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Download 3.6.1 Reproduction of the Flowering Plant 2.3.7 Functions of Meiosis
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3.6 REPRODUCTION AND GROWTH 3.6.1 Reproduction of the Flowering Plant Sexual Reproduction Sexual reproduction involves two parents. Each parent makes sex cells called gametes. Gametes are haploid cells capable of fusion. Two gametes join (one from each parent) to form a diploid zygote. The zygote develops into an embryo which goes on to become the new individual Sexual reproduction involves the production and transfer of gametes, fertilisation, and development of an embryo Remember 2.3.7 Functions of Meiosis in multicellular organisms (the role of meiosis) Meiosis is important in multicellular organisms for : (a) keeping the parental chromosome number the same by forming haploid cells (gametes) in sexual reproduction Meiosis halves the chromosome number when gametes are formed. This means that the normal chromosome number is restored again at fertilisation. (b) introducing variation in the species by exchange of genetic material between homologous chromosomes The daughter cells produced in meiosis are not identical due to crossing over or the exchange of genetic material that takes place when the haploid cells or gametes are formed. This results in variations or differences in organisms made as a result of sexual reproduction. As variations are the basis for evolution, sexual reproduction is more beneficial to a species than asexual reproduction Ms. B. Fennessy Loreto Secondary School Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Flower structure The function of the flower is reproduction Structure and function of the floral parts Structure Function Sepal Protect the flower when it is a bud Petal Large and brightly coloured to attract insects. (Fragrance and nectar might also be present) Stamen (male part) Carpel (female part) Anther Produces pollen grains (microscopes). Filament Supports the anther Stigma Traps pollen grains Style Holds stigma in position to trap pollen Ovary Site of fertilisation and becomes the fruit Receptacle Supports the flower Flowers usually have a number of carpels and stamens. Monocotyledonous plants (monocots) have flower parts in multiples of 3 e.g. 3, 6 or 9 petals, stamens etc Dicotyledonous plants (dicots) have flower parts in multiples of 4 or 5 e.g. 4, 8, 12 petals etc or 5, 10, 15 petals etc Ms. B. Fennessy Loreto Secondary School Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Stamens The stamen is the male part of the flower. The anther makes the pollen grains (microspores) by meiosis and releases them when it breaks open. (Pollen grains causes hay fever in some people) The Pollen grain produces male gametes The filament holds the anther so that the pollen grains can be transferred away from the flower. It contains a vascular bundle to bring food and water up to the anther Carpals The carpel is the female part of the flower. The stigma is where the pollen grain lands and is stimulated to germinate. The style connects the stigma to the ovary. The ovary contains one or more ovules. Each ovule makes an embryo sac (megaspore) by meiosis. The embryo sac produces an egg cell and polar nuclei. After fertilisation the ovule becomes the seed and the ovary becomes the fruit. Petals Petals are large and colourful to attract insects. They may have a fragrance and a nectary containing sugary nectar to further attract the insects. In some flowers the petals may be missing or small and green and the pollen is carried away on the wind Wind Pollinated Flower e.g. grass Ms. B. Fennessy Loreto Secondary School Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Pollination Pollination is the transfer of pollen from an anther to the stigma of a flower of the same species Self- pollination Self-pollination is the transfer of pollen from an anther of one flower to stigma of a flower on the same plant This leads to self fertilisation or inbreeding and weak offspring Cross-pollination Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of a flower on a different plant of the same species This leads to cross fertilisation and stronger healthier offspring with variation from the parent plants. Methods of Pollination Pollen is transferred either by insects and animals or by the wind Insect Pollinated Wind Pollinated The structure of the flower helps pollination and it is adapted to one or other type of pollination Insect/Animal pollinated flowers Wind pollinated flowers Petals Large & colourful Small & green Scent Scented No Scent Nectar Nectaries to produce nectar No nectaries or nectar Pollen Small amounts, large & sticky Large amounts, small & smooth Stigmas Sticky & inside petals Feathery & outside petals Anthers Short & inside petals Long & outside petals Examples Daisies, Dandelions, Buttercups Ms. B. Fennessy Grasses, conifers Loreto Secondary School Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Fertilisation