed that more lateral buds lower down the stem sprout, producing many branches. This shows that high concentrations of auxins have an inhibitor}' effect on sprouting of lateral buds and therefore hinders growth of many branches. This forms the basis of pruning in agriculture where more branches are required for increased harvest particularly on crops like coffee and tea.
  • The failure of lateral buds to develop in the presence of an apical bud is due to the diffusion of auxins from the shoot apex downwards in concentrations higher than that promoting lateral bud development.

Practical Activity 6

To investigate apical dominance in plants

Requirements

  • Tomato seedlings growing in a tin.

Procedure

  1. Cut off the terminal buds from 3 seedlings in the tin, leaving the other seedlings with the terminal buds intact,
  2. Leave the seedlings to continue growing for five more days.

Study Questions 8

  1. list the differences noticed between the two groups of seedlings? Explain how the differences come about.
  2. From your observations, explain the basis for pruning tea and coffee.



Growth and Development in Animals

  • In higher animals, most cells with the exception of the nerve cells, retain their power of division.
  • Thus, there is a continued breakdown and replacement of cells.
  • Animal cells undergo rapid cell division and cell differentiation but, unlike plant cells, they undergo very little cell enlargement.
  • In most animals growth occurs through: their life till they die.
  • This type of growth called continuous growth.
  • Arthropods e.g. insects show rapid growth immediately after moulting with periods when no growth increase occurs.
  • This is called discontinuous growth.
  • Insects exhibit two types of reproducti processes.
  • In some insects, the ova in t female are fertilised by the spermatozoa frc the male.
  • This is a typical example of sexi reproduction, common in butterflies ai moths.
  • In other insects like the black and t green aphids, the eggs are usually product without being fertilised and are able to --- into adult insects.
  • This type of asexual reproduction is referred to ; parthenogenesis.


Growth and Development in Insects

  • Majority of insects lay eggs that hatch int larvae, which is an immature stage, usual! quite different from the adults in morpholog and behaviour.
  • Depending on the insec species a larva is referred to as a grub, maggot or a caterpillar.
  • Generally the larv eats a lot, grows rapidly and sheds its cuticl several times until it reaches full size to become a pupa.
  • The pupa is an inactive, non feeding stage during which extensive breakdown and re-organisation of body tissue occur, eventually giving rise to the imago adult form.
  • Such changes, called metamorphosis, do occur in butterflies moths, bees, wasps and flies.
  • Insects which pass through these stages, namely, egg-larva-pupa, into imago/adult in their developmenl are said to undergo complete metamorphosis.

Development in a Housefly (An example of complete metamorphosis)

  • When the egg of a housefly is laid, it measures about 1mm in length.
  • The eggs are laid in batches of between 100 to 150.
  • The larvae which hatch from the eggs grow and feed on decaying matter.
  • After several moults and increase in size, a larva reaches about 1cm in length.
  • This takes about 5 days.
  • After this, the larva changes into a pupa encased in a pupal case called die puparium, from which the adult fly later emerges.
  • After emergence, the adult tgkes about two weeks of feeding and growing to attain sexual maturity, i.e. the males can mate and the females are able to lay eggs.
    housefly life cycle

Incomplete Metamorphosis

  • Development in some insects like the locust and cockroaches, involves the.egg hatching into a nymph which e!cie!y resembles the adult in every form, except for size and lack of sexual maturity.
  • For such insects to reach the adult, stages, they undergo a series of moults. before fully acquiring the adult size and attaining the sexual maturity.
  • These insects are said to undergo incomplete metamorphosis.
Development in a Cockroach (An example of incomplete metamorphosis)
  • Cockroaches produce eggs enclosed in a case in groups of between 10 - 15.
  • The case known as ootheca is made up of cfaitm.
  • The ootheca is usually deposited in moist dark and warm places, for example in cracks of furniture or crevices in walls.
  • It takes about a month before the small wingless nymphs emerge.
  • The nymphs feed, and moult about ten times with the total nymphal period lasting about 16 days for all the adult structure to become fully developed.
    life cycle of a cockroach

Role of Hormones in Insect Metamorphosis

  • In insects metamorphosis is controlled by hormones.
  • The hormones are produced in three glands namely;
  • Neurosecretory cells in the brain ganglia, a pair of corpora allata (singular Corpus allatum) located in the mandibular segment and prothoracic glands in the thorax.
  • During larval stages of the insect the corpora ailata produces juvenile hormone,
  • This leads to formation of larval cuticle., therefore moulting does not go beyond the larval stage.
  • When the larva matures, the corpus allatum disintegrates-
  • At this time the neurosecretory cells stimulate the prothoracic glands to produce moulting hormone (ecdysone).
  • Ecdysone is responsible for moulting in insects leading to the laying of the adult cuticle.
    role of hormones in insects

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