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Transcript
Sexual reproduction in flowering plants Lesson objectives By the end of this lesson you should be able to: • Distinguish between sexual and asexual reproduction with respect to plants • Describe the structures and functions of the parts of a flower • Discuss the formation male and female gametes Introduction • The ability to produce offspring is one of the characteristics of life • Reproduction ensures that parental traits are transmitted to successive generations • Reproduction has 2 main functions 1. Replacing organisms that die 2. Allows for an increases in numbers when conditions are favourable Asexual and sexual reproduction • Asexual reproduction involves 1 parent and requires no gametes – does not require meiosis – offspring are clones of parent (genetically identical) • Sexual reproduction involves 2 parents – fusion of gametes (fertilisation) – offspring show variations as they are a mixture of genes from both parents (driving force of evolution) Structures and functions of a flower Male gamete formation Female gamete formation What have you learned? Can you. . . • Distinguish between sexual and asexual reproduction with respect to plants? • Describe the structures and functions of the parts of a flower? • Discuss the formation male and female gametes? Lesson objectives • • • • By the end of this lesson you should be able to: Distinguish between self and cross pollination Discuss methods of pollination Discuss the causes, effects and treatments for hay fever Discuss fertilisation and seed formation I flowering plants Pollination “Pollination is the transfer of pollen from an anther to a stigma of a flower from the same species” • There are 2 types of pollination: 1. Self pollination 2. Cross pollination Methods of pollination – 2 types Animal Wind pollination pollination Adaptations of flowers to wind or animal pollination Wind pollination Animal pollination Petals: small or absent, not brightly coloured, no scent, no nectaries Pollen: large amounts, light, small, dry, smooth Anthers: large, outside petals, loosely attached to filament Stigmas: large and feathery, outside petals Petals: large, brightly coloured, have nectaries Pollen: small amounts, large, heavy, sticky, spiny Anthers: usually small, inside petals, firmly attached to filament Stigmas: usually small and sticky, inside petals Hay fever • Is an allergic reaction to the inhalation of particles of certain harmless substances • The substance that triggers the reaction is known as the allergen • The most common allergens are pollen grains but include fungal spores, animal skin or scales and house dust • The symptoms of hay fever include the inflammation of the mucous membranes of the nose, sneezing, a blocked and runny nose along with watery and irritated eyes • It affects 10% of the population and can be reduced by limiting contact with the allergen • Treatment include anti – histamine drugs and other drugs that partially inhibit the allergic response Fertilisation “Fertilisation is the union of male and female Growth of a pollen tube gametes to form a diploid zygote” Double fertilisation • Flowering plants are unique in having a double fertilisation: 1. 1st sperm nucleus (n) joins with the egg nucleus (n) to form a diploid (2n) zygote) – develops into the embryo 2. 2nd sperm nucleus (n) joins with the 2 polar nuclei (both n) to form a triploid (3n) endosperm nucleus Seed development Types of seeds Number of cotyledons • Plants that form a seed with a single cotyledon are called monocots, while plants with 2 cotyledons are called dicots Presence or absence of endosperm • In monocots the cotyledon absorbs food from the endosperm and normally passes it onto the embryo (i.e. the cotyledons do not store food) – at maturity monocot seeds have an endosperm – endospermic (grasses, cereals and maize) • In dicots the cotyledons absorb food from the endosperm and act as food stores – at maturity dicot seeds do not have an endosperm – non endospermic A non – endospermic seed (broadbean) An endospermic dicot seed (castor oil bean) LS of a monocot seed (maize). Note that maize is not a typical monocot seed in that it stores food in the cotyledon, as shown above. What have you learned? • • • • Can you. . . Distinguish between self and cross pollination? Discuss methods of pollination? Discuss the causes, effects and treatments for hay fever? Discuss fertilisation and seed formation I flowering plants? Lesson objectives • • • • • By the end of this lesson you should be able to: Discuss fruit formation Distinguish between true and false fruits Discuss the development of seedless fruit Discuss fruit and seed dispersal Discuss seed dormancy Development of a fruit Two types of fruit Changes in a flower after fertilisation Before fertilisation After fertilisation 1 Ovule Seed 2 Integumants Testa (seed coat) 3 Nucellus Endosperm→Cotyledons 4 Egg Zygote→Embryo 5 Polar nuclei 6 Ovary Endosperm Fruit 7 Ovary wall Pericarp (fruit coat/wall) Seedless fruit • The development of a fruit without a seed is called parthenocarpy (or parthenocarpic fruiting) – it can be formed in 2 ways 1. Genetically, either naturally or through special breeding programmes – bananas, grapefruit, pineapples, oranges and grapes 2. To spray plants with growth regulators e.g. auxins – grapes, peppers, cherries and some types of seedless tomatoes Fruit and seed dispersal “Dispersal is the transfer of a seed or a fruit away from the parent plant” Dispersal is necessary to : • Avoid large numbers of seeds competing with each other and the parent plant • Increase the chance of survival for the plant • Find new areas for growth • Increase the numbers of species The main methods of dispersal are wind, water, animal and self - dispersal Wind dispersal Water dispersal Animal dispersal Edible, Sticky fruits fleshy- or goosegrass succulent fruits Self - dispersal Examples include peas, beans and gorse Dormancy “Dormancy is a resting period when seeds undergo no growth and have reduced cell activity or metabolism” Dormancy is brought about by a number of ways: 1. Growth inhibitors (e.g. abscisic acid) may be present in the outer part of the seed 2. The testa may be impermeable to water or oxygen 3. The testa may be to tough to allow the embryo to emerge 4. There may be a lack of suitable growth regulator present to stimulate growth Dormancy in agriculture or horticulture • • • 1. 