Fertilisation is the joining of a haploid female gamete with a haploid male gamete to form a diploid zygote which develops into an embryo The pollen grain lands on the stigma and germinates to form a pollen tube The pollen tube grows down through the style to the opening of the embryo sac. The generative nucleus (originally in the pollen grain) divides by mitosis in the tube to form two male gametes. One of the gametes fertilises the egg cell or female gamete to form a diploid zygote The diploid zygote develops into an embryo A second fertilisation between the other male gamete and the polar nuclei results in the formation of the endosperm (3n) Double Fertilisation On sperm nucleus or gamete (n) joins with the egg cell (n) to form a diploid zygote (2n) which develops into an embryo The second sperm nucleus (n) joins with the two polar nuclei (both n) for form the triploid (3n) endosperm nucleus. The pollen tube means that the male gametes of flowering plants can move towards the egg without the need for external water. This is a major adaptation towards life on land Aside: When the pollen grain lands on the stigma it is stimulated to grow by sugars made by the stigma The growth of the pollen tube is controlled by the tube nucleus which dies when the pollen tube reaches the opening of the embryo sac (micropyle) The pollen tube grows towards chemicals released by the ovule – chemotropism The haploid generative nucleus divides by mitosis as it moves down the pollen tube to form two haploid sperm nuclei or male gametes When the pollen tube reaches the micropyle the male gametes go into the embryo sac Ms. B. Fennessy Loreto Secondary School Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Seed structure The entire ovule becomes the seed. The outer wall of the ovule (integuments) gets thicker and hardens into the seed coat or testa. The zygote becomes an embryo plant which has a plumule (shoot), a radicle (root) and one or two cotyledons (seed leaves). The cotyledon(s) store food. The epicotyl is the part of the plumule between the cotyledons and the first true leaf. The hypocotyl connects the radicle with the cotyledons. The triploid endosperm nucleus becomes the endosperm. This is a triploid food storage tissue which nourishes the developing embryo. Sometimes the cotyledons absorb all of the endosperm and hence non-endospermic seeds are formed e.g. broad bean Other times the cotyledons will absorb only some of the endosperm and hence endospermic seeds are formed e.g.maize. Eventually the embryo stops growing and becomes dormant. So as the ovule develops, a seed is formed which consists of an embryo and a food supply. The food supply is contained either in an endosperm or in seed leaves (the cotyledons). Ms. B. Fennessy Loreto Secondary School Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Monocotyledon, dicotyledon classification Plants are classified based upon the number of seed leaves (cotyledons) in the seed. Monocotyledons (Monocots) contain one seed leaf (cotyledon) and dicotyledons (Dicots) contain two seed leaves (cotyledons) The cotyledons are food reserves for the young plant after it germinates in the soil. It uses these food reserves until it is able to make its own food. In monocots, the cotyledon rarely stores food; rather it absorbs food molecules from the endosperm and passes them on to the embryo. Monocot seeds are endospermic because they still use the endosperm to store their food. In dicots, the cotyledons usually store the food that the embryo uses. Dicot seeds are non-endospermic because they no longer have endosperm. It has been absorbed by the cotyledons e.g. broad bean seeds. Remember Feature Monocotyledons Dicotyledons Cotyledons (seed leaves) One cotyledon Two cotyledons Flower parts Usually in multiples of 3 Usually in multiples of 4 or 5 Venation (Veins in leaves) Usually parallel Usually reticulate (netted) Stems Herbaceous (non woody) Woody or herbaceous Vascular bundle arrangement Scattered In a ring Ms. B. Fennessy Loreto Secondary School Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Fruit Formation After pollination, the ovule develops into a seed and the ovary develops into a fruit. The developing seeds produce growth regulators (auxins) to stimulate growth of the fruit tissues A fruit is a mature ovary or sometimes a modified floral part (e.g. the receptacle). The seeds are protected by a seed coat and may be contained within the fruit. The function of the fruit is to Protect the seeds Help with seed dispersal Once the fruit has formed the rest of the flower (e.g. the petals) dies and falls away. ©BF Seedless fruit production Seedless fruit production is caused by : The genetic variety of plants Growth regulators. The genetic variety of plants Seedless fruits can be produced either naturally or by special breeding programmes In nature there are some plants where the fruit develops without pollination or fertilisation (i.e. pathenocarpy). Growing such plants allows the production of seedless