3.6 Reproduction & Growth 3.6.1 Reproduction of The Flowering Plant Chapter 40 Flowering Plant Sexual Reproduction Learning Objectives 1. Give the structure and function of the floral parts. 2. Outline what is produced by pollen grains and embryo sacs. 3. Define and discuss the methods of pollination : 4. Define fertilisation (1st and 2nd). 5. Detail seed structure and function of each part. 6. Mention the function/purpose of cotyledons as a food store. 7. Monocotyledon, dicotyledon classification and distinguishing features. Reference to nonendospermic seed. 8. Describe the process of fruit formation. 9. Discuss the production of seedless fruit. 10. Explain the different methods of fruit and seed dispersal & discuss the need for dispersal. 11. Define and give the advantages of dormancy with reference to agriculture and horticulture. 12. Define germination and list the factors necessary. 13. Discuss the events of germination, the role of digestion and respiration. 14. Describe the stages of seedling growth. 15. Investigate the effect of water, oxygen and temperature on germination. 16. Use starch agar or skimmed milk plates to show digestive activity during germination. 17. Describe the stepwise development of pollen grains from microspore mother cells. 18. Describe the stepwise development of an embryo sac. There are 2 types of reproduction; Sexual and Asexual Asexual reproduction: Involves only one parent Does not require meiosis Does not involve gametes or fertilisation Offspring are genetically identical to the parent Genetically identical offspring produced by asexual reproduction form a family called a clone. Sexual reproduction: Involves two parents Requires meiosis Fertilisation involves the fusion of 2 gametes to form a zygote. Involves gametes & fertilisation Offspring are genetically different to the parents, they show variation Sexual reproduction is more beneficial to a species as it allows for evolution to occur.
Flowering Plant Structure - Receptacle: Supports flowering parts of plant - Sepal: Protect flower when it is a bud - Petal: Brightly coloured to attract insects (pollinators) - Stamen (Male part): Consists of Anther: Produces pollen grains by meiosis & Filament: Supports anther and contains vascular bundle to provide water and food to anther - Carpel (Female part) - Consists of Stigma: Where pollen lands, Style: Where pollen tube develops & Ovary: Produces ovules, where fertilisation occurs. Stages of Reproduction 1. Gamete formation, 2. Pollination, 3. Fertilisation, 4. Seed Formation 5. Fruit Formation, 6. Seed Dispersal, 7. Dormancy 8. Germination Male Gamete Formation (O.L & H.L) The male gametes are the pollen grains, produced by the anther. Cells inside the anther are diploid. Meiosis occurs producing haploid pollen grains. Each of these pollen grains then divides by mitosis to produce large numbers of gametes. When the pollen grains are fully developed, the anther splits and peels back exposing the pollen grains outside the anther. *Male Gamete Formation (H.L) Every anther consists of 4 internal chambers called pollen sacs. The tapetum supplies food and energy for cell division inside the pollen sac. Diploid cells called microspore mother cells (pollen mother cells) divide by meiosis to form 4 haploid cells called a tetrad. Each tetrad divides by mitosis to produce 4 individual pollen grains (haploid) called microspores. Each microspore contains 2 haploid nuclei (1 generative & 1 tube nucleus). Each pollen grain has a protective thick outer wall called an exine & a thin inner coat called the intine. *Female Gamete Development (H.L) The female gametes are the ovules, produced by the ovary. The integuments (ovary walls) have a small micropyle (opening) which allows the pollen tube to enter.
A single diploid megaspore mother cell in the ovule divides by meiosis to form 4 haploid cells, three degenerate, one remains as the embryo sac (Megaspore). The haploid nucleus of the embryo sac divides by mitosis 3 times forming 8 haploid nuclei. Of the 8 nuclei, 5 degenerate, the remaining 3 form the female gametes. 2 of these are polar nuclei, one forms a thin cell wall and becomes the egg. Nucellus cells supply nutrients for the swelling of the ovary after fertilisation. LS of Carpel with Mature Embryo Pollination Pollination: 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. Form of inbreeding resulting in less sturdy seeds. E.g. cereals (corn, maize) Cross-pollination involves the transfer of pollen from an anther to a different plant of the same species. Most common form, plants adapt to provide best chance of successful pollination. Seeds show more vigour and variation. Because plants can t move around they need to adapt other means to transfer the male pollen to the female eggs. [Wind (Grass, Conifers) or Animal (Orchid, Daisy, Daffodil)
Fertilisation Fertilisation: The union of male and female gametes to form a diploid zygote. Pollen lands on a stigma, a pollen tube is stimulated by the tube nucleus to grow towards the ovule & degenerates once it reaches the ovule (micropyle). The pollen tube is attracted towards chemicals produced by the ovule (chemotropism). As the generative nucleus moves down the pollen tube it divides by mitosis into two haploid sperm, which move toward the egg. In plants double fertilisation occurs. One haploid sperm joins with the haploid egg nucleus forming a diploid zygote, which will develop into an embryo (seed). The other haploid sperm joins with both haploid polar nuclei to form a triploid endosperm nucleus. Seed Formation The fertilised ovule becomes the seed. The integuments dry up and become the testa (seed coat). The zygote divides by mitosis to form the embryo (young plant). The embryo consists of the radicle (future root) and plumule (future shoot). Some embryo cells develop into the cotyledons (seed leaves). The endosperm nucleus (3n) divides rapidly by mitosis to produce the endosperm which acts as a food store (oils & fats).