2. 3. 4. Many seeds need a cold break to break dormancy – cause the breakdown of growth inhibitors or the production of growth promoters such as auxins Special conditions are necessary to break dormancy in seeds before they are planted for agricultural or horticultural use These conditions include: Soaking seeds in water Physical damage Exposure to light or dark Exposure to cold temperatures Advantages of dormancy • Allows the plant to avoid the harsh conditions of winter • Gives the embryo time to develop fully • Provides time for the seed to be dispersed • Maximises the growing season for the young seedling, i.e. by starting growth in the spring the plant is well developed by the autumn • Helps survival of the species because the duration of the dormancy varies What do you know? • • • • • Can you. . . Discuss fruit formation? Distinguish between true and false fruits? Discuss the development of seedless fruit? Discuss fruit and seed dispersal? Discuss seed dormancy? Lesson objectives • • • • • • By the end of this lesson you should be able to: Discuss the conditions necessary for seed germination Discuss the events in seed germination Account for changes in dry weights of germinating seeds Discuss the stages in seedling growth MA: To investigate the effect of water, oxygen and temperature on seed germination MA: To use starch agar or skimmed milk plates to show digestive activity during germination Germination “Germination is the re - growth of the embryo after a period of dormancy if the environmental conditions are suitable” Conditions for germination • Water is needed to allow enzyme reactions to occur – the seeds absorb water from the soil • Oxygen is needed for aerobic respiration • A suitable temperature is needed to allow enzyme reactions to take place • Dormancy must be complete • Light may be required Events in germination 1. Seeds store food in the form of oils, starch (especially 7.4.Dry weight (mass) of the falls the food used Products of digestion areseed moved todue theto embryo in cereals and grasses) and protein (especially in in 10.respiration Thelegumes plumulesuch emerge above thebeans) ground and leaves form as peas and 5.2.Glucose and amino acids areseeds used to makewater new structures Germination begins when absorb through 11.Once the first leaves start to photosynthesize the dry 8. Weight of food stores falls, the weight of the embryo such as cell and enzymes a tiny holewalls called the micropyle allowing enzymes to be weight of the seedling increases again increases activated 6. Fats and some of the glucose are used in respiration Oils bursts are digested fatty acids and glycerol, starch to 9. 3. Radicle throughtothe testa glucose and proteins to amino acids Changes in dry weights of germinating seed parts The stages in seedling growth Cotyledons remain below the soil – Broad beans In radicle this type of germination seed absorbs water, 6. 4.1. The The plumule develops grows up into through thethe primary the soil or and tap its root delicate which enzymes become active the seed begins to grow forms leaves many are protected lateral roots by theand plumule being hooked over Radicle bursts out through the testa and grows down 7. 5.2. Once The cotyledons above the (and ground endosperm, the plumule if present) straightens shrivel up and as produces food is transferred the first true from leaves them whichbetween become the green and 3. Plumule emerges and the region plumule start photosynthesize andtocotyledon (epicotyl) grows The stages in seedling growth Cotyledons move above the soil – Sunflower seeds This form of germination is similar to the previous form with the following differences: The above region the between thefruit emerging radicle and 2.• Once soil, the wall (pericarp) fallsthe to the cotyledons (hypocotyl)open grows the emerging ground, the cotyledons outcausing and become green and cotyledons to be carried above the soil photosynthetic. The plumule emerges from between the cotyledons and forms the first foliage leaves What do you know? Can you . . . • Discuss the conditions necessary for seed germination? • Discuss the events in seed germination? • Account for changes in dry weights of germinating seeds? • Discuss the stages in seedling growth? • MA: Describe an investigation of the effect of water, oxygen and temperature on seed germination • MA: Describe an investigation to use starch agar or skimmed milk plates to show digestive activity during germination Lesson Objectives - HL By the end of this lesson you should know how to: • Discuss male gamete formation and pollen grain development in detail • Discuss female gamete formation and the development of the embryo sac in detail • Discuss the events of fertilisation in detail Male gamete formation Pollen grain development Formation of a pollen grain Development of the embryo sac Female gamete formation LS of a carpal with mature embryo sac Fertilisation “Fertilisation is the union of male and female Growth of a pollen tube gametes to form a diploid zygote” What have you learned? Can you. . . • Discuss male gamete formation and pollen grain development in detail? • Discuss female gamete formation and the development of the embryo sac in detail? • Discuss the events of fertilisation in detail?