fruits like grapes, bananas etc Seedless fruit can also be produced by carrying out special breeding programmes. This involves changing the chromosome number of gametes so they cannot produce a functional seed but good fruit is still produced e.g. water melons Growth regulators Developing seeds produce growth regulators (auxins) to stimulate growth of the fruit tissues Therefore seedless fruit can be produced by spraying flowers with growth regulators or auxins. The growth regulators stimulate the ovary to swell with food and become a fruit before fertilisation even occurs e.g. seedless tomatoes, grapes Genetics and growth regulators also play a role in the production of bigger and larger fruit and vegetables. Growth regulators increase the cluster, number and sizes of berries and grapes Ethene is used to ripen bananas, melons, tomatoes, and to degreen oranges, lemons, grapefruit. Ms. B. Fennessy Loreto Secondary School Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Fruit and seed dispersal Dispersal is when a plant spreads its seeds away from the parent plant and scatters them over a wide area Need for dispersal Seeds are dispersed to ensure a better chance of survival. Dispersal helps Avoid overcrowding and poor growth Minimises competition for light, water, minerals and space Allows colonisation of new areas Examples of wind, water, animal and self-dispersal Plants have a wide variety of seed dispersal techniques. The fruit helps with seed dispersal Wind dispersal Small, light seeds easily blown by wind. Fruits are often winged e.g. sycamore parachutes or have feathery parachutes e.g. dandelion and they fly to a new place Animal dispersal Fruits attach to animal using hooks e.g. goose grass, burdock and are carried away to a new place. Animals eat edible fruits. The seeds are not digested but pass through the birds system and land in a new place e.g. blackberries, strawberries Water dispersal Fruits are buoyant and float on water to a new place e.g. coconut Self dispersal Seeds are formed in pods. The pods shrivel and burst, scattering the seeds away from the parent plant Ms. B. Fennessy Loreto Secondary School Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Dormancy Dormancy is a resting period for a seed where it does not germinate or grow even though environmental conditions are suitable During dormancy there is very little cell activity or metabolism. The seeds usually do not germinate until the following spring. Advantages of dormancy Allows time for seed dispersal Allows time for embryo to develop properly Allows seedling to avoid harsh winter weather Provides a bank of seeds in the soil for the future The seeds germinate in the spring which gives them the maximum growing season Dormancy in agricultural and horticultural practices Dormancy can be a problem for farmers and gardeners because they often want seeds to germinate at times that suit them and not leave it as nature intended. Therefore, they often treat seeds before planting in order to break dormancy, maximise germination and get the seeds to germinate at the same time. Agricultural and horticultural practices to break dormancy: Soaking the seeds in water to soften the seed coat (testa) which allows water and oxygen to enter Exposing the seeds to cold temperatures for some time e.g. putting the seeds into a fridge Scraping the seed surface with sandpaper to break the tough testa Spraying the seeds with growth regulator (e.g. giberellin) to encourage germination Aside: Causes of dormancy There might be plant growth inhibitors e.g. abscisic acid in the testa which stop the embryo from growing. These might need to be washed away by the rain or broken down by the cold before the seed can germinate The testa might be too tough to let water and oxygen into the seed and it needs time to break down Some seeds need to experience a long period of cold before they will germinate e.g. apple seeds Ms. B. Fennessy Loreto Secondary School Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Germination Germination is the restart of growth by the plant embryo when environmental conditions are suitable Factors necessary for germination Factors necessary for germination are: Water Oxygen Suitable temperature. The effect of water on germination It helps to swell the seed and break the seed coat (testa). It allows the dry seed to start metabolism again. It acts as a medium or a place for the chemical reactions to occur. It activates the enzymes which break down the food reserves (lipids, carbohydrates and proteins) in the endosperm or cotyledons. It is a transport medium for digested products The effect of oxygen on germination Oxygen is needed for metabolism. It is used in aerobic respiration – so it helps release energy from the food stores. The oxygen is an atmospheric gas found in the soil air spaces. If the seed is buried too deep or if the soil is waterlogged, the seed will be oxygen starved so germination or growth will not take place Suitable temperature A