Non-Endosperm Seed ENDOSPERMIC & NON-ENDOSPERMIC SEEDS As embryos develop the cotyledons will absorb the endosperm. If all endosperm is absorbed a non-endospermic seed is formed (Broad Bean, Peanut, Sunflower). Non-Endospermic Seed has no endosperm when fully formed. (Usually dicots) Endospermic Seed contains some endosperm when fully formed (Maize & Corn). (Usually monocots) Endosperm Seed In the initial stages, the nucellus food allows growth of the endosperm & embryo. Fruit Formation After fertilisation auxins stimulate the ovary to swell to protect the seed, this forms the fruit. The outside of the carpel becomes the outer layer of the fruit (pericarp). Some fruits are fleshy (tomatoes, peaches, grapes), others are dried (pea pods, green beans, peanuts). Once the fruit is formed the rest of the flower parts die away. False Fruits o o False fruits are fruits that develop from a part of the plant other than the ovary. Parts at the base of the flower (such as receptacle) swell with stored food and join. E.g. Apple (core is ovary) and strawberry. Seedless/Parthenocarpic Fruits Parthenocarpy: The development of fruit without a seed. o o 2 methods: Genetically using special breeding programmes (bananas, grapes, grapefruit). Addition of growth regulators (auxins) to flowers (peppers, cherries, grapes, peaches)
Fruit & Seed Dispersal Dispersal is the transfer of a seed or fruit away from the parent plant. It is important to; Avoid large numbers of seeds competing for resources Increase the chance of survival for plants Spread to new growth areas Increase species numbers 4 methods; Wind: tiny light seeds. E.g. Dandelion, Sycamore, Thistle, Ash Water: air-filled fruits that float. E.g. Coconut, Water Lily Animal: Sticky. E.g. Burdock, Buttercup. Edible. E.g. Strawberries, Acorns, Blackberries Self: Fruit dries and bursts spreading seeds. E.g. Peas, Lupins, Beans Dormancy Dormancy is a resting period when seeds undergo no growth and have reduced activity or metabolism. It can occur due to a number of factors; Growth inhibitors (abscisic acid) may be present on the outside of the seed until they get broken down by water or cold. The testa may be impermeable to water or oxygen, it will eventually breakdown allowing them to enter. Lack of suitable growth regulator due to lack of light, levels rise in spring due to increased light and temperature. Benefits: Allows plants to avoid harsh weather conditions. Gives more time for the embryo to develop fully. Provides time for dispersal. Maximises a young plants growing season from spring to autumn. Allows seed banks to form in the soil (seeds remain dormant for many years). They may only grow after elimination of all mature plants after a natural disaster. Germination Germination is the regrowth of the embryo, after a period of dormancy, if environmental conditions are suitable. - Dormancy must have occurred, even for a very short period of time to allow the embryo to develop, hence it is the regrowth or resumed growth of the embryo.
Conditions Necessary: Water: Allows enzyme reactions to occur. Absorbed from the soil. Oxygen: Required for aerobic respiration. Absorbed from the soil. Suitable Temperature: To allow enzyme action. Every plant is different but usually between 5 o C and 30 o C. Events: 1. The seeds absorb water 2. Stored foods are digested to simpler forms (oils to fatty acids & glycerol, Starch to glucose and proteins to amino acids) by the now active enzymes in the seed 3. Digested foods are transferred from the endosperm or cotyledon to the embryo 4. Some digested foods make new structures and some are used in respiration 5. The radicle grows and bursts through the testa 6. The plumule emerges above ground and new leaves form During germination, seed weight falls between days 0-6 due to respiration. From day 6 on it increases due to photosynthesis occurring in the newly formed leaves. As the weight of the embryo increases, the weight of the endosperm reduces accordingly. This shows that food is passing from the endosperm to the embryo. Epicotyl Germination:COTYLEDONS REMAIN BELOW SOIL Example; Broad Bean Seed absorbs water, activating enzymes and stimulating radicle growth. The radicle bursts through the testa and grows due to geotropism. The plumule emerges and the epicotyl (region between cotyledon and plumule) grows. The cotyledons & endosperm shrivel. The radicle develops into the primary root. The plumule straightens and produces the first true leaves.
Hypocotyl Germination: COTYLEDONS MOVE ABOVE SOIL Example; Sunflower Seed It differs from epicotyl germination due to; The region between the radicle and cotyledons is called the hypocotyl and it carries the cotyledons out of the soil. Once above ground the pericarp falls off and the cotyledons open, become green and photosynthesize. The plumule emerges from between the cotyledons and forms the first true leaves. To Investigate the Effect of Water, Oxygen and Temperature on Germination. Set up 4 tests tubes of radish/cress seeds under the following conditions: 1. Tube A: Control (Water, Suitable Temperature, Oxygen) [Germinated] 2. Tube B: No Water (Suitable Temperature, Oxygen) [No Germination] 3. Tube C: No Oxygen (Boiled Water, Layer of Oil, Suitable Temperature) [No Germination] 4. Tube D: In Cold (Water, Oxygen, Stored in Fridge) [No Germination] To Show Digestive Activity Occurs During Germination 1. Place halved, sterile, soaked seeds on starch agar 2. Leave in a warm place 3. Add iodine after a few days 4. The starch is digested by enzymes in live seeds (clear areas around the seeds) 5. The starch is not digested by dead (boiled) seeds, as no clear areas appear