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Sexual Reproduction in Flowering Plants Sexual Reproduction

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Sexual Reproduction in Flowering Plants Sexual Reproduction Sexual Reproduction in Flowering Plants Sexual Reproduction Sexual reproduction Anther Gametes Stamen Carpel Stigma Keywords Pollen grain Pollination Fertilisation Seed formation Seed dispersal Germination Aims • State the structure & function of the floral parts including: Sepal, petal, stamen, carpel) • Outline pollen grain development from microspore mother cells • Outline embryonic sac development What is meant by sexual reproduction? • It involves the fusion of a male and female gamete to form a zygote. • Gametes are haploid cells. • Sexual reproduction allows for variation and is more advantageous to a species than asexual reporduction. Structure of a flower Structure of a flower Stamen and Carpel Functions of the flower: Receptacle: Supports the floral parts of the flower. Sepal: To protect the flower Petals: To attract the insects and animals for pollination (to attract insects the flower is large and bright) Functions of the flower: • Stamen : To produce the pollen grains in the anthers. (Male part of flower) Functions of the flower: Filament: Holds the anther in place Anther: produces pollen (male gamete) Functions of the flower: Carpel : The female part of the flower Functions of the flower: Stigma: Where pollen lands after pollination Style: Pollen travels down this. Ovary Contains ovules Male Gamete formation Meiosis takes place in some cells to produce pollen grains which have a haploid nucleus. Each pollen grain divides by mitosis to form the male gametes. Pollen Grain development Anther consists of 4 chambers called pollen sacs Each pollen sac is surrounded by a protective layer called the fibrous layer. Pollen grain development Tapetum is a layer of cells used to provide food for cell division in the pollen sac. Pollen grain development There are diploid microspore mother cells in each pollen sac which are diploid. These cells divide by meiosis to produce haploid tetrad cells. Each tetrad forms four separate haploid pollen grains (microspores) Pollen grain development Each pollen grain has a thick outer wall called the exine (allows it to survive for long periods) Pollen grain then divides by mitosis to produce two haploid nuclei (the tube and generative nuclei). Once pollen grains have matured the anther dries and shrivels and the pollen are released. Development of the embryo sac The ovary contains an ovule. The ovule has a small opening called a micropyle for the pollen tube to enter. The ovule has a megaspore mother cell which divides by meiosis to form four haploid cells. Three of these cells degenerate and the remaining one is called the embryo sac. Development of embryo sac Structure of the ovule Megaspore mother cell (2n) Meiosis Integuments Embryo sac (n) micropyle Development of the embryo sac The haploid nucleus in the embryo sac divides by mitosis three times to form 8 haploid nucleus. Mitosis happens 3 times Polar nuclei (n) Egg cell (n) Development of the embryo sac Five of these nuclei degenerate. The three remaining nuclei form the female gametes. Two polar nuclei Egg cell Development of the embryo sac Pollination Pollination is the transfer of pollen from an anther to a stigma of a flower from the same species. Self pollination: involves the transfer of pollen from an anther to a stigma on the same plant. Cross fertilisation: involves the transfer of pollen from an anther to a stigma on a different plant of the same species. Pollination Anther Stigma Methods of Pollination Wind: wasteful of pollen (e.g grasses and oak) Animals: Less pollen wasted, insects are most common pollinators (e.g dandelions, primroses and buttercups) Wind Pollinated Plants Petals: small and green, no scent or nectar Pollen: large amounts, light, small and dry Anther: large and outside petal Stigma: large and feathery and outside petal. Wind Pollinated plants Adaptions of wind pollinated plants Animal Pollintion Petals: large, brightly coloured, scented and have nectar. Pollen: small amounts, heavy, large and sticky Anthers: small and inside petal Stigmas: small and sticky and inside petal Animal Pollination Animal Pollination Pollination Video Wild orchid wasp mimic - David Attenborough - BBC – YouTube Plant Reproduction: Methods of Pollination – YouTube nilu's classes:- class xii , chem.., Plant Reproduction Methods of Pollination - YouTube Did you Know: Hay Fever: An allergic reaction to allergens such as pollen grains. Affects up to 10% of population. Causing sneezing, blocked or runny nose. Fertilisation Fertilisation is the fusion of the male and female gametes to form a diploid zygote. Male gamete is the pollen grain Female gamete = 2 polar nuclei and the egg cell Fertilisation • Pollen grains lands on a stigma and a pollen tube begins to grow down the style towards the ovule. • Growth of the pollen tube is controlled by the tube nucleus which degenerates when it reaches the micropyle. • Ovule releases chemicals to cause pollen tube to gro towards the micropyle. Fertilisation Pollen grain Stigma Style Polar nuclei Ovary Egg cell Fertilisation Fertilisation Fertilisation • The haploid generative nucleus divides by mitosis to form two haploid sperm nuclei (male gametes) • The presence of the pollen tube means that water is not needed for the male gametes to move towards the egg. Double Fertilisation 1) One sperm nucleus (n) fuses with the egg nucleus (n) to form a diploid zygote (2n). 