suitable temperature is needed to allow enzyme reactions to take place. The rate of enzyme action is affected by temperature, therefore temperature affects metabolism and growth rates in the germinating seed. It affects chemical reactions like respiration, digestion and protein synthesis. Seeds often have a temperature range (usually between 5oC and 30oC) within which they will germinate, and they will not germinate above or below this range. I’ve soaked too much! I am fit to burst! Ms. B. Fennessy Loreto Secondary School Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Outline the role of digestion and respiration (in germination) As a result of taking in water the seed is hydrated. This activates the enzymes and starts metabolism. The food storage centres (cotyledons and/or endosperm) and the embryo are the main areas of activity. Role of Digestion Germination starts with the digestion of the food stored in the cotyledons and/or endosperm The enzymes digest the food stores to provide the materials for the growth of the embryo. Digestion is a catabolic reaction and needs water. The carbohydrate starch is converted to glucose, lipids (oils) are converted to fatty acids and glyercol and proteins are converted to amino acids. These soluble products are translocated (moved) to the growing regions of the embryo. This means that the dry weight (mass)* of the cotyledons and/or endosperm falls Some of the glucose is used to make cell walls. The amino acids are used to make enzymes and protein and so the embryo grows. This means that the dry weight of the embryo increases. Respiration When the food products have been moved from the cotyledons and/or endosperm to the embryo, respiration takes place. The lipids (oils) and glucose release energy when they are broken down in respiration (catabolic reaction). This energy is used by the embryo to carry out the anabolic reactions of making protein and growing. This means that the overall dry mass of the seed falls due to energy losses in respiration Once the seedling forms the first true leaves, photosynthesis then becomes the main source of glucose. Some of this glucose is stored as starch and the embryo continues to grow. This means that the dry weight of the seedling increases. * Dry weight is the weight without water. Dry weight is used for biological tissues to eliminate the variable of water. Digestion of Food Reserves The digested food stores are used to make cell walls and proteins Cotyledon dry weight decreases Glucose Starch Embryo dry weight increases Amino Acids Protein Fatty acids & glycerol Lipids Ms. B. Fennessy Loreto Secondary School Respiration of lipids & glucose Overall dry mass falls due to energy loss Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Stages of seedling growth A seedling is a young plant that is grown from a seed There are two types of seedling growth Cotyledons stay below the soil e.g. broad bean seed Cotyledons move above the soil e.g. sunflower The Cotyledons stay below the soil The seed absorbs water and swells and the enzymes become active The radical emerges and grow down due to gravity The plumule emerges and the epicotyl begins to grow The epicotyl pushes the plumule up through the soil. It is curved over for protection The cotyledons shrivel and wither as food is translocated out of them and sent to the embryo The radicle becomes a tap root (primary root) with many side roots Once above the ground, the plumule straightens up and the first true foliage leaves are formed which start to photosynthesise. Ms. B. Fennessy Loreto Secondary School Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH The cotyledons move above the soil The seed absorbs water and swells and the enzymes become active The radical emerges and grow down due to gravity The hypocotyl begins to elongates The hypocotyl grows upwards carrying the cotyledons with it The cotyledons come above the ground Once above the ground, the pericarp falls off, the hypocotyl straightens and the cotyledons turn green (they look like leaves, store food, help protect the plumule and carry out photosynthesis) The plumule comes out from between the cotyledons and the first true foliage leaves are formed which start to photosynthesise Seed An embryo (embryonic plant) A food supply (cotyledon or endosperm) A covering (seed coat/testa Ms. B. Fennessy Loreto Secondary School Germination Seedling Plant Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Vegetative propagation: asexual reproduction in plants Vegetative propagation (vegetative reproduction) is a type of asexual reproduction for plants. It is a process where new growth occurs without the need for seeds. It involves only one parent and the new plants are genetically identical to each other and to the parent plant. They are clones which means that there is no variation. However, the new plants are produced much faster than with sexual reproduction. They are produced naturally from modified stems, roots, leaves and buds Examples of vegetative propagation Modified stem e.g. Strawberry runners ©BF Runners are stems that run over the surface of the soil from plants like strawberries. When the runner is far enough from the parent plant, the terminal bud produces a daughter shoot and roots. When the new plant is established the runner between parent and daughter plant degenerates Modified root e.g. Dahlia root tubers A root tuber is a fibrous root that swells up with stored food. It survives under ground over the winter when the aerial shoots die away. In the following spring, the root-tubers produce new shoots from buds at the base of the old stem. The tubers can also break off and the bud near the top grows into a new separate plant. Ms. B. Fennessy Loreto Secondary School ©BF Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Modified leaf e.g. Mother of Thousands plantlets Small plantlets grow along the edge of the leaf. The plantlets fall off when they are fully developed. The roots grow and take hold in the soil and a new plant is formed ©BF Modified bud e.g. Onion bulb An onion bulb has a reduced stem. Roots extend from the underside of the stem into the soil. There are a number of leaves attached to the stem. These are swollen with food. It survives underground over the winter when the aerial shoots die away. In the following spring, the apical bud produce new shoots and a new plant is formed. ©BF Comparison of reproduction by seed and by vegetative propagation. Advantages Disadvantages Reproduction by seed Reproduction by vegetative propagation Variation in offspring (allows evolution) Simple reliable process only dependent on mitosis Less competition due to seed dispersal Rapid growth as offspring attached to parent Seeds stay dormant forming a seed bank in soil No external agents needed e.g. insects not needed Complex process dependent on both meiosis and mitosis No variation (evolution is slower) Depends on external agents for pollination & dispersal Overcrowding & competition for water, space & light Wasteful of flower parts e.g. pollen, seeds & fruit No seeds formed so there is no seed bank in the soil Slow growth of young plants e.g. long time for adult plant Identical offspring (clones) so susceptible to same disease Ms. B. Fennessy Loreto Secondary School Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH Artificial propagation in flowering plants. Asexual methods are used in agriculture and horticulture to grow new plants that are genetically identical to the parent plants and each other. These methods are used to grow plants with desirable characteristics that do not come true from seed. Exemplify any four methods used by horticulturalists to artificially propagate plants 1. Cuttings e.g. Germanium A cutting is taking part of a shoot and getting it to grow into a new plant. The shoot is cut at an angle just below a node. The cut end is dipped in rooting hormone powder to encourage new roots to develop. It is planted in moist soil in a pot. New roots grow from the base of the cutting . When the plant is established, it can be planted out. (The cutting can be put directly into sand to prevent waterlogging the new roots. The shoot can be covered with a plastic bag to prevent transpiration) 2. Grafting e.g. Apple tree Grafting is where the shoot system (scion) of one plant is joined to the root system (stock) of another. The main shoot of a crab apple tree is cut off at an angle. The root system is used as the stock. A branch of an eating apple tree is cut at similar angle (scion) The cut surfaces of the scion and stock are lined up and bound together The process is successful if the meristematic tissue merges Ms. B. Fennessy Loreto Secondary School ©BF Fermoy Co. Cork 3.6 REPRODUCTION AND GROWTH 3. Layering e.g. Carnations Layering involves bending over a long stem so that it touches the ground A stem of the parent plant is bent down and pegged into the soil. A cut is made at the bend to encourage roots to grow. The piece on the ground is covered in soil When a root system has developed the daughter plant can be separated from the parent plant ©BF 4. Tissue culturing (micro-propagation) e.g. Orchids Micropropagation is the growth of a large number of plants from very small plant pieces or cells. A plant is cut into many small pieces The cells are grown in-vitro on a suitable medium A clump of similar cells called a callus is formed The growing conditions are changed so that the callus grows into a young plant When the plants are large enough they can be planted out like normal plants This section reappears in the course ©BF In this way large numbers of identical plants can be grown. The plants are all genetically identical to each other and to the parent plant and are clones. The advantage of micropropagation is that a large number of plants can be produced in a short time however because there is no variation, when a disease strikes, all the plants are equally susceptible and they could all die. Ms. B. Fennessy Loreto Secondary School Fermoy Co. Cork