2) The second sperm nucleus (n) fuses with the two polar nuclei (both n) to form a triploid (3n) endosperm. Double Fertilization in Flowering Plants - YouTube Double fertilisation Double Fertilization in Angiosperms - YouTube Fertilisation Seed Formation • After fertilisation, the ovule becomes the seed. • Zygote divides by mitosis to form an embryo which will form the plumule (future shoot) and the radical (future root). • Some of embryo also develops into the cotyledon. Seed formation • The endosperm (3n) divides by mitosis to produce many endosperm cells. • These absorb the nucellus and act as a food store (starch and oils). Seed Development Seed Formation Before Fertilisation After Fertilisation Ovule Seed Integuments Testa (seed coat) Nucellus Endopserm (cotyledon) Egg Embryo (plumule, radicle) Polar nuclei Endosperm Ovary Fruit Ovary wall Pericarp (fruit coat or wall) Non endospermic seed It has no endosperm when fully formed. All the endosperm is absorbed by the cotyledon. Example: Broad bean, peanut and sunflower. Endospermic Seed It contains some endosperm when fully formed. Cotyledon only absorbed some of the endosperm. Example: Maize and corn NOTE: when your eating popcorn you are eating the endosperm of a seed. Non endospermic and endospermic seeds Summary of seed development Plumule (future shoot) Embryo Radical (future root) Cotyledon (food store or seed leaf) Seed Endosperm Food store (Monocots) Endospermic seed Monocot and dicot seeds Monocot seeds have one cotyledon Example: cereals, grasses and daffodils. Dicots have two cotyledons Example: broad bean and sunflower. NOTE: A cotyledon is a seed leaf used for food storage Monocots and Dicots Monocots: food is not stored in cotyledon but is instead stored in the endosperm. Dicots: food is stored in the cotyledons. Fruit Formation The ovary becomes the fruit. Wall of the ovary becomes the wall of the fruit (pericarp). Fruits are used to protect the seed and help in seed dispersal. Fleshy Fruits: Peaches, Tomatoes and grapes Dry Fruits: Green beans, monkey nuts and popcorn Fruit Formation Once the fruit has formed the other flower parts disappear and fall away. False Fruits In some exceptions, some fruits develop form other parts of flower apart from the ovary. These fruits are known as false fruits. Example: apples develop when base of flower join together and swell. Strawberries develop from a swollen receptacle. Seedless Fruit • Development of a fruit without a seed is called parthenocrapy. • The egg is not fertilised. How seedless fruits are formed? 1) Can be formed genetically (bananas, oranges and grapes) 2) Spray plants with growth regulators (auxins) which allows fruits to form without seeds and fertilisation occurring (grapes, peppers, peaches) Fruit and seed dispersal Dispersal is the transfer of a seed or fruit away from the parent plant. Why is dispersal important: 1) Avoids competition between each seed and also with the parent plant. 2) Increases the chance of survival. 3) Finds new area to grow. 4) Increases the numbers of the species. Types of dispersal: 1) Wind 2) Water 3) Animal 4) Self dispersal Wind dispersal Normally these seeds are small, light and have wings. Some of these seeds may also have parachutes His allows them to be easily dispersed. However, the young plant has very little food to keep it nourished in the early days of growth. Wind dispersal Water dispersal These have light air filled fruits which float. Allows them to be dispersed over large distances by rivers and the sea. Animal dispersal Animals carry seeds over large distances. These seeds have two adaptions: 1) Sticky fruits: may stick to animals hair or fur and be carried away. Animal dispersal 2) Edible and fleshy fruits: Brightly coloured, plenty of food with strong scents to attract animals. These seeds are released with faeces and acts as a fertiliser for the seed. Examples: strawberries and acorns. Self dispersal Some fruits have an explosive mechanism that sprays seeds a short distance away from parent plant. Examples: peas and beans Video on Seed dispersal • Seed Dispersal - YouTube Dormancy • Dormancy is a resting period when seeds under go no growth and have reduced cell activity or metabolism Causes of Dormancy 1) Growth inhibitors (abscisic acid) slow down growth until they are broken down by water. 2) Testa maybe impermeable to water which prevents growth. 3) Testa maybe to tough to allow embryo to grow. Advantages of Dormancy: 1) Allows the plant to avoid harsh conditions in the winter. 2) Give the embryo time to develop fully. 3) Provides time for seed to be dispersed. 4) Maximises the growing season by starting to grow in spring. Dormancy in agriculture and horticulture Some seeds need a cold period to break dormancy which allows for the production of growth promoters (auxins).
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